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Cells regularly experience fluid flow in natural systems. However, most experimental systems rely on batch cell culture and fail to consider the effect of flow-driven dynamics on cell physiology. Using microfluidics and single-cell imaging, we discover that the interplay of physical shear rate (a measure of fluid flow) and chemical stress trigger a transcriptional response in the human pathogen Pseudomonas aeruginosa . In batch cell culture, cells protect themselves by quickly scavenging the ubiquitous chemical stressor hydrogen peroxide (H 2 O 2 ) from the media. In microfluidic conditions, we observe that cell scavenging generates spatial gradients of H 2 O 2 . High shear rates replenish H 2 O 2 , abolish gradients, and generate a stress response. Combining mathematical simulations and biophysical experiments, we find that flow triggers an effect like “wind-chill” that sensitizes cells to H 2 O 2 concentrations 100 to 1,000 times lower than traditionally studied in batch cell culture. Surprisingly, the shear rate and H 2 O 2 concentration required to generate a transcriptional response closely match their respective values in the human bloodstream. Thus, our results explain a long-standing discrepancy between H 2 O 2 levels in experimental and host environments. Finally, we demonstrate that the shear rate and H 2 O 2 concentration found in the human bloodstream trigger gene expression in the blood-relevant human pathogen Staphylococcus aureus , suggesting that flow sensitizes bacteria to chemical stress in natural environments.

Cells regularly experience fluid flow in natural systems. However, most experimental systems rely on batch cell culture and fail to consider the effect of flow-driven dynamics on cell physiology. Using microfluidics and single-cell imaging, we discover that the interplay of physical shear rate (a measure of fluid flow) and chemical stress trigger a transcriptional response in the human pathogen Pseudomonas aeruginosa. In batch cell culture, cells protect themselves by quickly scavenging the ubiquitous chemical stressor hydrogen peroxide (H2O2) from the media. In microfluidic conditions, we observe that cell scavenging generates spatial gradients of H2O2. High shear rates replenish H2O2, abolish gradients, and generate a stress response. Combining mathematical simulations and biophysical experiments, we find that cells in flow are sensitive to a H2O2 concentration that is 100-1000 times lower than traditionally studied in batch cell culture. Surprisingly, the shear rate and H2O2 concentration required to trigger a transcriptional response closely match their respective values in the human bloodstream. Thus, our results explain a long-standing discrepancy between H2O2 levels in experimental and natural systems. Finally, we demonstrate that the shear rate and H2O2 concentration found in the human bloodstream trigger gene expression in the blood-relevant human pathogen Staphylococcus aureus, suggesting that flow sensitizes bacteria to chemical stress in natural environments.

Superoxide dismutases (SODs) are ancient enzymes of widespread importance present in all domains of life. Many insights have been gained into these important enzymes over the 50 years since their initial description, but recent studies in the context of microbial pathogenesis have resulted in findings that challenge long established dogmas. The repertoire of SODs that bacterial pathogens encode is diverse both in number and in metal dependencies, including copper, copper and zinc, manganese, iron, and cambialistic enzymes. Other bacteria also possess nickel dependent SODs. Compartmentalization of SODs only partially explains their diversity. The need for pathogens to maintain SOD activity across distinct hostile environments encountered during infection, including those limited for essential metals, is also a driver of repertoire diversity. SOD research using pathogenic microbes has also revealed the apparent biochemical ease with which metal specificity can change within the most common family of SODs. Collectively, these studies are revealing the dynamic nature of SOD evolution, both that of individual SOD enzymes that can change their metal specificity to adapt to fluctuating cellular metal availability, and of a cell’s repertoire of SOD isozymes that can be differentially expressed to adapt to fluctuating environmental metal availability in a niche.

Staphylococcus aureus is a versatile opportunistic pathogen whose success is driven by its ability to adapt to diverse environments and host-imposed stresses. Two-component signal transduction systems, such as ArlRS, often mediate these adaptations. Loss of ArlRS or the response regulator ArlR alone impairs the ability of S. aureus to respond to host-imposed manganese starvation and glucose limitation. As sensor histidine kinases and response regulators frequently work as pairs, it has been assumed that ArlS senses and activates ArlR in response to these stimuli. However, recent work suggests that the sensor histidine kinase GraS can also activate ArlR, calling the contribution of ArlS in responding to manganese and glucose availability into question. The results of current studies reveal that ArlS is necessary to activate ArlR in response to manganese sequestration by the host immune effector calprotectin and glucose limitation. Although the loss of ArlS does not completely eliminate ArlR activity, this response regulator is no longer responsive to manganese or glucose availability in the absence of its cognate histidine kinase. Despite the residual activity of ArlR in the absence of ArlS, ArlR phosphorylation by ArlS is required for S. aureus to resist calprotectin-imposed metal starvation. Cumulatively, these findings contribute to the understanding of S. aureus signal transduction in response to nutritional immunity and support the previous observation indicating that ArlRS is activated by a common signal derived from host-imposed manganese and glucose limitation. IMPORTANCE The ability of pathogens, including Staphylococcus aureus, to sense and adapt to diverse environments partially relies on two-component systems, such as ArlRS. Recent work revealed that the response regulator ArlR can be cross-activated by the sensor histidine kinase GraS, rendering the role of its cognate partner, ArlS, in response to manganese and glucose limitation uncertain. The results of this study reveal that ArlS is necessary for the activation of ArlR in response to calprotectin and glucose limitation. Although a low level of ArlR activity remains in the absence of ArlS, ArlS phosphotransfer to ArlR is required for S. aureus to overcome calprotectin-induced nutritional stress. Collectively, this study provides fundamental information to understand how ArlRS mediates staphylococcal adaptation during infection.

Yersinia pestis causes human plague and colonizes both a mammalian host and a flea vector during its transmission cycle. A key barrier to bacterial infection is the host's ability to actively sequester key biometals (e.g., iron, zinc, and manganese) required for bacterial growth. This is referred to as nutritional immunity. Mechanisms to overcome nutritional immunity are essential virulence factors for bacterial pathogens. Y. pestis produces an iron-scavenging siderophore called yersiniabactin (Ybt) that is required to overcome iron-mediated nutritional immunity and cause lethal infection. Recently, Ybt has been shown to bind to zinc, and in the absence of the zinc transporter ZnuABC, Ybt improves Y. pestis growth in zinc-limited medium. These data suggest that, in addition to iron acquisition, Ybt may also contribute to overcoming zinc-mediated nutritional immunity. To test this hypothesis, we used a mouse model defective in iron-mediated nutritional immunity to demonstrate that Ybt contributes to virulence in an iron-independent manner. Furthermore, using a combination of bacterial mutants and mice defective in zinc-mediated nutritional immunity, we identified calprotectin as the primary barrier for Y. pestis to acquire zinc during infection and that Y. pestis uses Ybt to compete with calprotectin for zinc. Finally, we discovered that Y. pestis encounters zinc limitation within the flea midgut, and Ybt contributes to overcoming this limitation. Together, these results demonstrate that Ybt is a bona fide zinc acquisition mechanism used by Y. pestis to surmount zinc limitation during the infection of both the mammalian and insect hosts.

Yersinia pestis causes human plague and colonizes both a mammalian host and a flea vector during its transmission cycle. A key barrier to bacterial infection is the host’s ability to actively sequester key biometals (e.g., iron, zinc, and manganese) required for bacterial growth. This is referred to as nutritional immunity. Mechanisms to overcome nutritional immunity are essential virulence factors for bacterial pathogens. Y. pestis produces an iron-scavenging siderophore called yersiniabactin (Ybt) that is required to overcome iron-mediated nutritional immunity and cause lethal infection. Recently, Ybt has been shown to bind to zinc, and in the absence of the zinc transporter ZnuABC, Ybt improves Y. pestis growth in zinc-limited medium. These data suggest that, in addition to iron acquisition, Ybt may also contribute to overcoming zinc-mediated nutritional immunity. To test this hypothesis, we used a mouse model defective in iron-mediated nutritional immunity to demonstrate that Ybt contributes to virulence in an iron-independent manner. Furthermore, using a combination of bacterial mutants and mice defective in zinc-mediated nutritional immunity, we identified calprotectin as the primary barrier for Y. pestis to acquire zinc during infection and that Y. pestis uses Ybt to compete with calprotectin for zinc. Finally, we discovered that Y. pestis encounters zinc limitation within the flea midgut, and Ybt contributes to overcoming this limitation. Together, these results demonstrate that Ybt is a bona fide zinc acquisition mechanism used by Y. pestis to surmount zinc limitation during the infection of both the mammalian and insect hosts.

Group B Streptococcus (GBS) asymptomatically colonizes the female reproductive tract but is a common causative agent of meningitis. GBS meningitis is characterized by extensive infiltration of neutrophils carrying high concentrations of calprotectin, a metal chelator. To persist within inflammatory sites and cause invasive disease, GBS must circumvent host starvation attempts. Here, we identified global requirements for GBS survival during calprotectin challenge, including known and putative systems involved in metal ion transport. We characterized the role of zinc import in tolerating calprotectin stress in vitro and in a mouse model of infection. We observed that a global zinc uptake mutant was less virulent than the parental GBS strain and found calprotectin knockout mice to be equally susceptible to infection by wild-type (WT) and mutant strains. These findings suggest that calprotectin production at the site of infection results in a zinc-limited environment and reveals the importance of GBS metal homeostasis to invasive disease., Nutritional immunity is an elegant host mechanism used to starve invading pathogens of necessary nutrient metals. Calprotectin, a metal-binding protein, is produced abundantly by neutrophils and is found in high concentrations within inflammatory sites during infection. Group B Streptococcus (GBS) colonizes the gastrointestinal and female reproductive tracts and is commonly associated with severe invasive infections in newborns such as pneumonia, sepsis, and meningitis. Although GBS infections induce robust neutrophil recruitment and inflammation, the dynamics of GBS and calprotectin interactions remain unknown. Here, we demonstrate that disease and colonizing isolate strains exhibit susceptibility to metal starvation by calprotectin. We constructed a mariner transposon (Krmit) mutant library in GBS and identified 258 genes that contribute to surviving calprotectin stress. Nearly 20% of all underrepresented mutants following treatment with calprotectin are predicted metal transporters, including known zinc systems. As calprotectin binds zinc with picomolar affinity, we investigated the contribution of GBS zinc uptake to overcoming calprotectin-imposed starvation. Quantitative reverse transcriptase PCR (qRT-PCR) revealed a significant upregulation of genes encoding zinc-binding proteins, adcA, adcAII, and lmb, following calprotectin exposure, while growth in calprotectin revealed a significant defect for a global zinc acquisition mutant (ΔadcAΔadcAIIΔlmb) compared to growth of the GBS wild-type (WT) strain. Furthermore, mice challenged with the ΔadcAΔadcAIIΔlmb mutant exhibited decreased mortality and significantly reduced bacterial burden in the brain compared to mice infected with WT GBS; this difference was abrogated in calprotectin knockout mice. Collectively, these data suggest that GBS zinc transport machinery is important for combatting zinc chelation by calprotectin and establishing invasive disease.

Phosphate is an essential nutrient that Staphylococcus aureus and other pathogens must acquire from the host during infection. While inorganic monophosphate (Pi) is the preferred source of this nutrient, bacteria can also obtain it from phosphate-containing organic molecules. The Pi-responsive regulator PhoPR is necessary for S. aureus to cause infection, suggesting that Pi is not freely available during infection and that this nutrient must be obtained from other sources. However, the organophosphates from which S. aureus can obtain phosphate are unknown. We evaluated the ability of 58 phosphorus-containing molecules to serve as phosphate sources for S. aureus. Forty-six of these compounds, including phosphorylated amino acids, sugars, and nucleotides, supported growth. Among the organophosphate sources was glycerol-3-phosphate (G3P), which is commonly found in the mammalian host. Differing from the model organism Escherichia coli, S. aureus does not import G3P intact to obtain Pi. Instead, S. aureus relies on the phosphatase PhoB to release Pi from G3P, which is subsequently imported by Pi transporters. To determine if this strategy is used by S. aureus to extract phosphate from other phosphate sources, we assessed the ability of PhoB- and Pi transporter-deficient strains to grow on the same library of phosphorus-containing molecules. Sixty percent of the substrates (28/46) relied on the PhoB/Pi transporter pathway, and an additional 10/46 (22%) were PhoB independent but still required Pi transport through the Pi transporters. Cumulatively, these results suggest that in Pi-limited environments, S. aureus preferentially generates Pi from organophosphates and then relies on Pi transporters to import this nutrient. IMPORTANCE For bacteria, the preferred form of the essential nutrient phosphate is inorganic monophosphate (Pi), but phosphate can also be extracted from a variety of phosphocompounds. Pathogens, including Staphylococcus aureus, experience Pi limitation within the host, suggesting that the use of alternative phosphate sources is important during infection. However, the alternative phosphate sources that can be used by S. aureus and others remain largely unexplored. We screened a library of phosphorus-containing compounds for the ability to support growth as a phosphate source. S. aureus could use a variety of phosphocompounds, including nucleotides, phosphosugars, and phosphoamino acids. Subsequent genetic analysis determined that a majority of these alternative phosphate sources are first processed extracellularly to liberate Pi, which is then imported through Pi transporters.

The host restricts the availability of zinc to prevent infection. To overcome this defense, Staphylococcus aureus and Pseudomonas aeruginosa rely on zincophore-dependent zinc importers. Synthesis of the zincophore staphylopine by S. aureus and its import are both necessary for the bacterium to cause infection. In this study, we sought to elucidate how loss of zincophore efflux impacts bacterial resistance to host-imposed zinc starvation. In culture and during infection, mutants lacking CntE, the staphylopine efflux pump, were more sensitive to zinc starvation imposed by the metal-binding immune effector calprotectin than those lacking the ability to import staphylopine. However, disruption of staphylopine synthesis reversed the enhanced sensitivity phenotype of the ΔcntE mutant to calprotectin, indicating that intracellular toxicity of staphylopine is more detrimental than the impaired ability to acquire zinc. Unexpectedly, intracellular accumulation of staphylopine does not increase the expression of metal importers or alter cellular metal concentrations, suggesting that, contrary to prevailing models, the toxicity associated with staphylopine is not strictly due to intracellular chelation of metals. As P. aeruginosa and other pathogens produce zincophores with similar chemistry, our observations on the crucial importance of zincophore efflux are likely to be broadly relevant. IMPORTANCEStaphylococcus aureus and many other bacterial pathogens rely on metal-binding small molecules to obtain the essential metal zinc during infection. In this study, we reveal that export of these small molecules is critical for overcoming host-imposed metal starvation during infection and prevents toxicity due to accumulation of the metal-binding molecule within the cell. Surprisingly, we found that intracellular toxicity of the molecule is not due to chelation of cellular metals.

The success of Staphylococcus aureus as a pathogen is due to its capability of fine‐tuning its cellular physiology to meet the challenges presented by diverse environments, which allows it to colonize multiple niches within a single vertebrate host. Elucidating the roles of energy‐yielding metabolic pathways could uncover attractive therapeutic strategies and targets. In this work, we seek to determine the effects of disabling NADH‐dependent aerobic respiration on the physiology of S. aureus. Differing from many pathogens, S. aureus has two type‐2 respiratory NADH dehydrogenases (NDH‐2s) but lacks the respiratory ion‐pumping NDHs. Here, we show that the NDH‐2s, individually or together, are not essential either for respiration or growth. Nevertheless, their absence eliminates biofilm formation, production of α‐toxin, and reduces the ability to colonize specific organs in a mouse model of systemic infection. Moreover, we demonstrate that the reason behind these phenotypes is the alteration of the fatty acid metabolism. Importantly, the SaeRS two‐component system, which responds to fatty acids regulation, is responsible for the link between NADH‐dependent respiration and virulence in S. aureus.

To control infection, mammals actively withhold essential nutrients, including the transition metal manganese, by a process termed nutritional immunity. A critical component of this host response is the manganese-chelating protein calprotectin. While many bacterial mechanisms for overcoming nutritional immunity have been identified, the intersection between metal starvation and other essential inorganic nutrients has not been investigated. Here, we report that overexpression of an operon encoding a highly conserved inorganic phosphate importer, PstSCAB, increases the sensitivity of Staphylococcus aureus to calprotectin-mediated manganese sequestration. Further analysis revealed that overexpression of pstSCAB does not disrupt manganese acquisition or result in overaccumulation of phosphate by S. aureus. However, it does reduce the ability of S. aureus to grow in phosphate-replete defined medium. Overexpression of pstSCAB does not aberrantly activate the phosphate-responsive two-component system PhoPR, nor was this two-component system required for sensitivity to manganese starvation. In a mouse model of systemic staphylococcal disease, a pstSCAB-overexpressing strain is significantly attenuated compared to wild-type S. aureus. This defect is partially reversed in a calprotectin-deficient mouse, in which manganese is more readily available. Given that expression of pstSCAB is regulated by PhoPR, these findings suggest that overactivation of PhoPR would diminish the ability of S. aureus to resist nutritional immunity and cause infection. As PhoPR is also necessary for bacterial virulence, these findings imply that phosphate homeostasis represents a critical regulatory node whose activity must be precisely controlled in order for S. aureus and other pathogens to cause infection.

Two-component regulatory systems enable bacteria to adapt to changes in their environment during infection by altering gene expression and coordinating antihost responses. Despite the critical role of two-component systems in bacterial survival and pathogenesis, the activating signals for most of these regulators remain unidentified. This is exemplified by ArlRS, a Staphylococcus aureus global regulator that contributes to virulence and to resisting host-mediated restriction of essential nutrients, such as manganese. In this report, we demonstrate that manganese starvation and the absence of glycolytic substrates activate ArlRS. Further investigations revealed that ArlRS is activated when the latter half of glycolysis is disrupted, suggesting that S. aureus monitors flux through the second half of this pathway. Host-imposed manganese starvation also induced the expression of pore-forming toxins in an ArlRS-dependent manner. Cumulatively, this work reveals that ArlRS acts as a sensor that links nutritional status, cellular metabolism, and virulence regulation., During infection, bacteria use two-component signal transduction systems to sense and adapt to the dynamic host environment. Despite critically contributing to infection, the activating signals of most of these regulators remain unknown. This also applies to the Staphylococcus aureus ArlRS two-component system, which contributes to virulence by coordinating the production of toxins, adhesins, and a metabolic response that enables the bacterium to overcome host-imposed manganese starvation. Restricting the availability of essential transition metals, a strategy known as nutritional immunity, constitutes a critical defense against infection. In this work, expression analysis revealed that manganese starvation imposed by the immune effector calprotectin or by the absence of glycolytic substrates activates ArlRS. Manganese starvation imposed by calprotectin also activated the ArlRS system even when glycolytic substrates were present. A combination of metabolomics, mutational analysis, and metabolic feeding experiments revealed that ArlRS is activated by alterations in metabolic flux occurring in the latter half of the glycolytic pathway. Moreover, calprotectin was found to induce expression of staphylococcal leukocidins in an ArlRS-dependent manner. These studies indicated that ArlRS is a metabolic sensor that allows S. aureus to integrate multiple environmental stresses that alter glycolytic flux to coordinate an antihost response and to adapt to manganese starvation. They also established that the latter half of glycolysis represents a checkpoint to monitor metabolic state in S. aureus. Altogether, these findings contribute to understanding how invading pathogens, such as S. aureus, adapt to the host during infection and suggest the existence of similar mechanisms in other bacterial species.

The ability of Staphylococcus aureus and other pathogens to consume glucose is critical during infection. However, glucose consumption increases the cellular demand for manganese sensitizing S. aureus to host-imposed manganese starvation. The current investigations were undertaken to elucidate how S. aureus copes with the need to consume glucose when metal-limited by the host. A critical component of host defense is production of the manganese binding protein calprotectin. S. aureus has two variants of phosphoglycerate mutase, one of which is manganese-dependent, GpmI, and another that is manganese-independent, GpmA. Leveraging the ability to impose metal starvation in culture utilizing calprotectin revealed that the loss of GpmA, but not GpmI, sensitized S. aureus to manganese starvation. Metabolite feeding experiments revealed that the growth defect of GpmA when manganese-starved was due to a defect in glycolysis and not gluconeogenesis. Loss of GpmA reduces the ability of S. aureus to cause invasive disease in wild type mice. However, GpmA was dispensable in calprotectin-deficient mice, which have defects in manganese sequestration, indicating that this isozyme contributes to the ability of S. aureus to overcome manganese limitation during infection. Cumulatively, these observations suggest that expressing a metal-independent variant enables S. aureus to consume glucose while mitigating the negative impact that glycolysis has on the cellular demand for manganese. S. aureus is not the only bacterium that expresses manganese-dependent and -independent variants of phosphoglycerate mutase. Similar results were also observed in culture with Salmonella enterica serovar Typhimurium mutants lacking the metal-independent isozyme. These similar observations in both Gram-positive and Gram-negative pathogens suggest that expression of metal-independent glycolytic isozymes is a common strategy employed by bacteria to survive in metal-limited environments, such as the host., Author summary Pathogens, such as Staphylococcus aureus and Salmonella species, must be able to consume glucose in order to cause infection. However, glycolysis can increase the need for manganese and sensitize invaders to the manganese-withholding defense of the host, known as nutritional immunity. How pathogens manage these conflicting pressures is currently unknown. The current investigations revealed that a second metal-independent variant of phosphoglycerate mutase possessed by both S. aureus and Salmonella enables them to grow and consume glycolytic substrates in the presence of the manganese-binding immune effector calprotectin. Infection experiments revealed that the manganese-independent isozyme critically contributes to the ability of S. aureus to overcome manganese starvation during infection. Together, these results suggest that using metal-independent isozymes to enable the consumption of sugars within the host or other metal-limited environments is a common strategy employed by diverse bacteria.

During infection, the host utilizes a diverse array of processes to combat invaders, including the restriction of availability of essential nutrients such as manganese. Similarly to many other pathogens, Staphylococcus aureus possesses two manganese importers, MntH and MntABC. Several infection models have revealed a critical role for MntABC during staphylococcal infection. However, culture-based studies have suggested parity between the two transporters when cells are resisting manganese starvation imposed by the manganese binding immune effector calprotectin. In this investigation, initial elemental analysis revealed that MntABC is the primary transporter responsible for obtaining manganese in culture in the presence of calprotectin. MntABC was also necessary to maintain wild-type levels of manganese-dependent superoxide dismutase activity in the presence of calprotectin. Building on this framework, we investigated if MntABC enabled S. aureus to resist the synergistic actions of nutritional immunity and other host defenses. This analysis revealed that MntABC critically contributes to staphylococcal growth when S. aureus is subjected to manganese limitations and exposed to oxidative stress. This transporter was also important for growth in manganese-limited environments when S. aureus was forced to consume glucose as an energy source, which occurs when it encounters nitric oxide. MntABC also expanded the pH range conducive for S. aureus growth under conditions of manganese scarcity. Collectively, the data presented in this work provide a robust molecular basis for the crucial role of MntABC in staphylococcal virulence. Further, this work highlights the importance of synergy between host defenses and the necessity of evaluating the contribution of virulence factors to pathogenesis in the presence of multiple stressors.

Microbial pathogens must obtain all essential nutrients, including phosphate, from the host. To optimize phosphate acquisition in diverse and dynamic environments, such as mammalian tissues, many bacteria use the PhoPR two-component system. Despite the necessity of this system for virulence in several species, PhoPR has not been studied in the major human pathogen Staphylococcus aureus. To illuminate its role in staphylococcal physiology, we initially assessed whether PhoPR controls the expression of the three inorganic phosphate (P(i)) importers (PstSCAB, NptA, and PitA) in S. aureus. This analysis revealed that PhoPR is required for the expression of pstSCAB and nptA and can modulate pitA expression. Consistent with a role in phosphate homeostasis, PhoPR-mediated regulation of the transporters is influenced by phosphate availability. Further investigations revealed that PhoPR is necessary for growth under P(i)-limiting conditions, and in some environments, its primary role is to induce the expression of pstSCAB or nptA. Interestingly, in other environments, PhoPR is necessary for growth independent of P(i) transporter expression, indicating that additional PhoPR-regulated factors promote S. aureus adaptation to low-P(i) conditions. Together, these data suggest that PhoPR differentially contributes to growth in an environment-specific manner. In a systemic infection model, a mutant of S. aureus lacking PhoPR is highly attenuated. Further investigation revealed that PhoPR-regulated factors, in addition to P(i) transporters, are critical for staphylococcal pathogenesis. Cumulatively, these findings point to an important role for PhoPR in orchestrating P(i) acquisition as well as transporter-independent mechanisms that contribute to S. aureus virulence.

Copper is universally toxic in excess, a feature exploited by the human immune system to facilitate bacterial clearance. The mechanism of copper intoxication remains unknown for many bacterial species. Here, we demonstrate that copper toxicity in Streptococcus pneumoniae is independent from oxidative stress but, rather, is the result of copper inhibiting the aerobic dNTP biosynthetic pathway. Furthermore, we show that copper-intoxicated S. pneumoniae is rescued by manganese, which is an essential metal in the aerobic nucleotide synthesis pathway. These data provide insight into new targets to enhance copper-mediated toxicity during bacterial clearance.

The opportunistic fungal pathogen Candida albicans acquires essential metals from the host, yet the host can sequester these micronutrients through a process known as nutritional immunity. How the host withholds metals from C. albicans has been poorly understood; here we examine the role of calprotectin (CP), a transition metal binding protein. When CP depletes bioavailable Zn from the extracellular environment, C. albicans strongly upregulates ZRT1 and PRA1 for Zn import and maintains constant intracellular Zn through numerous cell divisions. We show for the first time that CP can also sequester Cu by binding Cu(II) with subpicomolar affinity. CP blocks fungal acquisition of Cu from serum and induces a Cu starvation stress response involving SOD1 and SOD3 superoxide dismutases. These transcriptional changes are mirrored when C. albicans invades kidneys in a mouse model of disseminated candidiasis, although the responses to Cu and Zn limitations are temporally distinct. The Cu response progresses throughout 72 h, while the Zn response is short-lived. Notably, these stress responses were attenuated in CP null mice, but only at initial stages of infection. Thus, Zn and Cu pools are dynamic at the host-pathogen interface and CP acts early in infection to restrict metal nutrients from C. albicans .

During infection, the host sequesters essential nutrients, such as zinc, to combat invading microbes. Despite the ability of the immune effector protein calprotectin to bind zinc with subpicomolar affinity, Staphylococcus aureus is able to successfully compete with the host for zinc. However, the zinc importers expressed by S. aureus remain unknown. Our investigations have revealed that S. aureus possesses two importers, AdcABC and CntABCDF, which are induced in response to zinc limitation. While AdcABC is similar to known zinc importers in other bacteria, CntABCDF has not previously been associated with zinc acquisition. Concurrent loss of the two systems severely impairs the ability of S. aureus to obtain zinc and grow in zinc-limited environments. Further investigations revealed that the Cnt system is responsible for the ability of S. aureus to compete with calprotectin for zinc in culture and contributes to acquisition of zinc during infection. The cnt locus also enables S. aureus to produce the broad-spectrum metallophore staphylopine. Similarly to the Cnt transporter, loss of staphylopine severely impairs the ability of S. aureus to resist host-imposed zinc starvation, both in culture and during infection. Further investigations revealed that together staphylopine and the Cnt importer function analogously to siderophore-based iron acquisition systems in order to facilitate zinc acquisition by S. aureus. Analogous systems are found in a broad range of Gram-positive and Gram-negative bacterial pathogens, suggesting that this new type of zinc importer broadly contributes to the ability of bacteria to cause infection., IMPORTANCE A critical host defense against infection is the restriction of zinc availability. Despite the subpicomolar affinity of the immune effector calprotectin for zinc, Staphylococcus aureus can successfully compete for this essential metal. Here, we describe two zinc importers, AdcABC and CntABCDF, possessed by S. aureus, the latter of which has not previously been associated with zinc acquisition. The ability of S. aureus to compete with the host for zinc is dependent on CntABCDF and the metallophore staphylopine, both in culture and during infection. These results expand the mechanisms utilized by bacteria to obtain zinc, beyond Adc-like systems, and demonstrate that pathogens utilize strategies similar to siderophore-based iron acquisition to obtain other essential metals during infection. The staphylopine synthesis machinery is present in a diverse collection of bacteria, suggesting that this new family of zinc importers broadly contributes to the ability of numerous pathogens to cause infection.

During infection, pathogens must obtain all inorganic nutrients, such as phosphate, from the host. Despite the essentiality of phosphate for all forms of life, how Staphylococcus aureus obtains this nutrient during infection is unknown. Differing from Escherichia coli , the paradigm for bacterial phosphate acquisition, which has two inorganic phosphate (P i ) importers, genomic analysis suggested that S. aureus possesses three distinct P i transporters: PstSCAB, PitA, and NptA. While pitA and nptA are expressed in phosphate-replete media, expression of all three transporters is induced by phosphate limitation. The loss of a single transporter did not affect S. aureus . However, disruption of any two systems significantly reduced P i accumulation and growth in divergent environments. These findings indicate that PstSCAB, PitA, and NptA have overlapping but nonredundant functions, thus expanding the environments in which S. aureus can successfully obtain P i . Consistent with this idea, in a systemic mouse model of disease, loss of any one transporter did not decrease staphylococcal virulence. However, loss of NptA in conjunction with either PstSCAB or PitA significantly reduced the ability of S. aureus to cause infection. These observations suggest that P i acquisition via NptA is particularly important for the pathogenesis of S. aureus . While our analysis suggests that NptA homologs are widely distributed among bacteria, closely related less pathogenic staphylococcal species do not possess this importer. Altogether, these observations indicate that P i uptake by S. aureus differs from established models and that acquisition of a third transporter enhances the ability of the bacterium to cause infection.

Hydrogen sulfide (H2S) is thought to protect bacteria from oxidative stress, but a comprehensive understanding of its function in bacteria is largely unexplored. In this study, we show that the human pathogen Staphylococcus aureus (S. aureus) harbors significant effector molecules of H2S signaling, reactive sulfur species (RSS), as low molecular weight persulfides of bacillithiol, coenzyme A, and cysteine, and significant inorganic polysulfide species. We find that proteome S- sulfhydration, a post-translational modification (PTM) in H2S signaling, is widespread in S. aureus. RSS levels modulate the expression of secreted virulence factors and the cytotoxicity of the secretome, consistent with an S-sulfhydration-dependent inhibition of DNA binding by MgrA, a global virulence regulator. Two previously uncharacterized thioredoxin-like proteins, denoted TrxP and TrxQ, are S-sulfhydrated in sulfide-stressed cells and are capable of reducing protein hydrodisulfides, suggesting that this PTM is potentially regulatory in S. aureus. In conclusion, our results reveal that S. aureus harbors a pool of proteome- and metabolite-derived RSS capable of impacting protein activities and gene regulation and that H2S signaling can be sensed by global regulators to affect the expression of virulence factors.

Hydrogen sulfide (H2S) is a toxic molecule and a recently described gasotransmitter in vertebrates whose function in bacteria is not well understood. In this work, we describe the transcriptomic response of the major human pathogen Staphylococcus aureus to quantified changes in levels of cellular organic reactive sulfur species, which are effector molecules involved in H2S signaling. We show that nitroxyl (HNO), a recently described signaling intermediate proposed to originate from the interplay of H2S and nitric oxide, also induces changes in cellular sulfur speciation and transition metal homeostasis, thus linking sulfide homeostasis to an adaptive response to antimicrobial reactive nitrogen species., Staphylococcus aureus is a commensal human pathogen and a major cause of nosocomial infections. As gaseous signaling molecules, endogenous hydrogen sulfide (H2S) and nitric oxide (NO·) protect S. aureus from antibiotic stress synergistically, which we propose involves the intermediacy of nitroxyl (HNO). Here, we examine the effect of exogenous sulfide and HNO on the transcriptome and the formation of low-molecular-weight (LMW) thiol persulfides of bacillithiol, cysteine, and coenzyme A as representative of reactive sulfur species (RSS) in wild-type and ΔcstR strains of S. aureus. CstR is a per- and polysulfide sensor that controls the expression of a sulfide oxidation and detoxification system. As anticipated, exogenous sulfide induces the cst operon but also indirectly represses much of the CymR regulon which controls cysteine metabolism. A zinc limitation response is also observed, linking sulfide homeostasis to zinc bioavailability. Cellular RSS levels impact the expression of a number of virulence factors, including the exotoxins, particularly apparent in the ΔcstR strain. HNO, like sulfide, induces the cst operon as well as other genes regulated by exogenous sulfide, a finding that is traced to a direct reaction of CstR with HNO and to an endogenous perturbation in cellular RSS, possibly originating from disassembly of Fe-S clusters. More broadly, HNO induces a transcriptomic response to Fe overload, Cu toxicity, and reactive oxygen species and reactive nitrogen species and shares similarity with the sigB regulon. This work reveals an H2S/NO· interplay in S. aureus that impacts transition metal homeostasis and virulence gene expression. IMPORTANCE Hydrogen sulfide (H2S) is a toxic molecule and a recently described gasotransmitter in vertebrates whose function in bacteria is not well understood. In this work, we describe the transcriptomic response of the major human pathogen Staphylococcus aureus to quantified changes in levels of cellular organic reactive sulfur species, which are effector molecules involved in H2S signaling. We show that nitroxyl (HNO), a recently described signaling intermediate proposed to originate from the interplay of H2S and nitric oxide, also induces changes in cellular sulfur speciation and transition metal homeostasis, thus linking sulfide homeostasis to an adaptive response to antimicrobial reactive nitrogen species.

During infection, vertebrates limit access to manganese and zinc, starving invading pathogens, such as Staphylococcus aureus , of these essential metals in a process termed “nutritional immunity.” The manganese and zinc binding protein calprotectin is a key component of the nutrient-withholding response, and mice lacking this protein do not sequester manganese from S. aureus liver abscesses. One potential mechanism utilized by S. aureus to minimize host-imposed manganese and zinc starvation is the expression of the metal transporters MntABC and MntH. We performed transcriptional analyses of both mntA and mntH , which revealed increased expression of both systems in response to calprotectin treatment. MntABC and MntH compete with calprotectin for manganese, which enables S. aureus growth and retention of manganese-dependent superoxide dismutase activity. Loss of MntABC and MntH results in reduced staphylococcal burdens in the livers of wild-type but not calprotectin-deficient mice, suggesting that these systems promote manganese acquisition during infection. During the course of these studies, we observed that metal content and the importance of calprotectin varies between murine organs, and infection leads to profound changes in the anatomical distribution of manganese and zinc. In total, these studies provide insight into the mechanisms utilized by bacteria to evade host-imposed nutrient metal starvation and the critical importance of restricting manganese availability during infection.

Staphylococcus aureus is a significant infectious threat to global public health. Acquisition or synthesis of heme is required for S. aureus to capture energy through respiration, but an excess of this critical cofactor is toxic to bacteria. S. aureus employs the heme sensor system (HssRS) to overcome heme toxicity; however, the mechanism of heme sensing is not defined. Here, we describe the identification of a small molecule activator of HssRS that induces endogenous heme biosynthesis by perturbing central metabolism. This molecule is toxic to fermenting S. aureus , including clinically relevant small colony variants. The utility of targeting fermenting bacteria is exemplified by the fact that this compound prevents the emergence of antibiotic resistance, enhances phagocyte killing, and reduces S. aureus pathogenesis. Not only is this small molecule a powerful tool for studying bacterial heme biosynthesis and central metabolism; it also establishes targeting of fermentation as a viable antibacterial strategy.

Transition metals such as manganese are essential nutrients for both pathogen and host. Vertebrates exploit this necessity to combat invading microbes by restricting access to these critical nutrients, a defense known as nutritional immunity. During infection, the host uses several mechanisms to impose manganese limitation. These include removal of manganese from the phagolysosome, sequestration of extracellular manganese, and utilization of other metals to prevent bacterial acquisition of manganese. In order to cause disease, pathogens employ a variety of mechanisms that enable them to adapt to and counter nutritional immunity. These adaptations include, but are likely not limited to, manganese-sensing regulators and high-affinity manganese transporters. Even though successful pathogens can overcome host-imposed manganese starvation, this defense inhibits manganese-dependent processes, reducing the ability of these microbes to cause disease. While the full impact of host-imposed manganese starvation on bacteria is unknown, critical bacterial virulence factors such as superoxide dismutases are inhibited. This chapter will review the factors involved in the competition for manganese at the host-pathogen interface and discuss the impact that limiting the availability of this metal has on invading bacteria.

SummaryNeutrophils are innate immune cells that counter pathogens by many mechanisms, including release of antimicrobial proteins such as calprotectin to inhibit bacterial growth. Calprotectin sequesters essential micronutrient metals such as zinc, thereby limiting their availability to microbes, a process termed nutritional immunity. We find that while calprotectin is induced by neutrophils during infection with the gut pathogen Salmonella Typhimurium, calprotectin-mediated metal sequestration does not inhibit S. Typhimurium proliferation. Remarkably, S. Typhimurium overcomes calprotectin-mediated zinc chelation by expressing a high affinity zinc transporter (ZnuABC). A S. Typhimurium znuA mutant impaired for growth in the inflamed gut was rescued in the absence of calprotectin. ZnuABC was also required to promote the growth of S. Typhimurium over that of competing commensal bacteria. Thus, our findings indicate that Salmonella thrives in the inflamed gut by overcoming the zinc sequestration of calprotectin and highlight the importance of zinc acquisition in bacterial intestinal colonization.

All strains of Staphylococcus aureus encode a putative copper-sensitive operon repressor (CsoR) and one other CsoR-like protein of unknown function. We show here that NWMN_1991 encodes a bona fide Cu(I)-inducible CsoR of a genetically unlinked copA-copZ copper resistance operon in S. aureus strain Newman. In contrast, an unannotated open reading frame found between NWMN_0027 and NWMN_0026 (denoted NWMN_0026.5) encodes a CsoR-like regulator that represses expression of adjacent genes by binding specifically to a pair of canonical operator sites positioned in the NWMN_0027-0026.5 intergenic region. Inspection of these regulated genes suggests a role in assimilation of inorganic sulfur from thiosulfate and vectorial sulfur transfer, and we designate NWMN_0026.5 as CstR (CsoR-like sulfur transferase repressor). Expression analysis demonstrates that CsoR and CstR control their respective regulons in response to distinct stimuli with no overlap in vivo. Unlike CsoR, CstR does not form a stable complex with Cu(I); operator binding is instead inhibited by oxidation of the intersubunit cysteine pair to a mixture of disulfide and trisulfide linkages by a likely metabolite of thiosulfate assimilation, sulfite. CsoR is unreactive toward sulfite under the same conditions. We conclude that CsoR and CstR are paralogs in S. aureus that function in the same cytoplasm to control distinct physiological processes.

Kingella kingae is a gram-negative bacterium that is being recognized increasingly as a cause of septic arthritis and osteomyelitis in young children. Previous work established that K. kingae expresses type IV pili that mediate adherence to respiratory epithelial and synovial cells. PilA1 is the major pilus subunit in K. kingae type IV pili and is essential for pilus assembly. To develop a better understanding of the role of K. kingae type IV pili during colonization and invasive disease, we examined a collection of clinical isolates for pilus expression and in vitro adherence. In addition, in a subset of isolates we performed nucleotide sequencing to assess the level of conservation of PilA1. The majority of respiratory and nonendocarditis blood isolates were piliated, while the majority of joint fluid, bone, and endocarditis blood isolates were nonpiliated. The piliated isolates formed either spreading/corroding or nonspreading/noncorroding colonies and were uniformly adherent, while the nonpiliated isolates formed domed colonies and were nonadherent. PilA1 sequence varied significantly from strain to strain, resulting in substantial variability in antibody reactivity. These results suggest that type IV pili may confer a selective advantage on K. kingae early in infection and a selective disadvantage on K. kingae at later stages in the pathogenic process. We speculate that PilA1 is immunogenic during natural infection and undergoes antigenic variation to escape the immune response.

Kingella kingae is a gram-negative bacterium that colonizes the respiratory tract and is a common cause of septic arthritis and osteomyelitis. Despite the increasing frequency of K. kingae disease, little is known about the mechanism by which this organism adheres to respiratory epithelium and seeds joints and bones. Previous work showed that K. kingae expresses long surface fibers that vary in surface density. In the current study, we found that these fibers are type IV pili and are necessary for efficient adherence to respiratory epithelial and synovial cells and that the number of pili expressed by the bacterium correlates with the level of adherence to synovial cells but not with the level of adherence to respiratory cells. In addition, we established that the major pilin subunit is encoded by a pilA homolog in a conserved region of the chromosome that also contains a second pilin gene and a type IV pilus accessory gene, both of which are dispensable for pilus assembly and pilus-mediated adherence. Upon examination of the K. kingae genome, we identified two genes in physically separate locations on the chromosome that encode homologs of the Neisseria PilC proteins and that have only a low level homology to each other. Examination of mutant strains revealed that both of the K. kingae PilC homologs are essential for a wild-type level of adherence to both respiratory epithelial and synovial cells. Taken together, these results demonstrate that type IV pili and the two PilC homologs play important roles in mediating K. kingae adherence.

Kingella kingae is an emerging bacterial pathogen that is increasingly recognized as the causative agent of a variety of pediatric diseases, including septic arthritis and osteomyelitis. The pathogenesis of K. kingae disease is believed to begin with colonization of the upper respiratory tract. In the present study, we examined interactions between K. kingae and cultured respiratory epithelial cells and observed potent cytotoxicity, detected by both microscopy and lactic acid dehydrogenase (LDH) release assays. Experiments with synovial and macrophage cell lines revealed cytotoxicity for these cell types as well. Using mariner mutagenesis and a screen for loss of cytotoxicity, a genetic locus encoding an RTX toxin system was identified. Disruption of the K. kingae RTX locus resulted in a loss of cytotoxicity for respiratory epithelial, synovial, and macrophage cell lines. DNA sequence analysis demonstrated that the RTX locus is flanked by insertion elements and has a reduced G+C content compared to that of the whole genome. Two relatively less invasive Kingella species, K. oralis and K. denitrificans , were found to be noncytotoxic and to lack the RTX region, as determined by LDH release assays and Southern blotting. We concluded that K. kingae expresses an RTX toxin that has wide cellular specificity and was likely acquired horizontally. The possible roles for this toxin in the pathogenesis of K. kingae disease include breaching of the epithelial barrier and destruction of target tissues, such as synovium (joint lining).

First-row transition elements are essential for all forms of life. During infection invading microbes must obtain these nutrients from their host. Vertebrates take advantage of this fact to combat invaders by sequestering essential nutrients, a defense known as nutritional immunity. The most well-characterized aspect of this defense is the iron-withholding response. Advances in elemental imaging have revealed that zinc and manganese are also sequestered during infection. The importance of nutritional immunity to host defense is emphasized by the increased susceptibility to infection when levels of these metals are elevated. This chapter will discuss iron, zinc, and manganese availability during infection, the impact of withholding these metals from invading pathogens, and the antimicrobial peptides utilized by the host to restrict the availability of these essential nutrients.

Summary Diet, and specifically dietary metals, can modify the risk of infection. However, the mechanisms by which manganese (Mn), a common dietary supplement, alters infection remain unexplored. We report that dietary Mn levels dictate the outcome of systemic infections caused by Staphylococcus aureu s, a leading cause of bacterial endocarditis. Mice fed a high Mn diet display alterations in Mn levels and localization within infected tissues, and S. aureus virulence and infection of the heart are enhanced. Although the canonical mammalian Mn-sequestering protein calprotectin surrounds staphylococcal heart abscesses, calprotectin is not released into the abscess nidus and does not limit Mn in this organ. Consequently, excess Mn is bioavailable to S. aureus in the heart. Bioavailable Mn is utilized by S. aureus to detoxify reactive oxygen species and protect against neutrophil killing, enhancing fitness within the heart. Therefore, a single dietary modification overwhelms vital host antimicrobial strategies, leading to fatal staphylococcal infection.

Staphylococcus aureus is a devastating mammalian pathogen for which the development of new therapeutic approaches is urgently needed due to the prevalence of antibiotic resistance. During infection pathogens must overcome the dual threats of host-imposed manganese starvation, termed nutritional immunity, and the oxidative burst of immune cells. These defenses function synergistically, as host-imposed manganese starvation reduces activity of the manganese-dependent enzyme superoxide dismutase (SOD). S. aureus expresses two SODs, denoted SodA and SodM. While all staphylococci possess SodA, SodM is unique to S. aureus, but the advantage that S. aureus gains by expressing two apparently manganese-dependent SODs is unknown. Surprisingly, loss of both SODs renders S. aureus more sensitive to host-imposed manganese starvation, suggesting a role for these proteins in overcoming nutritional immunity. In this study, we have elucidated the respective contributions of SodA and SodM to resisting oxidative stress and nutritional immunity. These analyses revealed that SodA is important for resisting oxidative stress and for disease development when manganese is abundant, while SodM is important under manganese-deplete conditions. In vitro analysis demonstrated that SodA is strictly manganese-dependent whereas SodM is in fact cambialistic, possessing equal enzymatic activity when loaded with manganese or iron. Cumulatively, these studies provide a mechanistic rationale for the acquisition of a second superoxide dismutase by S. aureus and demonstrate an important contribution of cambialistic SODs to bacterial pathogenesis. Furthermore, they also suggest a new mechanism for resisting manganese starvation, namely populating manganese-utilizing enzymes with iron., Author Summary During infection, pathogens must overcome the restriction of essential nutrients such as manganese by the host, while simultaneously coping with other host defenses such as the oxidative burst. Using the host protein that limits manganese availability during infection and mice lacking this effector, we determined that acquisition of a second superoxide dismutase that is capable of using either manganese or iron enhances the ability of Staphylococcus aureus to cause infection. When manganese-starved by the host, this cambialistic enzyme enables S. aureus to maintain superoxide dismutase activity and survive when exposed to oxidative stress. These results reveal the important contribution of cambialistic superoxide dismutases to bacterial pathogenesis and represent a new mechanism for resisting manganese starvation during infection.

During infection invading pathogens must acquire all essential nutrients, including first row transition metals, from the host. To combat invaders, the host exploits this fact and restricts the availability of these nutrients using a defense mechanism known as nutritional immunity. While iron sequestration is the most well-known aspect of this defense, recent work has revealed that the host restricts the availability of other essential elements, notably manganese, during infection. Furthermore, these studies have revealed that the host utilizes multiple strategies that extend beyond metal sequestration to prevent bacteria from obtaining these metals. This review will discuss the mechanisms by which bacteria attempt to obtain the essential first row transition metal ion manganese during infection, and the approaches utilized by the host to prevent this occurrence. In addition, this review will discuss the impact of host-imposed manganese starvation on invading bacteria.

How cells regulate the bioavailability of utilizable sulfur while mitigating the effects of hydrogen sulfide toxicity is poorly understood. CstR [Copper-sensing operon repressor (CsoR)-like sulfurtransferase repressor] represses the expression of the cst operon encoding a putative sulfide oxidation system in Staphylococcus aureus. Here, we show that the cst operon is strongly and transiently induced by cellular sulfide stress in an acute phase and specific response and that cst-encoded genes are necessary to mitigate the effects of sulfide toxicity. Growth defects are most pronounced when S. aureus is cultured in chemically defined media with thiosulfate (TS) as a sole sulfur source, but are also apparent when cystine is used or in rich media. Under TS growth conditions, cells fail to grow as a result of either unregulated expression of the cst operon in a ΔcstR strain or transformation with a non-inducible C31A/C60A CstR that blocks cst induction. This suggests that the cst operon contributes to cellular sulfide homeostasis. Tandem high-resolution mass spectrometry reveals derivatization of CstR by both inorganic tetrasulfide and an organic persulfide, glutathione persulfide, to yield a mixture of Cys31-Cys60' interprotomer cross-links, including di-, tri- and tetrasulfide bonds, which allosterically inhibit cst operator DNA binding by CstR.

The small Rho GTPase Cdc42 regulates key signaling pathways required for multiple cell functions, including maintenance of shape, polarity, proliferation, invasion, migration, differentiation, and morphogenesis. As the role of Cdc42-dependent signaling in fibroblasts in vivo is unknown, we attempted to specifically delete it in these cells by crossing the Cdc42 fl/fl mouse with an fibroblast-specific protein 1 (FSP1)-Cre mouse, which is thought to mediate recombination exclusively in fibroblasts. Surprisingly, the FSP1-Cre;Cdc42 fl/fl mice died at 3 weeks of age due to overwhelming suppurative upper airway infections that were associated with neutrophilia and lymphopenia. Even though major aberrations in lymphoid tissue development were present in the mice, the principal cause of death was severe migration and killing abnormalities of the neutrophil population resulting in an inability to control infection. We also show that in addition to fibroblasts, FSP1-Cre deleted Cdc42 very efficiently in all leukocytes. Thus, by using this nonspecific Cre mouse, we inadvertently demonstrated the importance of Cdc42 in host protection from lethal infections and suggest a critical role for this small GTPase in innate immunity.

During infection the host imposes manganese and zinc starvation on invading pathogens. Despite this, Staphylococcus aureus and other successful pathogens remain capable of causing devastating disease. However, how these invaders adapt to host-imposed metal starvation and overcome nutritional immunity remains unknown. We report that ArlRS, a global staphylococcal virulence regulator, enhances the ability of S. aureus to grow in the presence of the manganese-and zinc-binding innate immune effector calprotectin. Utilization of calprotectin variants with altered metal binding properties revealed that strains lacking ArlRS are specifically more sensitive to manganese starvation. Loss of ArlRS did not alter the expression of manganese importers or prevent S. aureus from acquiring metals. It did, however, alter staphylococcal metabolism and impair the ability of S. aureus to grow on amino acids. Further studies suggested that relative to consuming glucose, the preferred carbon source of S. aureus, utilizing amino acids reduced the cellular demand for manganese. When forced to use glucose as the sole carbon source S. aureus became more sensitive to calprotectin compared to when amino acids are provided. Infection experiments utilizing wild type and calprotectin-deficient mice, which have defects in manganese sequestration, revealed that ArlRS is important for disease when manganese availability is restricted but not when this essential nutrient is freely available. In total, these results indicate that altering cellular metabolism contributes to the ability of pathogens to resist manganese starvation and that ArlRS enables S. aureus to overcome nutritional immunity by facilitating this adaptation., Author Summary The ubiquitous pathogen Staphylococcus aureus is a serious threat to human health due to the continued spread of antibiotic resistance. This spread has made it challenging to treat staphylococcal infections and led to the call for new approaches to treat this devastating pathogen. One approach is to disrupt the ability of S. aureus to adapt to nutrient availability during infection. During infection, the host imposes manganese and zinc starvation on invading pathogens. However, the mechanisms utilized by Staphylococcus aureus to overcome this host defense are unknown. We report that ArlRS, a global staphylococcal virulence regulator, is important for resisting manganese starvation during infection. Loss of ArlRS does not prevent S. aureus from acquiring metals but instead renders the bacterium incapable of adapting to limited manganese availability. ArlRS mutants also have metabolic defects and a reduced ability to grow on amino acids. When using glucose as a carbon source S. aureus is more sensitive to manganese starvation and increases the expression of manganese transporters relative to when amino acids are provided suggesting a higher demand for manganese. These observations indicate that ArlRS contributes to resisting nutritional immunity by altering metabolism to reduce the staphylococcal demand for manganese.

Nutritional immunity is the name given to the sequestration of essential metals by vertebrates to limit growth of invading pathogens such as Staphylococcus aureus. While iron sequestration is the canonical example of nutritional immunity, recent work has highlighted the importance of manganese and zinc sequestration by vertebrates in controlling both bacterial and fungal infections. Our laboratories are investigating the biochemical basis for the sequestration of essential metals and the impact of this nutrient deprivation on the physiology of S. aureus and other pathogens. We have shown that during staphylococcal infection the manganese and zinc binding protein calprotectin (CP) is a critical component of nutritional immunity. Furthermore, this neutrophil protein is required to render the staphylococcal abscess devoid of manganese. However, the structural features responsible for binding these metals were unknown as was the impact of this CP-induced nutrient deprivation on S. aureus. In our recently published manuscript, we showed using isothermal titration calorimetry and mutagenesis experiments that CP contains two transition metal binding sites, and is capable of binding two zinc ions or one manganese ion with nanomolar affinity. Moreover, CP was found to reduce staphylococcal superoxide dismutase activity and other antioxidant defenses resulting in the accumulation of intracellular superoxide and increased sensitivity to oxidative stress. The inhibition of superoxide defenses by CP renders S. aureus more sensitive to neutrophil-mediated killing while animal modeling suggests that manganese sequestration by the host during infection reduces staphylococcal superoxide dismutase activity. In total, these results suggest a two hit model for the antimicrobial activity of CP whereby this protein both inhibits bacterial growth and renders microbial invaders more sensitive to other immune effectors.