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Pathogen avoidance behaviour has been observed across animal taxa as a vital host-microbe interaction mechanism. The nematode Caenorhabditis elegans has evolved multiple diverse mechanisms for pathogen avoidance under natural selection pressure. We summarise the current knowledge of the stimuli that trigger pathogen avoidance, including alterations in aerotaxis, intestinal bloating, and metabolites. We then survey the neural circuits involved in pathogen avoidance, transgenerational epigenetic inheritance of pathogen avoidance, signalling crosstalk between pathogen avoidance and innate immunity, and C. elegans avoidance of non-Pseudomonas bacteria. In this review, we highlight the latest advances in understanding host-microbe interactions and the gutbrain axis. [ABSTRACT FROM AUTHOR]

Agrobacterium tumefaciens is a soil-borne pathogenic bacterium that causes crown gall disease in many plants. Chemotaxis offers A. tumefaciens the ability to find its host and establish infection. Being an aerobic bacterium, A. tumefaciens possesses one chemotaxis system with multiple potential chemoreceptors. Chemoreceptors play an important role in perceiving and responding to environmental signals. However, the studies of chemoreceptors in A. tumefaciens remain relatively restricted. Here, we characterized a cytoplasmic chemoreceptor of A. tumefaciens C58 that contains an N-terminal globin domain. The chemoreceptor was designated as Atu1027. The deletion of Atu1027 not only eliminated the aerotactic response of A. tumefaciens to atmospheric air but also resulted in a weakened chemotactic response to multiple carbon sources. Subsequent site-directed mutagenesis and phenotypic analysis showed that the conserved residue His100 in Atu1027 is essential for the globin domain's function in both chemotaxis and aerotaxis. Furthermore, deleting Atu1027 impaired the biofilm formation and pathogenicity of A. tumefaciens. Collectively, our findings demonstrated that Atu1027 functions as an aerotaxis receptor that affects agrobacterial chemotaxis and the invasion of A. tumefaciens into its host. [ABSTRACT FROM AUTHOR]

This document is an editorial that highlights four original research articles on cell migration dynamics. The first article explores the effect of surface roughness on cell migration using human osteosarcoma cells. The second article focuses on the classification of Triple-Negative Breast Cancers (TNBCs) and the role of pseudopod dynamics in cell migration. The third article investigates the aerotaxis of cells using a model system. The fourth article examines the mechanisms of corticogenesis in a mouse model. The editorial emphasizes the significance of these studies and their relevance to a wide audience of scientists. Additionally, the document discusses two separate research studies on single cell migration. The first study investigates the aerotaxis of Dictyostelium discoideum cells and finds that oxidative and nitrosative stress are not involved in this process. The second study identifies a distinct cell population that undergoes a direct neuronal shift during fetal cortical development, suggesting differences in cortical evolution between mice and humans. These studies highlight the importance of analyzing the dynamic nature of single cell migration and suggest further avenues for research in the field. [Extracted from the article]

Abstract Background Aerotaxis, the chemotactism to oxygen, is well documented in prokaryotes. We previously reported for the first time that non-tumorigenic breast epithelial cells also display unequivocal directional migration towards oxygen. This process is independent of the hypoxia-inducible factor (HIF)/prolyl hydroxylase domain (PHD) pathway but controlled by the redox regulation of epidermal growth factor receptor (EGFR), with a reactive oxygen species (ROS) gradient overlapping the oxygen gradient at low oxygen concentration. Since hypoxia is an acknowledged hallmark of cancers, we addressed the putative contribution of aerotaxis to cancer metastasis by studying the directed migration of cancer cells from an hypoxic environment towards nearby oxygen sources, modelling the in vivo migration of cancer cells towards blood capillaries. Methods We subjected to the aerotactic test described in our previous papers cells isolated from fresh breast tumours analysed by the Pathology Department of the Saint-Etienne University Hospital (France) over a year. The main selection criterion, aside from patient consent, was the size of the tumour, which had to be large enough to perform the aerotactic tests without compromising routine diagnostic tests. Finally, we compared the aerotactic properties of these primary cells with those of commonly available breast cancer cell lines. Results We show that cells freshly isolated from sixteen human breast tumour biopsies, representative of various histological characteristics and grades, are endowed with strong aerotactic properties similar to normal mammary epithelial cell lines. Strikingly, aerotaxis of these primary cancerous cells is also strongly dependent on both EGFR activation and ROS. In addition, we demonstrate that aerotaxis can trigger directional invasion of tumour cells within the extracellular matrix contrary to normal mammary epithelial cells. This contrasts with results obtained with breast cancer cell lines, in which aerotactic properties were either retained or impaired, and in some cases, even lost during the establishment of these cell lines. Conclusions Altogether, our results support that aerotaxis may play an important role in breast tumour metastasis. In view of these findings, we discuss the prospects for combating metastatic spread. Trial registration IRBN1462021/CHUSTE. Graphical abstract: EGFR-dependent aerotaxis of primary breast cancer cells

Spatial and temporal variations of oxygen environments affect the behaviors of various cells and are involved in physiological and pathological events. Our previous studies with Dictyostelium discoideum as a model of cell motility have demonstrated that aerotaxis toward an oxygen-rich region occurs below 2% O2. However, while the aerotaxis of Dictyostelium seems to be an effective strategy to search for what is essential for survival, the mechanism underlying this phenomenon is still largely unclear. One hypothesis is that an oxygen concentration gradient generates a secondary oxidative stress gradient that would direct cell migration towards higher oxygen concentration. Such mechanism was inferred but not fully demonstrated to explain the aerotaxis of human tumor cells. Here, we investigated the role on aerotaxis of flavohemoglobins, proteins that can both act as potential oxygen sensors and modulators of nitric oxide and oxidative stress. The migratory behaviors of Dictyostelium cells were observed under both self-generated and imposed oxygen gradients. Furthermore, their changes by chemicals generating or preventing oxidative stress were tested. The trajectories of the cells were then analyzed through time-lapse phase-contrast microscopic images. The results indicate that both oxidative and nitrosative stresses are not involved in the aerotaxis of Dictyostelium but cause cytotoxic effects that are enhanced upon hypoxia.

BackgroundAn efficient signal transduction system allows a bacterium to sense environmental cues and then to respond positively or negatively to those signals; this process is referred to as taxis. In addition to external cues, the internal metabolic state of any bacterium plays a major role in determining its ability to reside and thrive in its current environment. Similar to external signaling molecules, cytoplasmic signals are also sensed by methyl-accepting chemotaxis proteins (MCPs) via diverse ligand binding domains. Myxobacteria are complex soil-dwelling social microbes that can perform a variety of physiologic and metabolic activities ranging from gliding motility, sporulation, biofilm formation, carotenoid and secondary metabolite biosynthesis, predation, and slime secretion. To live such complex lifestyles, they have evolved efficient signal transduction systems with numerous one- and two-component regulatory system along with a large array of chemosensory systems to perceive and integrate both external and internal cues.ResultsHere we report the in silico characterization of a putative energy taxis cluster, Cc-5, which is present in only one amongst 34 known and sequenced myxobacterial genomes, Corallococcus coralloides. In addition, we propose that this energy taxis cluster is involved in oxygen sensing, suggesting that C. coralloides can sense (either directly or indirectly) and then respond to changing concentrations of molecular oxygen.ConclusionsThis hypothesis is based on the presence of a unique MCP encoded in this gene cluster that contains two different oxygen-binding sensor domains, PAS and globin. In addition, the two monooxygenases encoded in this cluster may contribute to aerobic respiration via ubiquinone biosynthesis, which is part of the cytochrome bc1 complex. Finally, we suggest that this cluster was acquired from Actinobacteria, Gammaproteobacteria or Cyanobacteria. Overall, this in silico study has identified a potentially innovative and evolved mechanism of energy taxis in only one of the myxobacteria, C. coralloides.

Bacterial chemotaxis is the phenomenon in which bacteria migrate toward a more favorable niche in response to chemical cues in the environment. The methyl-accepting chemotaxis proteins (MCPs) are the principal sensory receptors of the bacterial chemotaxis system. Aerotaxis is a special form of chemotaxis in which oxygen serves as the signaling molecule; the process is dependent on the aerotaxis receptors (Aer) containing the Per-Arnt-Sim (PAS) domain. Over 40 MCPs are annotated on the genome of Vibrio cholerae; however, little is known about their functions. We investigated six MCPs containing the PAS domain in V. cholerae El Tor C6706, namely aer2, aer3, aer4, aer5, aer6, and aer7. Deletion analyses of each aer homolog gene indicated that these Aer receptors are involved in aerotaxis, chemotaxis, biofilm formation, and intestinal colonization. Swarming motility assay indicated that the aer2 gene was responsible for sensing the oxygen gradient independent of the other five homologs. When bile salts and mucin were used as chemoattractants, each Aer receptor influenced the chemotaxis differently. Biofilm formation was enhanced by overexpression of the aer6 and aer7 genes. Moreover, deletion of the aer2 gene resulted in better bacterial colonization of the mutant in adult mice; however, virulence gene expression was unaffected. These data suggest distinct roles for different Aer homologs in V. cholerae physiology. [ABSTRACT FROM AUTHOR]

Background: Aerotaxis, the chemotactism to oxygen, is well documented in prokaryotes. We previously reported for the first time that non-tumorigenic breast epithelial cells also display unequivocal directional migration towards oxygen. This process is independent of the hypoxia-inducible factor (HIF)/prolyl hydroxylase domain (PHD) pathway but controlled by the redox regulation of epidermal growth factor receptor (EGFR), with a reactive oxygen species (ROS) gradient overlapping the oxygen gradient at low oxygen concentration. Since hypoxia is an acknowledged hallmark of cancers, we addressed the putative contribution of aerotaxis to cancer metastasis by studying the directed migration of cancer cells from an hypoxic environment towards nearby oxygen sources, modelling the in vivo migration of cancer cells towards blood capillaries. Methods: We subjected to the aerotactic test described in our previous papers cells isolated from fresh breast tumours analysed by the Pathology Department of the Saint-Etienne University Hospital (France) over a year. The main selection criterion, aside from patient consent, was the size of the tumour, which had to be large enough to perform the aerotactic tests without compromising routine diagnostic tests. Finally, we compared the aerotactic properties of these primary cells with those of commonly available breast cancer cell lines. Results: We show that cells freshly isolated from sixteen human breast tumour biopsies, representative of various histological characteristics and grades, are endowed with strong aerotactic properties similar to normal mammary epithelial cell lines. Strikingly, aerotaxis of these primary cancerous cells is also strongly dependent on both EGFR activation and ROS. In addition, we demonstrate that aerotaxis can trigger directional invasion of tumour cells within the extracellular matrix contrary to normal mammary epithelial cells. This contrasts with results obtained with breast cancer cell lines, in which aerotactic properties were either retained or impaired, and in some cases, even lost during the establishment of these cell lines. Conclusions: Altogether, our results support that aerotaxis may play an important role in breast tumour metastasis. In view of these findings, we discuss the prospects for combating metastatic spread. Trial registration: IRBN1462021/CHUSTE. [ABSTRACT FROM AUTHOR]

Microorganisms often actively respond to multiple external stimuli to navigate toward their preferred niches. For example, unicellular magnetotactic bacteria integrate both oxygen sensory information and the Earth's geomagnetic field to help them locate anoxic conditions in a process known as magneto-aerotaxis. However, for multicellular magnetotactic prokaryotes (MMPs), the colonial structure of 4-16 cells places fundamental constraints on collective sensing, colony motility and directed swimming. To investigate how colonies navigate environments with multiple stimuli, we performed microfluidic experiments of MMPs with opposing magnetic fields and oxygen gradients. These experiments reveal unusual back-and-forth excursions called 'escape motility', in which colonies shuttle along magnetic field lines, punctuated by abrupt-yet highly coordinated-changes in collective ciliary beating. Through cell tracking and numerical simulations, we demonstrate that escape motility can arise through a simple magneto-aerotaxis mechanism, which includes the effect of magnetic torques and chemical sensing. At sufficiently high densities of MMPs, we observe the formation of dynamic crystal structures, whose stability is governed by the magnetic field strength and near-field hydrodynamic interactions. The results shed light on how some of the earliest multicellular organisms navigate complex physico-chemical landscapes.

In aerobic organisms, oxygen is essential for efficient energy production, and it acts as the last acceptor of the mitochondrial electron transport chain and as regulator of gene expression. However, excessive oxygen can lead to production of deleterious reactive oxygen species. Therefore, the directed migration of single cells or cell clumps from hypoxic areas toward a region of optimal oxygen concentration, named aerotaxis, can be considered an adaptive mechanism that plays a major role in biological and pathological processes. One relevant example is the development of O2 gradients when tumors grow beyond their vascular supply, leading frequently to metastasis. In higher eukaryotic organisms, aerotaxis has only recently begun to be explored, but genetically amenable model organisms suitable to dissect this process remain an unmet need. In this regard, we sought to assess whether Dictyostelium cells, which are an established model for chemotaxis and other motility processes, could sense oxygen gradients and move directionally in their response. By assessing different physical parameters, our findings indicate that both growing and starving Dictyostelium cells under hypoxic conditions migrate directionally toward regions of higher O2 concentration. This migration is characterized by a specific pattern of cell arrangement. A thickened circular front of high cell density (corona) forms in the cell cluster and persistently moves following the oxygen gradient. Cells in the colony center, where hypoxia is more severe, are less motile and display a rounded shape. Aggregation-competent cells forming streams by chemotaxis, when confined under hypoxic conditions, undergo stream or aggregate fragmentation, giving rise to multiple small loose aggregates that coordinately move toward regions of higher O2 concentration. By testing a panel of mutants defective in chemotactic signaling, and a catalase-deficient strain, we found that the latter and the pkbR1null exhibited altered migration patterns. Our results suggest that in Dictyostelium, like in mammalian cells, an intracellular accumulation of hydrogen peroxide favors the migration toward optimal oxygen concentration. Furthermore, differently from chemotaxis, this oxygen-driven migration is a G protein-independent process.

Abstract Background Bacterial chemotaxis, the ability of motile bacteria to navigate gradients of chemicals, plays key roles in the establishment of various plant-microbe associations, including those that benefit plant growth and crop productivity. The motile soil bacterium Azospirillum brasilense colonizes the rhizosphere and promotes the growth of diverse plants across a range of environments. Aerotaxis, or the ability to navigate oxygen gradients, is a widespread behavior in bacteria. It is one of the strongest behavioral responses in A. brasilense and it is essential for successful colonization of the root surface. Oxygen is one of the limiting nutrients in the rhizosphere where density and activity of organisms are greatest. The aerotaxis response of A. brasilense is also characterized by high precision with motile cells able to detect narrow regions in a gradient where the oxygen concentration is low enough to support their microaerobic lifestyle and metabolism. Results Here, we present a mathematical model for aerotaxis band formation that captures most critical features of aerotaxis in A. brasilense. Remarkably, this model recapitulates experimental observations of the formation of a stable aerotactic band within 2 minutes of exposure to the air gradient that were not captured in previous modeling efforts. Using experimentally determined parameters, the mathematical model reproduced an aerotactic band at a distance from the meniscus and with a width that matched the experimental observation. Conclusions Including experimentally determined parameter values allowed us to validate a mathematical model for aerotactic band formation in spatial gradients that recapitulates the spatiotemporal stability of the band and its position in the gradient as well as its overall width. This validated model also allowed us to capture the range of oxygen concentrations the bacteria prefer during aerotaxis, and to estimate the effect of parameter values (e.g. oxygen consumption rate), both of which are difficult to obtain in experiments.

Magnetospirillum gryphiswaldense employs iron-rich nanoparticles for magnetic navigation within environmental redox gradients. This behavior termed magnetoaerotaxis was previously shown to rely on the sensory pathway CheOp1, but the precise localization of CheOp1-related chemoreceptor arrays during the cell cycle and its possible interconnection with three other chemotaxis pathways have remained unstudied. Here, we analyzed the localization of chemoreceptor-associated adaptor protein CheW1 and histidine kinase CheA1 by superresolution microscopy in a spatiotemporal manner. CheW1 localized in dynamic clusters that undergo occasional segregation and fusion events at lateral sites of both cell poles. Newly formed smaller clusters originating at midcell before completion of cytokinesis were found to grow in size during the cell cycle. Bipolar CheA1 localization and formation of aerotactic swim halos were affected depending on the fluorescent protein tag, indicating that CheA1 localization is important for aerotaxis. Furthermore, polar CheW1 localization was independent of cheOp2 to cheOp4 but lost in the absence of cheOp1 or cheA1. Results were corroborated by the detection of a direct protein interaction between CheA1 and CheW1 and by the observation that cheOp2- and cheOp3-encoded CheW paralogs localized in spatially distinct smaller clusters at the cell boundary. Although the findings of a minor aerotaxis-related CheOp4 phenotype and weak protein interactions between CheOp1 and CheOp4 by two-hybrid analysis implied that CheW1 and CheW4 might be part of the same chemoreceptor array, CheW4 was localized in spatially distinct polar-lateral arrays independent of CheOp1, suggesting that CheOp1 and CheOp4 are also not connected at the molecular level. [ABSTRACT FROM AUTHOR]

Pathogen avoidance behaviour has been observed across animal taxa as a vital host-microbe interaction mechanism. The nematode Caenorhabditis elegans has evolved multiple diverse mechanisms for pathogen avoidance under natural selection pressure. We summarise the current knowledge of the stimuli that trigger pathogen avoidance, including alterations in aerotaxis, intestinal bloating, and metabolites. We then survey the neural circuits involved in pathogen avoidance, transgenerational epigenetic inheritance of pathogen avoidance, signalling crosstalk between pathogen avoidance and innate immunity, and C. elegans avoidance of non- Pseudomonas bacteria. In this review, we highlight the latest advances in understanding host-microbe interactions and the gut-brain axis., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Lei, Tan, Tu and Tan.)

Pseudomonas chlororaphis PCL1606 (PcPCL1606) is a rhizobacterium isolated from avocado roots, which is a favorable niche for its development. This strain extensively interacts with plant roots and surrounding microbes and is considered a biocontrol rhizobacterium. Genome sequencing has shown the presence of thirty-one potential methyl-accepting chemotaxis proteins (MCPs). Among these MCPs, two candidates are putative functional aerotaxis receptors, encoded at locus PCL1606_41090 (aer1-1) and locus PLC1606_20530 (aer1-2), that are homologous to the Aer receptor of Pseudomonas aeruginosa strain PaO1. Single- and double-deletion mutants in one or both genes have led to motility deficiencies in oxygen-rich areas, particularly reduced swimming motility compared with that of wildtype PcPCL1606. No differences in swarming tests were detected, and less adhesion by the aer double mutant was observed. However, the single and double mutants on avocado plant roots showed delayed biocontrol ability. During the first days of the biocontrol experiment, the aer-defective mutants also showed delayed root colonization. The current research characterizes the presence of aer transductors on P. chlororaphis. Thus, the functions of the PCL1606_41090 and PCL1606_20530 loci, corresponding to genes aer1-1 and aer1-2, respectively, are elucidated.

Abstract Background An efficient signal transduction system allows a bacterium to sense environmental cues and then to respond positively or negatively to those signals; this process is referred to as taxis. In addition to external cues, the internal metabolic state of any bacterium plays a major role in determining its ability to reside and thrive in its current environment. Similar to external signaling molecules, cytoplasmic signals are also sensed by methyl-accepting chemotaxis proteins (MCPs) via diverse ligand binding domains. Myxobacteria are complex soil-dwelling social microbes that can perform a variety of physiologic and metabolic activities ranging from gliding motility, sporulation, biofilm formation, carotenoid and secondary metabolite biosynthesis, predation, and slime secretion. To live such complex lifestyles, they have evolved efficient signal transduction systems with numerous one- and two-component regulatory system along with a large array of chemosensory systems to perceive and integrate both external and internal cues. Results Here we report the in silico characterization of a putative energy taxis cluster, Cc-5, which is present in only one amongst 34 known and sequenced myxobacterial genomes, Corallococcus coralloides. In addition, we propose that this energy taxis cluster is involved in oxygen sensing, suggesting that C. coralloides can sense (either directly or indirectly) and then respond to changing concentrations of molecular oxygen. Conclusions This hypothesis is based on the presence of a unique MCP encoded in this gene cluster that contains two different oxygen-binding sensor domains, PAS and globin. In addition, the two monooxygenases encoded in this cluster may contribute to aerobic respiration via ubiquinone biosynthesis, which is part of the cytochrome bc1 complex. Finally, we suggest that this cluster was acquired from Actinobacteria, Gammaproteobacteria or Cyanobacteria. Overall, this in silico study has identified a potentially innovative and evolved mechanism of energy taxis in only one of the myxobacteria, C. coralloides.

The alphaproteobacterium Magnetospirillum gryphiswaldense has the intriguing ability to navigate within magnetic fields, a behavior named magnetotaxis, governed by the formation of magnetosomes, intracellular membrane-enveloped crystals of magnetite. Magnetosomes are aligned in chains along the cell's motility axis by a dedicated multipart cytoskeleton ("magnetoskeleton"); however, precise estimates of its significance for magnetotaxis have not been reported. Here, we estimated the alignment of strains deficient in various magnetoskeletal constituents by live-cell motility tracking within defined magnetic fields ranging from 50 μT (reflecting the geomagnetic field) up to 400 μT. Motility tracking revealed that ΔmamY and ΔmamK strains (which assemble mispositioned and fragmented chains, respectively) are partially impaired in magnetotaxis, with approximately equal contributions of both proteins. This impairment was reflected by a required magnetic field strength of 200 μT to achieve a similar degree of alignment as for the wild-type strain in a 50-μT magnetic field. In contrast, the ΔmamJ strain, which predominantly forms clusters of magnetosomes, was only weakly aligned under any of the tested field conditions and could barely be distinguished from a nonmagnetic mutant. Most findings were corroborated by a soft agar swimming assay to analyze magnetotaxis based on the degree of distortion of swim halos formed in magnetic fields. Motility tracking further revealed that swimming speeds of M. gryphiswaldense are highest within the field strength equaling the geomagnetic field. In conclusion, magnetic properties and intracellular positioning of magnetosomes by a dedicated magnetoskeleton are required and optimized for bacterial magnetotaxis and most efficient locomotion within the geomagnetic field. [ABSTRACT FROM AUTHOR]

Chemotactic bacteria sense environmental changes via dedicated receptors that bind to extra- or intracellular cues and relay this signal to ultimately alter direction of movement toward beneficial cues and away from harmful environments. In complex environments, such as the rhizosphere, bacteria must be able to sense and integrate diverse cues. Azospirillum brasilense is a microaerophilic motile bacterium that promotes growth of cereals and grains. Root surface colonization is a prerequisite for the beneficial effects on plant growth but how motile A. brasilense navigates the rhizosphere is poorly studied. Previously only 2 out of 51 A. brasilense chemotaxis receptors have been characterized, AerC and Tlp1, and only Tlp1 was found to be essential for wheat root colonization. Here we describe another chemotaxis receptor, named Aer, that is homologous to the Escherichia coli Aer receptor, likely possesses an FAD cofactor and is involved in aerotaxis (taxis in an air gradient). We also found that the A. brasilense Aer contributes to sensing chemical gradients originating from wheat roots. In addition to A. brasilense Aer having a putative N-terminal FAD-binding PAS domain, it possesses a C-terminal PilZ domain that contains all the conserved residues for binding c-di-GMP. Mutants lacking the PilZ domain of Aer are altered in aerotaxis and are completely null in wheat root colonization and they also fail to sense gradients originating from wheat roots. The PilZ domain of Aer is also vital in integrating Aer signaling with signaling from other chemotaxis receptors to sense gradients from wheat root surfaces and colonizing wheat root surfaces.

Agrobacterium tumefaciens is a soil-borne pathogenic bacterium that causes crown gall disease in many plants. Chemotaxis offers A. tumefaciens the ability to find its host and establish infection. Being an aerobic bacterium, A. tumefaciens possesses one chemotaxis system with multiple potential chemoreceptors. Chemoreceptors play an important role in perceiving and responding to environmental signals. However, the studies of chemoreceptors in A. tumefaciens remain relatively restricted. Here, we characterized a cytoplasmic chemoreceptor of A. tumefaciens C58 that contains an N-terminal globin domain. The chemoreceptor was designated as Atu1027. The deletion of Atu1027 not only eliminated the aerotactic response of A. tumefaciens to atmospheric air but also resulted in a weakened chemotactic response to multiple carbon sources. Subsequent site-directed mutagenesis and phenotypic analysis showed that the conserved residue His100 in Atu1027 is essential for the globin domain's function in both chemotaxis and aerotaxis. Furthermore, deleting Atu1027 impaired the biofilm formation and pathogenicity of A. tumefaciens . Collectively, our findings demonstrated that Atu1027 functions as an aerotaxis receptor that affects agrobacterial chemotaxis and the invasion of A. tumefaciens into its host., Competing Interests: The authors declare there are no competing interests., (©2024 Huang et al.)

Pellicles are biofilms found at the air-liquid interface and are widely distributed in natural environments. In this study, a simple pellicle detection method was established, and using this new method, the pellicle formation activities of Shewanella oneidensis MR-1 and its 42 cytochrome c mutants were analysed. The results showed that the pellicle was initiated at very early stages of incubation. Aerotaxis was the major external factor, while energy acquirement was the main internal factor for pellicle initiation. Among the 42 cytochrome c mutants, 17 mutants, including those deficient in aerobic respiration, sulfur or sulfite/sulfate respiration, nitrite respiration, metal respiration, DMSO respiration and fumarate respiration, exhibited delayed pellicle initiation. The results suggest that S. oneidensis utilizes the electron acceptors simultaneously under anoxic conditions and that the disruption of any of these anaerobic respiration routes would retard pellicle initiation. Image 1 • A simple pellicle detection method was established. • Aerotaxis and the H 2 S diffusion are two external factors for pellicle initiation. • Energy acquirement is the main internal factor for pellicle initiation. • S. oneidensis utilizes electron acceptors simultaneously under anoxic conditions. • Disruption of any of respiration routes will retard pellicle formation. [ABSTRACT FROM AUTHOR]

Background: Bacterial chemotaxis, the ability of motile bacteria to navigate gradients of chemicals, plays key roles in the establishment of various plant-microbe associations, including those that benefit plant growth and crop productivity. The motile soil bacterium Azospirillum brasilense colonizes the rhizosphere and promotes the growth of diverse plants across a range of environments. Aerotaxis, or the ability to navigate oxygen gradients, is a widespread behavior in bacteria. It is one of the strongest behavioral responses in A. brasilense and it is essential for successful colonization of the root surface. Oxygen is one of the limiting nutrients in the rhizosphere where density and activity of organisms are greatest. The aerotaxis response of A. brasilense is also characterized by high precision with motile cells able to detect narrow regions in a gradient where the oxygen concentration is low enough to support their microaerobic lifestyle and metabolism. Results: Here, we present a mathematical model for aerotaxis band formation that captures most critical features of aerotaxis in A. brasilense. Remarkably, this model recapitulates experimental observations of the formation of a stable aerotactic band within 2 minutes of exposure to the air gradient that were not captured in previous modeling efforts. Using experimentally determined parameters, the mathematical model reproduced an aerotactic band at a distance from the meniscus and with a width that matched the experimental observation. Conclusions: Including experimentally determined parameter values allowed us to validate a mathematical model for aerotactic band formation in spatial gradients that recapitulates the spatiotemporal stability of the band and its position in the gradient as well as its overall width. This validated model also allowed us to capture the range of oxygen concentrations the bacteria prefer during aerotaxis, and to estimate the effect of parameter values (e.g. oxygen consumption rate), both of which are difficult to obtain in experiments. [ABSTRACT FROM AUTHOR]

Chemotactic bacteria sense environmental changes via dedicated receptors that bind to extra- or intracellular cues and relay this signal to ultimately alter direction of movement toward beneficial cues and away from harmful environments. In complex environments, such as the rhizosphere, bacteria must be able to sense and integrate diverse cues. Azospirillum brasilense is a microaerophilic motile bacterium that promotes growth of cereals and grains. Root surface colonization is a prerequisite for the beneficial effects on plant growth but how motile A. brasilense navigates the rhizosphere is poorly studied. Previously only 2 out of 51 A. brasilense chemotaxis receptors have been characterized, AerC and Tlp1, and only Tlp1 was found to be essential for wheat root colonization. Here we describe another chemotaxis receptor, named Aer, that is homologous to the Escherichia coli Aer receptor, likely possesses an FAD cofactor and is involved in aerotaxis (taxis in an air gradient). We also found that the A. brasilense Aer contributes to sensing chemical gradients originating from wheat roots. In addition to A. brasilense Aer having a putative N-terminal FAD-binding PAS domain, it possesses a C-terminal PilZ domain that contains all the conserved residues for binding c-di-GMP. Mutants lacking the PilZ domain of Aer are altered in aerotaxis and are completely null in wheat root colonization and they also fail to sense gradients originating from wheat roots. The PilZ domain of Aer is also vital in integrating Aer signaling with signaling from other chemotaxis receptors to sense gradients from wheat root surfaces and colonizing wheat root surfaces. [ABSTRACT FROM AUTHOR]

Abstract In this study, we developed a microfluidic system to elucidate the behavioral response of Euglena gracilis cells to simultaneous competing optical and chemical stimuli. The system illuminated a nonuniform blue light of 0.5–18 mW/cm2 on cells confined in a 2D microchamber to induce a photophobic response (step-up photo-shock). After cells accumulated in areas of weak blue light, a CO 2 gradient (0%–100%) was generated in the microchamber to induce counter chemotaxis (aerotaxis). E. gracilis cells showed negative chemotaxis for areas of higher CO 2 concentrations rather than strong blue light, suggesting that CO 2 chemotaxis is dominant over blue light photophobicity. We also examined phototaxis instead of photo-shock responses, using in-plane uniform blue light illumination in a counter direction to the CO 2 gradient. The cells exhibited both CO 2 chemotaxis and blue light phototaxis, and swam back and forth randomly. In both cases, some sensitive cells accumulated in niche areas, primarily with CO 2 concentration < 25% and secondarily with a weaker blue light intensity (further away from the blue light source). The results indicate that competing chemotactic and photophobic or phototactic stimuli induced superimposed responses, although E. gracilis was more strongly affected by CO 2 chemotaxis than by blue light. We propose that the two independent chemotaxis and photoresponse signal-transduction pathways are merged into one flagellar control mechanism. Moreover, a small proportion of cells were resistant to CO 2 or blue light, showing the diversity in cell metabolic status. Highlights • Development of a microfluidic system, where competing optical and chemical stimuli are simultaneously imposed to microalgae cells confined in a microchamber • Video observation of superimposed behavior of Euglena gracilis , with a non-absolute priority of CO 2 chemotaxis to photo-shock or phototaxis • Existence of diversity of cell characteristics involved in chemotaxis/phototaxis, originating from differences in metabolic status of the cells [ABSTRACT FROM AUTHOR]

Listeria monocytogenes is a food-borne pathogen that can grow as a biofilm on surfaces. Biofilm formation in food-processing environments is a big concern for food safety, as it can cause product contamination through the food-processing line. Although motile aerobic bacteria have been described to form biofilms at the air-liquid interface of cell cultures, to our knowledge, this type of biofilm has not been described in L . monocytogenes before. In this study we report L . monocytogenes biofilm formation at the air-liquid interface of aerobically grown cultures, and that this phenotype is specifically induced when the media is supplemented with glycerol as a carbon and energy source. Planktonic growth, metabolic activity assays and HPLC measurements of glycerol consumption over time showed that glycerol utilization in L . monocytogenes is restricted to growth under aerobic conditions. Gene expression analysis showed that genes encoding the glycerol transporter GlpF, the glycerol kinase GlpK and the glycerol 3-phosphate dehydrogenase GlpD were upregulated in the presence of oxygen, and downregulated in absence of oxygen. Additionally, motility assays revealed the induction of aerotaxis in the presence of glycerol. Our results demonstrate that the formation of biofilms at the air-liquid interface is dependent on glycerol-induced aerotaxis towards the surface of the culture, where L . monocytogenes has access to higher concentrations of oxygen, and is therefore able to utilize this compound as a carbon source. [ABSTRACT FROM AUTHOR]

In this paper, we consider Fokker–Planck type diffusion aerotaxis equations which resemble the usual aerotaxis equations. In view of well-posedness, its own special diffusion structure enables us to take advantage. In two dimensions, we show the existence of global classical solutions under some appropriate conditions. In addition, the stabilization property of the solution is studied, as time approaches infinity. In three dimensions, we prove the existence of global weak solutions. [ABSTRACT FROM AUTHOR]

Pseudomonas syringae pv. tabaci 6605 (Pta 6605) is a causal agent of wildfire disease in host tobacco plants. Pta 6605 is a foliar pathogen that requires chemotaxis and aerotaxis for plant infection. Pta 6605 has seven che gene clusters and 54 potential chemoreceptor genes. Pta 6605 is thought to have three chemosensory pathways, Che, Wsp, and Chp from analysis of the CheA/CheW binding motif. However, a chemoreceptor for the Che2 chemosensory pathway was not identified. Mutational analysis revealed that che genes in cluster I, chemoreceptor genes for γ-aminobutyric acid and proteinogenic amino acids, are required for plant infection. In this mini-review, we summarized the findings about chemotaxis and aerotaxis in Pta 6605 obtained so far. • Pseudomonas syringae pv. tabaci 6605 (Pta 6605) has 54 chemoreceptor genes. • Chemotaxis gene clusters were identified in Pta 6605. • Chemosensory pathways were predicted in Pta 6605. • Plant infection of Pta 6605 requires chemotaxis and aerotaxis. • CheA/CheW binding motifs of chemoreceptors were investigated in Pta 6605. [ABSTRACT FROM AUTHOR]

Spatial and temporal variations of oxygen environments affect the behaviors of various cells and are involved in physiological and pathological events. Our previous studies with Dictyostelium discoideum as a model of cell motility have demonstrated that aerotaxis toward an oxygen-rich region occurs below 2% O 2 . However, while the aerotaxis of Dictyostelium seems to be an effective strategy to search for what is essential for survival, the mechanism underlying this phenomenon is still largely unclear. One hypothesis is that an oxygen concentration gradient generates a secondary oxidative stress gradient that would direct cell migration towards higher oxygen concentration. Such mechanism was inferred but not fully demonstrated to explain the aerotaxis of human tumor cells. Here, we investigated the role on aerotaxis of flavohemoglobins, proteins that can both act as potential oxygen sensors and modulators of nitric oxide and oxidative stress. The migratory behaviors of Dictyostelium cells were observed under both self-generated and imposed oxygen gradients. Furthermore, their changes by chemicals generating or preventing oxidative stress were tested. The trajectories of the cells were then analyzed through time-lapse phase-contrast microscopic images. The results indicate that both oxidative and nitrosative stresses are not involved in the aerotaxis of Dictyostelium but cause cytotoxic effects that are enhanced upon hypoxia., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Hirose, Hesnard, Ghazi, Roussel, Voituron, Cochet-Escartin, Rieu, Anjard and Funamoto.)

As the closest unicellular relatives of animals, choanoflagellates serve as useful model organisms for understanding the evolution of animal multicellularity. An important factor in animal evolution was the increasing ocean oxygen levels in the Precambrian, which are thought to have influenced the emergence of complex multicellular life. As a first step in addressing these conditions, we study here the response of the colony-forming choanoflagellate Salpingoeca rosetta to oxygen gradients. Using a microfluidic device that allows spatio-temporal variations in oxygen concentrations, we report the discovery that S. rosetta displays positive aerotaxis. Analysis of the spatial population distributions provides evidence for logarithmic sensing of oxygen, which enhances sensing in low oxygen neighborhoods. Analysis of search strategy models on the experimental colony trajectories finds that choanoflagellate aerotaxis is consistent with stochastic navigation, the statistics of which are captured using an effective continuous version based on classical run-and-tumble chemotaxis.

Shewanella oneidensis (S. oneidensis) is a model bacterial strain for studies of bioelectrochemical systems (BESs). It has two extracellular electron transfer pathways: 1) shuttling electrons via an excreted mediator riboflavin; and 2) direct contact between the c-type cytochromes at the cell membrane and the electrode. Despite the extensive use of S. oneidensis in bioelectrochemical systems such as microbial fuel cells and biosensors, many basic microbiology questions about S. oneidensis in the context of BES remain unanswered. Here, we present studies of motility and chemotaxis of S. oneidensis under well controlled concentration gradients of two electron acceptors, oxygen and oxidized form of riboflavin (flavin+) using a newly developed microfluidic platform. Experimental results demonstrate that either oxygen or flavin+ is a chemoattractant to S. oneidensis. The chemotactic tendency of S. oneidensis in a flavin+ concentration gradient is significantly enhanced in an anaerobic in contrast to an aerobic condition. Furthermore, either a low oxygen tension or a high flavin+ concentration considerably enhances the speed of S. oneidensis. This work presents a robust microfluidic platform for generating oxygen and/or flavin+ gradients in an aqueous environment, and demonstrates that two important electron acceptors, oxygen and oxidized riboflavin, cooperatively regulate S. oneidensis migration patterns. The microfluidic tools presented as well as the knowledge gained in this work can be used to guide the future design of BESs for efficient electron production.

Model organisms are of great importance to our understanding of basic biology and to making advances in biomedical research. However, the influence of laboratory cultivation on these organisms is underappreciated, and especially how that environment can affect research outcomes. Recent experiments led to insights into how the widely used laboratory reference strain of the nematode Caenorhabditis elegans compares with natural strains. Here we describe potential selective pressures that led to the fixation of laboratory-derived alleles for the genes npr-1 , glb-5 , and nath-10 . These alleles influence a large number of traits, resulting in behaviors that affect experimental interpretations. Furthermore, strong phenotypic effects caused by these laboratory-derived alleles hinder the discovery of natural alleles. We highlight strategies to reduce the influence of laboratory-derived alleles and to harness the full power of C. elegans . [ABSTRACT FROM AUTHOR]

Abstract: Sensor proteins play crucial roles in maintaining homeostasis of cells by sensing changes in extra- and intracellular chemical and physical conditions to trigger biological responses. It has recently become clear that gas molecules function as signalling molecules in these biological regulatory systems responsible for transcription, chemotaxis, synthesis/hydrolysis of nucleotide second messengers, and other complex physiological processes. Haem-containing sensor proteins are widely used to sense gas molecules because haem can bind gas molecules reversibly. Ligand binding to the haem in the sensor proteins triggers conformational changes around the haem, which results in their functional regulation. Spectroscopic and crystallographic studies are essential to understand how these sensor proteins function in these biological regulatory systems. In this chapter, I discuss structural and functional relationships of haem-containing PAS and PAS-related families of the sensor proteins. [Copyright &y& Elsevier]

C. elegans shows robust and reproducible behavioral responses to oxygen. Specifically, worms prefer O 2 levels of 5-10% and avoid too high or too low O 2 . Their O 2 preference is not fixed but shows plasticity depending on experience, context, or genetic background. We recently showed that this experience-dependent plasticity declines with age, providing a useful behavioral readout for studying the mechanisms of age-related decline of neural plasticity. Here, we describe a technique to visualize behavioral O 2 preference and its plasticity in C. elegans , by creating spatial gradients of [O 2 ] in a microfluidic polydimethylsiloxane (PDMS) chamber and recording the resulting spatial distribution of the animals., Competing Interests: Competing interests The authors declare no competing interests., (Copyright © The Authors; exclusive licensee Bio-protocol LLC.)

Abstract: We propose a simple but reliable technology for the generation of oxygen gradient in microfluidic devices, which is useful for cell aerotaxis study. The technology is based on single layer patterning of polydimethylsiloxane (PDMS) and integration of a thin layer polyvinylidene chloride (PVDC). The PDMS layer was patterned to define a cell chamber embedded by two micro-channels, whereas the PVDC layer acts as barrier to prevent the diffusion of gas outside into the layer, thus forcing gas to diffuse along the PDMS layer. As a result, a linear oxygen gradient in the cell chamber can be established by controlling the gas contents inside the two micro-channels. By using 3-way valves, such a linear gradient could be changed from one state to another. To demonstrate the application potential of this approach, we used this device in the study of the aerotactic behaviour of magnetospirillum sp. strain AMB-1 and observed a stable and reversible cell migration by changing the gas distribution in the cell chamber. This system should be useful for better understanding of cell aerotactic mechanism. [Copyright &y& Elsevier]

Spatiotemporal variations of oxygen concentration affect the cell behaviors that are involved in physiological and pathological events. In our previous study with Dictyostelium discoideum (Dd) as a model of cell motility, aggregations of Dd cells exhibited long-lasting and highly stable migration in a self-generated hypoxic environment, forming a ring shape that spread toward the outer higher oxygen region. However, it is still unclear what kinds of changes in the migratory properties are responsible for the observed phenomena. Here, we investigated the migration of Dd to clarify the oxygen-dependent characteristics of aerokinesis and aerotaxis. Migratory behaviors of Dd cells were analyzed under various oxygen concentration gradients and uniform oxygen conditions generated in microfluidic devices. Under hypoxic conditions below 2% O2, corresponding to less than 25 µM O2 in the culture medium, the migration of Dd cells was enhanced (aerokinesis) and the oxygen gradient guided the cells toward the oxygen-rich region (aerotaxis). The aerotaxis was attributed to the increase in the frequency of migration associated with the direction of higher O2, the acceleration of migration velocity, and the enhancement of migration straightness. Thus, aerokinesis and aerotaxis are dependent on both the oxygen level and possibly relative gradient and are essential mechanisms for the migration of Dd. [ABSTRACT FROM AUTHOR]

Abstract: Here we report the results of transient absorption and photoacoustic calorimetry studies of CO photodissociation from the heme domain of the bacterial oxygen sensor HemAT-Bs. The results indicate that CO photolysis is accompanied by an overall ΔH of −19kcalmol−1 and ΔV of +4mlmol−1 as well as a red-shifted kinetic difference spectrum all occurring in <50ns. Analysis of the ΔH/ΔV reveals that a conformational change takes place with a ΔH conf of −40kcalmol−1 and ΔV conf of −22mlmol−1. These thermodynamic changes are consistent with an increase in the solvent accessible surface area of the protein upon ligand dissociation, as observed in the X-ray structure of the ferric CN-bound and CN free forms of HemAT-Bs. [Copyright &y& Elsevier]

Aerotactic band formation by Desulfovibrio desulfuricans (DSM 9104) was studied in a stopped-flow diffusion chamber. This chamber allowed us to create reproducible, steep oxygen gradients in a flat capillary, time-lapse video recordings and spatio-temporal analysis of band formation. The cells formed two types of bands. Bands of the first type evolved quickly after starting the experiment and were located near the oxic–anoxic interface. Bands of the second type typically appeared several minutes later and a few millimeters inside the initially anoxic volume of the capillary. Band formation depended on metabolism and could be stimulated by lactate addition, and thus appears to be energy taxis. Mathematical modeling of oxygen diffusion and respiration within the chamber revealed that bands formed preferentially at oxygen concentrations close to 4% air saturation. The swimming speed of the cells was determined by digital single-cell tracking and found to be highest (up to 58 μm s−1) close to the oxic–anoxic interfaces. Motility patterns were influenced by surfaces, at which cells accumulated. Bioconvection sometimes occurred if very dense bands had formed. The ecological implications of these two phenomena are unknown. [ABSTRACT FROM AUTHOR]

The regulation of aerotaxis in Pseudomonas aeruginosa is reported. P. aeruginosa possesses two aerotaxis transducers, Aer and Aer-2. The aerotactic responses of P. aeruginosa cells were induced during the transition from exponential to stationary growth phase. A deletion mutant for the anaerobic transcriptional regulator ANR showed decreased aerotaxis. The anr mutation eliminated Aer-mediated aerotaxis, but not Aer-2-mediated aerotaxis. Expression of an aer-lacZ transcriptional fusion was also induced during the transition from exponential to stationary growth phase. The anr mutant showed only background levels of aer-lacZ expression. Rapid amplification of cDNA ends (RACE) and DNA sequencing revealed that the 5′ end of the mRNA was located at an A nucleotide −67 nt upstream of aer. The aer promoter contained two putative FNR/ANR boxes at −42.5 and −93.5 bp upstream of the transcriptional start site of aer. Mutational analysis of the aer promoter region revealed that both FNR/ANR boxes were essential for the expression of the aer gene. These results indicate that ANR is required for the activation of aer expression but it is not essential for Aer-2-mediated aerotaxis in P. aeruginosa. [Copyright &y& Elsevier]

It was previously shown that the chemotaxis gene cluster 1 (cheYZABW) was required for chemotaxis. In this study, the involvement of the same cluster in aerotaxis is described and two transducer genes for aerotaxis are identified. Aerotaxis assays of a number of deletion–insertion mutants of Pseudomonas aeruginosa PAO1 revealed that the chemotaxis gene cluster 1 and cheR are required for aerotaxis. Mutant strains which contained deletions in the methyl-accepting chemotaxis protein-like genes tlpC and tlpG showed decreased aerotaxis. A double mutant deficient in tlpC and tlpG was negative for aerotaxis. TlpC has 45% amino acid identity with the Escherichia coli aerotactic transducer Aer. The TlpG protein has a predicted C-terminal segment with 89% identity to the highly conserved domain of the E. coli serine chemoreceptor Tsr. A hydropathy plot of TlpG indicated that hydrophobic membrane-spanning regions are missing in TlpG. A PAS motif was found in the N-terminal domains of TlpC and TlpG. On this basis, the tlpC and tlpG genes were renamed aer and aer-2, respectively. No significant homology other than the PAS motif was detected in the N-terminal domains between Aer and Aer-2. [Copyright &y& Elsevier]

Magnetotactic bacteria orient and migrate along magnetic field lines. Each cell is essentially a self-propelled magnetic dipole. The magnetic properties of these bacteria have been determined by a variety of techniques, including pulsed hysteresis measurements on single cells. [ABSTRACT FROM AUTHOR]

Bacterial aerotaxis (the movement of a cell toward oxygen) was evaluated for its potential use in detecting the toxicity of chemicals to microorganisms. The level of toxicity was determined by the concentration of test chemicals resulting in a 50% inhibition of aerotaxis of Pseudomonas aeruginosa PAO1 after 40 min of exposure. The aerotactic responses of P. aeruginosa were measured by using chemotaxis well chambers. Each clear acrylic chamber had a lower and upper well separated by a polycarbonate filter with a uniform pore size of 8.0 μm. To automatically detect bacterial cells that crossed the filter in response to a gradient of oxygen, P. aeruginosa PAO1 was marked with green fluorescent protein (GFP), and the GFP fluorescence intensity in the upper well was continuously monitored by using a fluorescence spectrometer. By using this technique, volatile chlorinated aliphatic compounds, including trichloroethylene (TCE), trichloroethane, and tetrachloroethylene, were found to be inhibitory to bacterial aerotaxis, suggesting their possible toxicity to microorganisms. We also examined more than 20 potential toxicants for their ability to inhibit the aerotaxis of P. aeruginosa. Based on these experimental results, we concluded that bacterial aerotaxis has potential for use as a fast and reliable indicator in assessing the toxicity of chemicals to microorganisms. [Copyright &y& Elsevier]

Bacterial aerotaxis is a rapid response towards or away from oxygen. Here we report on the use of computer-assisted motion analysis coupled to flash photolysis of caged oxygen to quantify aerotactic responses in bacteria. The caged compound (μ-peroxo)(μ-hydroxo)bis[bis(bipyridyl) cobalt(III)] perchlorate liberates molecular oxygen upon irradiation with near-UV light. A mixture of cells and the caged oxygen compound was placed in a capillary tube and challenged by discrete stimuli of molecular oxygen produced by photolysis. We then recorded the rate of change of direction (rcd) as an estimate of tumble frequency in response to liberated oxygen and measured the signal processing (excitation) times in Bacillus subtilis, Bacillus halodurans and Escherichia coli. This computer-assisted caged oxygen assay gives a unique physiological profile of different aerotaxis transducers in bacteria. [Copyright &y& Elsevier]

Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.