Search

Background We previously reported developing 2 anticapsular monoclonal antibodies (mAbs) as a novel therapy for Acinetobacter baumannii infections. We sought to determine whether a bispecific mAb (bsAb) could improve avidity and efficacy while maximizing strain coverage in one molecule. Methods Humanized mAb 65 was cloned into a single-chain variable fragment and attached to humanized mAb C8, combining their paratopes into a single bsAb (C73). We tested bsAb C73’s strain coverage, binding affinity, ex vivo opsonic activity, and in vivo efficacy compared to each mAb alone and combined. Results The bsAb demonstrated strain coverage, binding affinity, opsonization, and in vivo efficacy superior to either original mAb alone or combined. Conclusions A humanized bsAb targeting distinct A. baumannii capsule moieties enabled potent and effective coverage of disparate A. baumannii clinical isolates. The bsAb enhances feasibility of development by minimizing the number of components of a promising novel therapeutic for these difficult-to-treat infections.

Objective: To establish an antimicrobial stewardship program in the outpatient setting. Design: Prescribers of antimicrobials were asked to complete a survey regarding antimicrobial stewardship. We also monitored their compliance with appropriate prescribing practices, which were shared in monthly quality improvement reports. Setting: The study was performed at Loyola University Health System, an academic teaching healthcare system in a metropolitan suburban environment. Participants: Prescribers of antimicrobials across 19 primary care and 3 immediate- and urgent-care clinics. Methods: The voluntary survey was developed using SurveyMonkeyand was distributed via e-mail. Data were collected anonymously. Rates of compliance with appropriate prescribing practices were abstracted from electronic health records and assessed by 3 metrics: (1) avoidance of antibiotics in adult acute bronchitis and appropriate antibiotic treatment in (2) patients tested for pharyngitis and (3) children with upper respiratory tract infections. Results: Prescribers were highly knowledgeable about what constitutes appropriate prescribing; verified compliance rates were highly concordant with self-reported rates. Nearly all prescribers were concerned about resistance, but fewer than half believed antibiotics were overprescribed in their office. Among respondents, 74% reported intense pressure from patients to prescribe antimicrobials inappropriately. Immediate- and urgent-care prescribers had higher rates of compliance than primary-care prescribers, and the latter group responded well to monthly reports and online educational resources. Conclusions: Intense pressure from patients to prescribe antimicrobials when they are not indicated leads to overprescribing, an effect compounded by the importance of patient satisfaction scores. Compliance reporting improved the number of appropriate antibiotics prescribed in the primary care setting.

Extremely drug-resistant (XDR) Acinetobacter baumannii is a notorious and frequently encountered pathogen demanding novel therapeutic interventions. An initial monoclonal antibody (MAb), C8, raised against A. baumannii capsule, proved a highly effective treatment against a minority of clinical isolates. To overcome this limitation, we broadened coverage by developing a second antibody for use in a combination regimen. We sought to develop an additional anti-A. baumannii MAb through hybridoma technology by immunizing mice with sublethal inocula of virulent, XDR clinical isolates not bound by MAb C8. We identified a new antibacterial MAb, 65, which bound to strains in a pattern distinct from and complementary to that of MAb C8. MAb 65 enhanced macrophage opsonophagocytosis of targeted strains and markedly improved survival in lethal bacteremic sepsis and aspiration pneumonia murine models of A. baumannii infection. MAb 65 was also synergistic with colistin, substantially enhancing protection compared to monotherapy. Treatment with MAb 65 significantly reduced blood bacterial density, ameliorated cytokine production (interleukin-1β [IL-1β], IL-6, IL-10, and tumor necrosis factor), and sepsis biomarkers. We describe a novel MAb targeting A. baumannii that broadens immunotherapeutic strain coverage, is highly potent and effective, and synergistically improves outcomes in combination with antibiotics.

BackgroundNew strategies are needed to slow the emergence of antibiotic resistance among bacterial pathogens. In particular, society is experiencing a crisis of antibiotic-resistant infections caused by Gram-negative bacterial pathogens and novel therapeutics are desperately needed to combat such diseases. Acquisition of iron from the host is a nearly universal requirement for microbial pathogens—including Gram-negative bacteria—to cause infection. We have previously reported that apo-transferrin (lacking iron) can inhibit the growth of Staphylococcus aureus in culture and diminish emergence of resistance to rifampicin.ObjectivesTo define the potential of apo-transferrin to inhibit in vitro growth of Klebsiella pneumoniae and Acinetobacter baumannii, key Gram-negative pathogens, and to reduce emergence of resistance to antibiotics.MethodsThe efficacy of apo-transferrin alone or in combination with meropenem or ciprofloxacin against K. pneumoniae and A. baumannii clinical isolates was tested by MIC assay, time–kill assay and assays for the selection of resistant mutants.ResultsWe confirmed that apo-transferrin had detectable MICs for all strains tested of both pathogens. Apo-transferrin mediated an additive antimicrobial effect for both antibiotics against multiple strains in time–kill assays. Finally, adding apo-transferrin to ciprofloxacin or meropenem reduced the emergence of resistant mutants during 20 day serial passaging of both species.ConclusionsThese results suggest that apo-transferrin may have promise to suppress the emergence of antibiotic-resistant mutants when treating infections caused by Gram-negative bacteria.

Acinetobacter baumannii is a highly antibiotic-resistant bacterial pathogen for which novel therapeutic approaches are needed. Unfortunately, the drivers of virulence in A. baumannii remain uncertain. By comparing genomes among a panel of A. baumannii strains we identified a specific gene variation in the capsule locus that correlated with altered virulence. While less virulent strains possessed the intact gene gtr6, a hypervirulent clinical isolate contained a spontaneous transposon insertion in the same gene, resulting in the loss of a branchpoint in capsular carbohydrate structure. By constructing isogenic gtr6 mutants, we confirmed that gtr6-disrupted strains were protected from phagocytosis in vitro and displayed higher bacterial burden and lethality in vivo. Gtr6+ strains were phagocytized more readily and caused lower bacterial burden and no clinical illness in vivo. We found that the CR3 receptor mediated phagocytosis of gtr6+, but not gtr6-, strains in a complement-dependent manner. Furthermore, hypovirulent gtr6+ strains demonstrated increased virulence in vivo when CR3 function was abrogated. In summary, loss-of-function in a single capsule assembly gene dramatically altered virulence by inhibiting complement deposition and recognition by phagocytes across multiple A. baumannii strains. Thus, capsular structure can determine virulence among A. baumannii strains by altering bacterial interactions with host complement-mediated opsonophagocytosis., Author summary Acinetobacter baumannii is one of the most antibiotic-resistant pathogens in clinical medicine and is responsible for a significant number of deaths worldwide. We found that a highly virulent strain contained a mobile piece of DNA in one of its capsule assembly genes which rendered the gene inactive and thus removed a single sugar from the bacterium’s complex outer carbohydrate capsule. When we inactivated the same gene in a non-virulent related strain, it became virulent, and when we repaired the non-functional gene the virulent strain became non-virulent. We then determined that this single sugar was critical for innate immune cells to recognize and phagocytose bacteria, and that the cells depended on the deposition of host complement proteins on the capsule to recognize the strains with this extra sugar. This finding provides new insight into A. baumannii pathogenesis and may inform the development of future therapies against this insidious pathogen.

Monoclonal antibodies (mAbs) are gaining significant momentum as novel therapeutics for infections caused by antibiotic-resistant bacteria. We evaluated the mechanism by which antibacterial mAb therapy protects against Acinetobacter baumannii infections. Anticapsular mAb enhanced macrophage opsonophagocytosis and rescued mice from lethal infections by harnessing complement, macrophages, and neutrophils; however, the degree of bacterial burden did not correlate with survival. Furthermore, mAb therapy reduced proinflammatory (interleukin-1β [IL-1β], IL-6, tumor necrosis factor-α [TNF-α]) and anti-inflammatory (IL-10) cytokines, which correlated inversely with survival. Although disrupting IL-10 abrogated the survival advantage conferred by the mAb, IL-10–knockout mice treated with mAb could still survive if TNF-α production was suppressed directly (via anti–TNF-α neutralizing antibody) or indirectly (via macrophage depletion). Thus, even for a mAb that enhances microbial clearance via opsonophagocytosis, clinical efficacy required modulation of pro- and anti-inflammatory cytokines. These findings may inform future mAb development targeting bacteria that trigger the sepsis cascade.

Sustainable Discovery and Development of Antibiotics Shifting to a nonprofit-driven model for development of antibiotics could encourage discovery and development of truly needed drugs that improve...

SUMMARY Acinetobacter is a complex genus, and historically, there has been confusion about the existence of multiple species. The species commonly cause nosocomial infections, predominantly aspiration pneumonia and catheter-associated bacteremia, but can also cause soft tissue and urinary tract infections. Community-acquired infections by Acinetobacter spp. are increasingly reported. Transmission of Acinetobacter and subsequent disease is facilitated by the organism's environmental tenacity, resistance to desiccation, and evasion of host immunity. The virulence properties demonstrated by Acinetobacter spp. primarily stem from evasion of rapid clearance by the innate immune system, effectively enabling high bacterial density that triggers lipopolysaccharide (LPS)–Toll-like receptor 4 (TLR4)-mediated sepsis. Capsular polysaccharide is a critical virulence factor that enables immune evasion, while LPS triggers septic shock. However, the primary driver of clinical outcome is antibiotic resistance. Administration of initially effective therapy is key to improving survival, reducing 30-day mortality threefold. Regrettably, due to the high frequency of this organism having an extreme drug resistance (XDR) phenotype, early initiation of effective therapy is a major clinical challenge. Given its high rate of antibiotic resistance and abysmal outcomes (up to 70% mortality rate from infections caused by XDR strains in some case series), new preventative and therapeutic options for Acinetobacter spp. are desperately needed.

There has been renewed interest in combining traditional small-molecule antimicrobial agents with nontraditional therapies to potentiate antimicrobial effects. Apotransferrin, which decreases iron availability to microbes, is one such approach. We conducted a 48-h one-compartment in vitro infection model to explore the impact of apotransferrin on the bactericidal activity of ciprofloxacin. The challenge panel included four Klebsiella pneumoniae isolates with ciprofloxacin MIC values ranging from 0.08 to 32 mg/liter. Each challenge isolate was subjected to an ineffective ciprofloxacin monotherapy exposure (free-drug area under the concentration-time curve over 24 h divided by the MIC [AUC/MIC ratio] ranging from 0.19 to 96.6) with and without apotransferrin. As expected, the no-treatment and apotransferrin control arms showed unaltered prototypical logarithmic bacterial growth. We identified relationships between exposure and change in bacterial density for ciprofloxacin alone (R(2) = 0.64) and ciprofloxacin in combination with apotransferrin (R(2) = 0.84). Addition of apotransferrin to ciprofloxacin enabled a remarkable reduction in bacterial density across a wide range of ciprofloxacin exposures. For instance, at a ciprofloxacin AUC/MIC ratio of 20, ciprofloxacin monotherapy resulted in nearly 2 log(10) CFU increase in bacterial density, while the combination of apotransferrin and ciprofloxacin resulted in 2 log(10) CFU reduction in bacterial density. Furthermore, addition of apotransferrin significantly reduced the emergence of ciprofloxacin-resistant subpopulations compared to monotherapy. These data demonstrate that decreasing the rate of bacterial replication with apotransferrin in combination with antimicrobial therapy represents an opportunity to increase the magnitude of the bactericidal effect and to suppress the growth rate of drug-resistant subpopulations.

Inflammatory caspases (caspase-1, caspase-4, caspase-5 and caspase-11 (caspase-1/-4/-5/-11)) mediate host defense against microbial infections, processing pro-inflammatory cytokines and triggering pyroptosis. However, precise checkpoints are required to prevent their unsolicited activation. Here we report that serpin family B member 1 (SERPINB1) limited the activity of those caspases by suppressing their caspase-recruitment domain (CARD) oligomerization and enzymatic activation. While the reactive center loop of SERPINB1 inhibits neutrophil serine proteases, its carboxy-terminal CARD-binding motif restrained the activation of pro-caspase-1/-4/-5/-11. Consequently, knockdown or deletion of SERPINB1 prompted spontaneous activation of caspase-1/-4/-5/-11, release of the cytokine IL-1β and pyroptosis, inducing elevated inflammation after non-hygienic co-housing with pet-store mice and enhanced sensitivity to lipopolysaccharide- or Acinetobacter baumannii-induced endotoxemia. Our results reveal that SERPINB1 acts as a vital gatekeeper of inflammation by restraining neutrophil serine proteases and inflammatory caspases in a genetically and functionally separable manner.

Staphylococcus aureus infections represent a major public health threat, but previous attempts at developing a universal vaccine have been unsuccessful. We attempted to identify a vaccine that would be protective against both skin/soft tissue and bloodstream infections. We first tested a panel of staphylococcal antigens that are conserved across strains, combined with aluminum hydroxide as an adjuvant, for their ability to induce protective immunity in both skin and bacteremia infection models. Antigens were identified that reduced dermonecrosis during skin infection, and other non-overlapping antigens were identified that showed trends to protection in the bacteremia model. However, individual antigens were not identified that mediated substantial protection in both the skin and bacteremia infection models. We therefore tested a variety of combinations of proteins to seek a single combination that could mediate protection in both models. After iterative testing, a vaccine consisting of 3 antigens, ABC transporter protein (SACOL2451), ABC2 transporter protein (SACOL0695), and α-hemolysin (SACOL1173), was identified as the most effective combination. This combination vaccine provided protection in a skin infection model. However, these antigens were only partially protective in the bacteremia infection model. Even by testing multiple different adjuvants, optimized efficacy in the skin infection model did not translate into efficacy in the bacteremia model. Thus protective vaccines against skin/soft tissue infections may not enable effective protection against bloodstream infections.

In 2017, the WHO identified Acinetobacter baumannii as the top priority for the development of new antibiotics. Despite the need for new antibiotics, there remains a lack of well validated preclinical tools for A. baumannii. Here, we characterize and validate a mouse model for A. baumannii translational research. Antibiotic sensitivity for meropenem, amikacin, and polymyxin b was determined by the broth microdilution MIC assay. LD100 inoculums, in both blood and lung infection models, were determined in male and female C3HeB/FeJ mice that were challenged with various A. baumannii clinical isolates. Blood (blood infection model) or blood and lung tissue (lung infection model) were collected from infected mice at 2 and 18 hours and the bacterial burden was determined by quantitative culture. Blood chemistry was analyzed using the iStat system. Cytokines (IL-1s, TNF, IL-6, and IL-10) were measured in the blood and lung homogenate by ELISA assay. Lung sections (H&E stains) were scored by a pathologist. In the blood infection model, the cytokines and physiological data indicate that mice become moribund due to sepsis (low blood pH, falling bicarbonate, and a rising base deficit), whereas mice become moribund due to respiratory failure (low blood pH, rising bicarbonate, and a falling base deficit) in the oral aspiration pneumonia model. We also characterized the timing of changes in various clinical and biomarker endpoints, which can serve as a basis for future interventional studies. Susceptibility was generally similar across gender and infection route. However, we did observe that female mice were approximately 2-fold more sensitive to LAC-4 ColR in the blood infection model. We also observed that female mice were more than 10-fold more resistant to VA-AB41 in the oral aspiration pneumonia model. These results establish parameters to follow in order to assess efficacy of novel preventative and therapeutic approaches for these infections.

Background Antimicrobial-resistant infections lead to increased morbidity, mortality, and healthcare costs. Among the most facile modifiable risk factors for developing resistance is inappropriate prescribing. The CDC estimates that 47 million (or ≥30% of) outpatient antibiotic prescriptions in the United States are unnecessary. This has provided impetus for expanding our antimicrobial stewardship program (ASP) into the outpatient setting. Initial goals included the following: continuous evaluation and reporting of antibiotic prescribing compliance; minimize underuse of antibiotics from delayed diagnoses and misdiagnoses; ensure proper drug, dose, and duration; improve the percentage of appropriate prescriptions. Methods To achieve these goals, we first sent a baseline survey to outpatient prescribers, assessing their understanding of stewardship and antimicrobial resistance. Questions were modeled from the Illinois Department of Public Health (IDPH) Precious Drugs & Scary Bugs Campaign. The survey was sent to prescribers at 19 primary care and three immediate/urgent care clinics. Compliance rates for prescribing habits were subsequently tracked via electronic health records and reported to prescribers in accordance with IRB approval. Results Prescribers were highly knowledgeable about what constitutes appropriate prescribing, with verified compliance rates highly concordant with self-reported rates. However, 74% of respondents reported intense pressure from patients to inappropriately prescribe antimicrobials. Compliance rates have been tracked since December 2018 and comparing pre- with post-intervention rates shows improvement in primary care since reporting rates to prescribers in August 2019. Conclusion Reporting compliance rates has been helpful in avoiding inappropriate antimicrobial therapy. However, the survey data reinforce the importance of behavioral interventions to bolster ASP efficacy in the outpatient setting. Going forward, posters modeled off of the IDPH template will be conspicuously exhibited in exam rooms, indicating institutional commitment to the enumerated ASP guidelines. Future studies will allow for comparison of pre- and post-intervention knowledge and prescriber compliance. Disclosures All Authors: No reported disclosures

Murine infection models are critical for understanding disease pathogenesis and testing the efficacy of novel therapeutics designed to combat causative pathogens. Infectious pneumonia is among the most common infections presented by patients in the clinic and thus warrants an appropriate in vivo model. Typical pneumonia models use intranasal inoculation, which deposits excessive organisms outside the lung, causing off-target complications and symptoms, such as sinusitis, gastritis, enteritis, physical trauma, or microparticle misting to mimic aerosol spread more typical of viral, tuberculous, or fungal pneumonia. These models do not accurately reflect the pathogenesis of typical community- or healthcare-acquired bacterial pneumonia. In contrast, this murine model of oropharyngeal aspiration pneumonia mimics the droplet route in healthcare-acquired pneumonia. Inoculating 50 µL of the bacteria suspension into the oropharynx of anesthetized mice causes reflexive aspiration, which results in pneumonia. With this model, one can examine the pathogenesis of pneumonia-causing pathogens and new treatments to combat these diseases.

Antibody-mediated depletion of neutrophils is commonly used to study neutropenia. However, the mechanisms by which antibodies deplete neutrophils have not been well defined. We noticed that mice deficient in complement and macrophages had blunted neutrophil depletion in response to anti-Ly6G monoclonal antibody (MAb) treatment. In vitro, exposure of murine neutrophils to anti-Ly6G MAb in the presence of plasma did not result in significant depletion of cells, either in the presence or absence of complement. In vivo, anti-Ly6G-mediated neutrophil depletion was abrogated following macrophage depletion, but not complement depletion, indicating a requirement for macrophages to induce neutropenia by this method. These results inform the use and limitations of anti-Ly6G antibody as an experimental tool for depleting neutrophils in various immunological settings., Highlights • In vivo depletion of mouse neutrophils by anti-Ly6G antibody requires macrophages. • Plasma is not sufficient for anti-Ly6G-mediated neutrophil depletion in vitro. • Anti-Ly6G depletes neutrophils in mice without active complement.

Background: Extremely drug-resistant (XDR) Acinetobacter baumannii is one of the most commonly encountered, highly resistant pathogens requiring novel therapeutic interventions. Methods: We developed C8, a monoclonal antibody (mAb), by immunizing mice with sublethal inocula of a hypervirulent XDR clinical isolate. Results: C8 targets capsular carbohydrate on the bacterial surface, enhancing opsonophagocytosis. Treating with a single dose of C8 as low as 0.5 μg/mouse (0.0167 mg/kg) markedly improved survival in lethal bacteremic sepsis and aspiration pneumonia models of XDR A. baumannii infection. C8 was also synergistic with colistin, substantially improving survival compared to monotherapy. Treatment with C8 significantly reduced blood bacterial density, cytokine production (tumor necrosis factor α, interleukin [IL] 6, IL-1β, and IL-10), and sepsis biomarkers. Serial in vitro passaging of A. baumannii in the presence of C8 did not cause loss of mAb binding to the bacteria, but did result in emergence of less-virulent mutants that were more susceptible to macrophage uptake. Finally, we developed a highly humanized variant of C8 that retains opsonophagocytic activity in murine and human macrophages and rescued mice from lethal infection. Conclusions: We describe a promising and novel mAb as therapy for lethal, XDR A. baumannii infections, and demonstrate that it synergistically improves outcomes in combination with antibiotics.

Current initiatives to incentivize development of antibiotics for resistant pathogens have yielded disappointing results because many recently approved antibiotics do not address clinical unmet nee...

Background We have previously demonstrated that A. baumannii virulence is driven by avoidance of innate effector clearance, resulting in LPS-TLR4 triggering of excess inflammation in the host. We also raised a monoclonal antibody (MAb) that improved survival of mice lethally infected with A. baumannii. Methods Mice were selectively depleted of innate effectors (macrophages with liposomal clodronate, neutrophils with cyclophosphamide, and/or complement with cobra venom factor), infected with an XDR clinical blood isolate of A. baumannii, and treated with placebo or anti-A. baumannii MAb. Results Single disruption of macrophages or neutrophils did not enhance lethality but complement deficiency did. In contrast, singly disrupting complement or neutrophils did not impact bacterial density but macrophage disruption markedly increased it. Thus, a dissociation of bacterial density and survival was observed. MAb therapy was completely protective in mice depleted of a single effector. While dual depletion resulted in diminished MAb efficacy in terms of survival, mice retaining neutrophils had marked improvements in survival with MAb therapy compared with other dual-depletion groups. The dissociation of bacterial density and survival suggested that inflammation was a primary driver of host outcome. Levels of IL-10 and TNFα had a reciprocal relationship in mice across effector depletion groups and were lower in mouse groups with higher survival when adjusted for bacterial density. IL-10 disruption completely abrogated the survival benefit of MAb therapy without altering bacterial clearance mediated by MAb. In contrast, TNFα disruption enhanced MAb efficacy for survival, and the presence of TNFα was antagonistic to MAb efficacy. Conclusion These results confirm that host outcomes from A. baumannii infection are driven by host inflammatory response rather than bacterial density alone. Furthermore, novel therapeutic approaches seeking to improve outcomes from such infections must seek to shift the balance of pro-/anti-inflammatory cytokines to favor a down-modulated inflammatory response. Disclosures All Authors: No reported Disclosures.

Background Microbiological assays require accurate and reproducible preparation of bacterial inocula. Inocula prepared on different days by different individuals can vary significantly from experiment to experiment. This variance is particularly problematic for Gram-negative bacterial infections, for which threshold effects can result in marked variations in host outcome with minor differences in inocula. Results We compared the accuracy of traditional methods versus using frozen stocks for preparing Acinetobacter baumannii inocula for infection in mice. Standard inoculum preparation resulted in substantial variability, both with respect to the actual inocula achieved compared to the targeted inocula, and with respect to the in vivo outcome resulting from similar inocula. Cryopreservation of the bacteria resulted in no significant decrement in growth of the bacteria. Furthermore, preparation of multiple infectious inocula from a frozen stock significantly improved the accuracy of the achieved inocula, and resulted in more reproducible in vivo outcomes from infection. Frozen stocks reduced inter-experiment variability associated with inoculum preparation, displayed no significant loss of growth capacity, and maintained virulence, increasing the reliability of infection. Conclusions Frozen stocks require considerably less time to prepare and enhance reproducibility of in vivo experimental results when infecting with A. baumannii.

In the last decade, Acinetobacter baumannii has emerged to become a predominant cause of nosocomial infections in the United States and across the globe [1–4] and is one of the few bacteria of which strains have developed resistance to all available antibiotics [5, 6]. In particular, bacteremia caused by carbapenem-resistant A. baumannii isolates results in >50% mortality rates owing to inadequate available therapies [7–11]. Furthermore, in contrast to other resistant bacteria, few antibiotics are in the pipeline to treat extremely drug-resistant A. baumannii [6, 12]. There is a critical need for new strategies to prevent and treat infections caused by highly resistant A. baumannii. Rational design of new prevention and/or treatment strategies requires a much deeper understanding of relevant host-pathogen interactions. In vitro virulence factors, such as surface hydrophobicity, iron siderophores, outer membrane proteins, biofilm-inducing polysaccharides [13–15], and phospholipase D [16] are described as being dominant during lung, skin, or ascites/abscess infection by A. baumannii. Regrettably, studies defining host factors that determine outcome are more limited. During lung infection, depletion of neutrophils converted survivable inocula to lethal inocula [17, 18], and macrophage depletion resulted in increased tissue bacterial burden [19]. However, the mechanisms that enable A. baumannii to cause disease during bacteremia are not well described. We sought to define microbe-host interactions that determined survival during bloodstream infection caused by a representative collection of clinical isolates of A. baumannii. Previously, we demonstrated a role for lipopolysaccharide (LPS) in A. baumannii virulence and showed that mice lacking functional Toll-like receptor 4 (TLR4) were resistant to sepsislike disease and death after bloodstream infection [20]. Here we extend those findings to demonstrate that early clearance kinetics of the bacteria by the host are critical in determining virulence and that distinct A. baumannii isolates display different susceptibilities to host innate effector mechanisms. These findings have important implications for vaccine and other immune-based therapies.

New strategies to treat antibiotic-resistant infections are urgently needed. We serendipitously discovered that stem cell conditioned media possessed broad antimicrobial properties. Biochemical, functional, and genetic assays confirmed that the antimicrobial effect was mediated by supra-physiological concentrations of transferrin. Human transferrin inhibited growth of gram-positive (Staphylococcus aureus), gram-negative (Acinetobacter baumannii), and fungal (Candida albicans) pathogens by sequestering iron and disrupting membrane potential. Serial passage in subtherapeutic transferrin concentrations resulted in no emergence of resistance. Infected mice treated with intravenous human transferrin had improved survival and reduced microbial burden. Finally, adjunctive transferrin reduced the emergence of rifampin-resistant mutants of S. aureus in infected mice treated with rifampin. Transferrin is a promising, novel antimicrobial agent that merits clinical investigation. These results provide proof of principle that bacterial infections can be treated in vivo by attacking host targets (ie, trace metal availability) rather than microbial targets.