Chemotaxis in a gliding bacterium

VirB11, a traffic ATPase, mediated flagella assembly and type IV pilus morphogenesis to control the motility and virulence of Xanthomonas albilineans

Xanthomonas albilineans (Xal) is a gram-negative bacterial pathogen responsible for developing sugarcane leaf scald disease, which engenders significant economic losses within the sugarcane industry. In the current study, homologous recombination exchange was carried out to induce mutations within the virB/D4-like type IV secretion system (T4SS) genes of Xal. The results revealed that the virB11-deletion mutant (ΔvirB11) exhibited a loss in swimming and twitching motility. Application of transmission electron microscopy analysis further demonstrated that the ΔvirB11 failed to develop flagella formation and type IV pilus morphology and exhibited reduced swarming behaviour and virulence. However, these alterations had no discernible impact on bacterial growth. Comparative transcriptome analysis between the wild-type Xal JG43 and the deletion-mutant ΔvirB11 revealed 123 differentially expressed genes (DEGs), of which 28 and 10 DEGs were notably associated with flagellar assembly and chemotaxis, respectively. In light of these findings, we postulate that virB11 plays an indispensable role in regulating the processes related to motility and chemotaxis in Xal.

Cell behaviors underlying Myxococcus xanthus aggregate dispersal

IMPORTANCE

Understanding the processes behind bacterial biofilm formation, maintenance, and dispersal is essential for addressing their effects on health and ecology. Within these multicellular communities, various cues can trigger differentiation into distinct cell types, allowing cells to adapt to their specific local environment. The soil bacterium Myxococcus xanthus forms biofilms in response to starvation, marked by cells aggregating into mounds. Some aggregates persist as spore-filled fruiting bodies, while others disperse after initial formation for unknown reasons. Here, we use a combination of cell tracking analysis and computational simulations to identify behaviors at the cellular level that contribute to aggregate dispersal. Our results suggest that cells in aggregates actively determine whether to disperse or persist and undergo a transition to sporulation based on a self-produced cue related to the aggregate size. Identifying these cues is an important step in understanding and potentially manipulating bacterial cell-fate decisions.

Plasmalogens and Photooxidative Stress Signaling in Myxobacteria, and How it Unmasked CarF/TMEM189 as the Δ1'-Desaturase PEDS1 for Human Plasmalogen Biosynthesis

Plasmalogens are glycerophospholipids with a hallmark sn-1 vinyl ether bond that endows them with unique physical-chemical properties. They have proposed biological roles in membrane organization, fluidity, signaling, and antioxidative functions, and abnormal plasmalogen levels correlate with various human pathologies, including cancer and Alzheimer’s disease. The presence of plasmalogens in animals and in anaerobic bacteria, but not in plants and fungi, is well-documented. However, their occurrence in the obligately aerobic myxobacteria, exceptional among aerobic bacteria, is often overlooked. Tellingly, discovery of the key desaturase indispensable for vinyl ether bond formation, and therefore fundamental in plasmalogen biogenesis, emerged from delving into how the soil myxobacterium Myxococcus xanthus responds to light. A recent pioneering study unmasked myxobacterial CarF and its human ortholog TMEM189 as the long-sought plasmanylethanolamine desaturase (PEDS1), thus opening a crucial door to study plasmalogen biogenesis, functions, and roles in disease. The findings demonstrated the broad evolutionary sweep of the enzyme and also firmly established a specific signaling role for plasmalogens in a photooxidative stress response. Here, we will recount our take on this fascinating story and its implications, and review the current state of knowledge on plasmalogens, their biosynthesis and functions in the aerobic myxobacteria.

Identification of Positive Chemotaxis in the Protozoan Pathogen Trypanosoma brucei

Almost all living things need to be able to move, whether it is toward desirable environments or away from danger. For vector-borne parasites, successful transmission and infection require that these organisms be able to sense where they are and use signals from their environment to direct where they go next, a process known as chemotaxis. Here, we show that Trypanosoma brucei, the deadly protozoan parasite that causes African sleeping sickness, can sense and move toward an attractive cue. To our knowledge, this is the first report of positive chemotaxis in these organisms. In addition to describing a new behavior in T. brucei, our findings enable future studies of how chemotaxis works in these pathogens, which will lead to deeper understanding of how they move through their hosts and may lead to new therapeutic or transmission-blocking strategies.

To complete its infectious cycle, the protozoan parasite Trypanosoma brucei must navigate through diverse tissue environments in both its tsetse fly and mammalian hosts. This is hypothesized to be driven by yet unidentified chemotactic cues. Prior work has shown that parasites engaging in social motility in vitro alter their trajectory to avoid other groups of parasites, an example of negative chemotaxis. However, movement of T. brucei toward a stimulus, positive chemotaxis, has so far not been reported. Here, we show that upon encountering Escherichia coli, socially behaving T. brucei parasites exhibit positive chemotaxis, redirecting group movement toward the neighboring bacterial colony. This response occurs at a distance from the bacteria and involves active changes in parasite motility. By developing a quantitative chemotaxis assay, we show that the attractant is a soluble, diffusible signal dependent on actively growing E. coli. Time-lapse and live video microscopy revealed that T. brucei chemotaxis involves changes in both group and single cell motility. Groups of parasites change direction of group movement and accelerate as they approach the source of attractant, and this correlates with increasingly constrained movement of individual cells within the group. Identification of positive chemotaxis in T. brucei opens new opportunities to study mechanisms of chemotaxis in these medically and economically important pathogens. This will lead to deeper insights into how these parasites interact with and navigate through their host environments.

IMPORTANCE Almost all living things need to be able to move, whether it is toward desirable environments or away from danger. For vector-borne parasites, successful transmission and infection require that these organisms be able to sense where they are and use signals from their environment to direct where they go next, a process known as chemotaxis. Here, we show that Trypanosoma brucei, the deadly protozoan parasite that causes African sleeping sickness, can sense and move toward an attractive cue. To our knowledge, this is the first report of positive chemotaxis in these organisms. In addition to describing a new behavior in T. brucei, our findings enable future studies of how chemotaxis works in these pathogens, which will lead to deeper understanding of how they move through their hosts and may lead to new therapeutic or transmission-blocking strategies.

Data-Driven Models Reveal Mutant Cell Behaviors Important for Myxobacterial Aggregation

Self-organization into spatial patterns is evident in many multicellular phenomena. Even for the best-studied systems, our ability to dissect the mechanisms driving coordinated cell movement is limited. While genetic approaches can identify mutations perturbing multicellular patterns, the diverse nature of the signaling cues coupled to significant heterogeneity of individual cell behavior impedes our ability to mechanistically connect genes with phenotype. Small differences in the behaviors of mutant strains could be irrelevant or could sometimes lead to large differences in the emergent patterns. Here, we investigate rescue of multicellular aggregation in two mutant strains of Myxococcus xanthus mixed with wild-type cells. The results demonstrate how careful quantification of cell behavior coupled to data-driven modeling can identify specific motility features responsible for cell aggregation and thereby reveal important synergies and compensatory mechanisms. Notably, mutant cells do not need to precisely recreate wild-type behaviors to achieve complete aggregation.

Single mutations frequently alter several aspects of cell behavior but rarely reveal whether a particular statistically significant change is biologically significant. To determine which behavioral changes are most important for multicellular self-organization, we devised a new methodology using Myxococcus xanthus as a model system. During development, myxobacteria coordinate their movement to aggregate into spore-filled fruiting bodies. We investigate how aggregation is restored in two mutants, csgA and pilC, that cannot aggregate unless mixed with wild-type (WT) cells. To this end, we use cell tracking to follow the movement of fluorescently labeled cells in combination with data-driven agent-based modeling. The results indicate that just like WT cells, both mutants bias their movement toward aggregates and reduce motility inside aggregates. However, several aspects of mutant behavior remain uncorrected by WT, demonstrating that perfect recreation of WT behavior is unnecessary. In fact, synergies between errant behaviors can make aggregation robust.

IMPORTANCE Self-organization into spatial patterns is evident in many multicellular phenomena. Even for the best-studied systems, our ability to dissect the mechanisms driving coordinated cell movement is limited. While genetic approaches can identify mutations perturbing multicellular patterns, the diverse nature of the signaling cues coupled to significant heterogeneity of individual cell behavior impedes our ability to mechanistically connect genes with phenotype. Small differences in the behaviors of mutant strains could be irrelevant or could sometimes lead to large differences in the emergent patterns. Here, we investigate rescue of multicellular aggregation in two mutant strains of Myxococcus xanthus mixed with wild-type cells. The results demonstrate how careful quantification of cell behavior coupled to data-driven modeling can identify specific motility features responsible for cell aggregation and thereby reveal important synergies and compensatory mechanisms. Notably, mutant cells do not need to precisely recreate wild-type behaviors to achieve complete aggregation.

Bacterial surface motility is modulated by colony-scale flow and granular jamming

Microbes routinely face the challenge of acquiring territory and resources on wet surfaces. Cells move in large groups inside thin, surface-bound water layers, often achieving speeds of 30 µm s^-1 within this environment, where viscous forces dominate over inertial forces (low Reynolds number). The canonical Gram-positive bacterium Bacillus subtilis is a model organism for the study of collective migration over surfaces with groups exhibiting motility on length-scales three orders of magnitude larger than themselves within a few doubling times. Genetic and chemical studies clearly show that the secretion of endogenous surfactants and availability of free surface water are required for this fast group motility. Here, we show that: (i) water availability is a sensitive control parameter modulating an abiotic jamming-like transition that determines whether the group remains fluidized and therefore collectively motile, (ii) groups self-organize into discrete layers as they travel, (iii) group motility does not require proliferation, rather groups are pulled from the front, and (iv) flow within expanding groups is capable of moving material from the parent colony into the expanding tip of a cellular dendrite with implications for expansion into regions of varying nutrient content. Together, these findings illuminate the physical structure of surface-motile groups and demonstrate that physical properties, like cellular packing fraction and flow, regulate motion from the scale of individual cells up to length scales of centimetres.

A Nodulation-Proficient Nonrhizobial Inhabitant of Pueraria phaseoloides

Pueraria phaseoloides is a legume cover crop, found chiefly in the wet zone of Sri Lanka. Nitrogen fixation is performed by nodular inhabitants of this cover crop, comparable to the nodule-dwelling bacteria of most other legume plants. We isolated a bacterium (Sub1) from Pueraria phaseoloides, of coccobacillus cell shape, that showed nodulation, when assessed by hydroponics, showing nodules as early as 3 weeks after reinfection. When a nifH fragment from the genome of this bacterium was amplified using a pair of nifH primers, it yielded an amplicon of 360 bp that, when sequenced, helped us identify the bacterium, as belonging to a species of Pseudacidovorax intermedius, at 99% sequence identity. When we constructed a phylogenetic tree with neighboring sequences, we encountered nifH sequences of Pseudacidovorax, forming a monophyletic cluster, which too contained a single Azospirillum species. The genus Pseudacidovorax is a bacterium that, so far, has not been associated with legume nodules. Sub1 secreted a pair of enzymes to the extracellular medium to degrade cellulose and milk proteins. The Sub1 bacterium showed biofilm formation and secreted into the extracellular medium, indole acetic acid. Sub1 also showed a "bulls eye" swarming pattern for the chemoattractant proline, while showing no significant chemotaxis movement, for naringenin, quercetin, and glutamate. Sub1 too possesses the basic genetic foundation (nifH and nifD) to produce a molybdenum-dependent nitrogenase enzyme. We finally show that this rare nonrhizobial bacterium is able to impact, positively, nodulation and shoot length of Pueraria plants, demonstrating that this beta-proteobacterium can abet the biological vigor of this legume cover crop.

Agent-Based Modeling Reveals Possible Mechanisms for Observed Aggregation Cell Behaviors

Myxococcus xanthus is a soil bacterium that serves as a model system for biological self-organization. Cells form distinct, dynamic patterns depending on environmental conditions. An agent-based model was used to understand how M. xanthus cells aggregate into multicellular mounds in response to starvation. In this model, each cell is modeled as an agent represented by a point particle and characterized by its position and moving direction. At low agent density, the model recapitulates the dynamic patterns observed by experiments and a previous biophysical model. To study aggregation at high cell density, we extended the model based on the recent experimental observation that cells exhibit biased movement toward aggregates. We tested two possible mechanisms for this biased movement and demonstrate that a chemotaxis model with adaptation can reproduce the observed experimental results leading to the formation of stable aggregates. Furthermore, our model reproduces the experimentally observed patterns of cell alignment around aggregates.

Assessing Travel Conditions: Environmental and Host Influences On Bacterial Surface Motility

The degree to which surface motile bacteria explore their surroundings is influenced by aspects of their local environment. Accordingly, regulation of surface motility is controlled by numerous chemical, physical, and biological stimuli. Discernment of such regulation due to these multiple cues is a formidable challenge. Additionally inherent ambiguity and variability from the assays used to assess surface motility can be an obstacle to clear delineation of regulated surface motility behavior. Numerous studies have reported single environmental determinants of microbial motility and lifestyle behavior but the translation of these data to understand surface motility and bacterial colonization of human host or environmental surfaces is unclear. Here, we describe the current state of the field and our understanding of exogenous factors that influence bacterial surface motility.

Data-driven modeling reveals cell behaviors controlling self-organization during Myxococcus xanthus development

Collective cell movement is critical to the emergent properties of many multicellular systems, including microbial self-organization in biofilms, embryogenesis, wound healing, and cancer metastasis. However, even the best-studied systems lack a complete picture of how diverse physical and chemical cues act upon individual cells to ensure coordinated multicellular behavior. Known for its social developmental cycle, the bacterium Myxococcus xanthus uses coordinated movement to generate three-dimensional aggregates called fruiting bodies. Despite extensive progress in identifying genes controlling fruiting body development, cell behaviors and cell-cell communication mechanisms that mediate aggregation are largely unknown. We developed an approach to examine emergent behaviors that couples fluorescent cell tracking with data-driven models. A unique feature of this approach is the ability to identify cell behaviors affecting the observed aggregation dynamics without full knowledge of the underlying biological mechanisms. The fluorescent cell tracking revealed large deviations in the behavior of individual cells. Our modeling method indicated that decreased cell motility inside the aggregates, a biased walk toward aggregate centroids, and alignment among neighboring cells in a radial direction to the nearest aggregate are behaviors that enhance aggregation dynamics. Our modeling method also revealed that aggregation is generally robust to perturbations in these behaviors and identified possible compensatory mechanisms. The resulting approach of directly combining behavior quantification with data-driven simulations can be applied to more complex systems of collective cell movement without prior knowledge of the cellular machinery and behavioral cues.

Regulations governing the multicellular lifestyle of Myxococcus xanthus

In living organisms, cooperative cell movements underlie the formation of differentiated tissues. In bacteria, Myxococcus xanthus uses cooperative group movements, to predate on prey and to form multicellular fruiting bodies, where the cells differentiate into dormant spores. Motility is controlled by a central signaling Che-like pathway, Frz. Single cell studies indicate Frz regulates the frequency at which cells reverse their direction of movement by transmitting signals to a molecular system that controls the spatial activity of the motility engines. This regulation is central to all Myxococcus multicellular behaviors but how Frz signaling generates ordered patterns is poorly understood. In this review, we first discuss the genetic structure of the Frz pathway and possible regulations that could explain its action during Myxococcus development.

Myxobacteria: Moving, Killing, Feeding, and Surviving Together

Myxococcus xanthus, like other myxobacteria, is a social bacterium that moves and feeds cooperatively in predatory groups. On surfaces, rod-shaped vegetative cells move in search of the prey in a coordinated manner, forming dynamic multicellular groups referred to as swarms. Within the swarms, cells interact with one another and use two separate locomotion systems. Adventurous motility, which drives the movement of individual cells, is associated with the secretion of slime that forms trails at the leading edge of the swarms. It has been proposed that cellular traffic along these trails contributes to M. xanthus social behavior via stigmergic regulation. However, most of the cells travel in groups by using social motility, which is cell contact-dependent and requires a large number of individuals. Exopolysaccharides and the retraction of type IV pili at alternate poles of the cells are the engines associated with social motility. When the swarms encounter prey, the population of M. xanthus lyses and takes up nutrients from nearby cells. This cooperative and highly density-dependent feeding behavior has the advantage that the pool of hydrolytic enzymes and other secondary metabolites secreted by the entire group is shared by the community to optimize the use of the degradation products. This multicellular behavior is especially observed in the absence of nutrients. In this condition, M. xanthus swarms have the ability to organize the gliding movements of 1000s of rods, synchronizing rippling waves of oscillating cells, to form macroscopic fruiting bodies, with three subpopulations of cells showing division of labor. A small fraction of cells either develop into resistant myxospores or remain as peripheral rods, while the majority of cells die, probably to provide nutrients to allow aggregation and spore differentiation. Sporulation within multicellular fruiting bodies has the benefit of enabling survival in hostile environments, and increases germination and growth rates when cells encounter favorable conditions. Herein, we review how these social bacteria cooperate and review the main cell–cell signaling systems used for communication to maintain multicellularity.

Myxobacterial vesicles death at a distance?

Outer membrane vesicles (OMVs) are produced from the outer membrane (OM) of myxobacterial cells and are found in large quantities within myxobacterial biofilms. It has been proposed that OMVs are involved in several of the social behaviors exhibited by the myxobacteria, including motility and predation. Proteomic data suggest that specific proteins are either selectively incorporated into or excluded from myxobacterial OMVs, as observed for OMVs of other organisms. Hydrolases are found in large numbers in OMVs, which then transport them to target bacteria. Fusion of OMVs with the OM of Gram-negative cells, or lysis of OMVs next to Gram-positive bacteria, is thought to deliver hydrolases to target cells, causing their lysis. The model myxobacterium Myxococcus xanthus is a predator of other bacteria, and OMVs are likely employed as predatory agents by this organism. The transfer of motility proteins between cells of M. xanthus has been documented, and OMV-mediated transfer provides a convenient mechanism to explain this phenomenon. This review describes the general principles of OMV biology, provides an overview of myxobacterial behavior, summarizes what is currently known about myxobacterial OMVs, and discusses the potential involvement of OMVs in many features of the myxobacterial life-cycle.

Gliding motility driven by individual cell-surface movements in a multicellular filamentous bacterium Chloroflexus aggregans

Chloroflexus aggregans is an unbranched multicellular filamentous bacterium having the ability of gliding motility. The filament moves straightforward at a constant rate, ∼3 μm sec(-1) on solid surface and occasionally reverses the moving direction. In this study, we successfully detected movements of glass beads on the cell-surface along long axis of the filament indicating that the cell-surface movement was the direct force for gliding. Microscopic analyses found that the cell-surface movements were confined to a cell of the filament, and each cell independently moved and reversed the direction. To understand how the cellular movements determine the moving direction of the filament, we proposed a discrete-time stochastic model; sum of the directions of the cellular movements determines the moving direction of the filament only when the filament pauses, and after moving, the filament keeps the same directional movement until all the cells pause and/or move in the opposite direction. Monte Carlo simulation of this model showed that reversal frequency of longer filaments was relatively fixed to be low, but the frequency of shorter filaments varied widely. This simulation result appropriately explained the experimental observations. This study proposed the relevant mechanism adequately describing the motility of the multicellular filament in C. aggregans.

Directional reversals enable Myxococcus xanthus cells to produce collective one-dimensional streams during fruiting-body formation

The formation of a collectively moving group benefits individuals within a population in a variety of ways. The surface-dwelling bacterium Myxococcus xanthus forms dynamic collective groups both to feed on prey and to aggregate during times of starvation. The latter behaviour, termed fruiting-body formation, involves a complex, coordinated series of density changes that ultimately lead to three-dimensional aggregates comprising hundreds of thousands of cells and spores. How a loose, two-dimensional sheet of motile cells produces a fixed aggregate has remained a mystery as current models of aggregation are either inconsistent with experimental data or ultimately predict unstable structures that do not remain fixed in space. Here, we use high-resolution microscopy and computer vision software to spatio-temporally track the motion of thousands of individuals during the initial stages of fruiting-body formation. We find that cells undergo a phase transition from exploratory flocking, in which unstable cell groups move rapidly and coherently over long distances, to a reversal-mediated localization into one-dimensional growing streams that are inherently stable in space. These observations identify a new phase of active collective behaviour and answer a long-standing open question in Myxococcus development by describing how motile cell groups can remain statistically fixed in a spatial location.

A two-component regulatory system modulates twitching motility in Dichelobacter nodosus

Dichelobacter nodosus is the essential causative agent of footrot in sheep and type IV fimbriae-mediated twitching motility has been shown to be essential for virulence. We have identified a two-component signal transduction system (TwmSR) that shows similarity to chemosensory systems from other bacteria. Insertional inactivation of the gene encoding the response regulator, TwmR, led to a twitching motility defect, with the mutant having a reduced rate of twitching motility when compared to the wild-type and a mutant complemented with the wild-type twmR gene. The reduced rate of twitching motility was not a consequence of a reduced growth rate or decreased production of surface located fimbriae, but video microscopy indicated that it appeared to result from an overall loss of twitching directionality. These results suggest that a chemotactic response to environmental factors may play an important role in the D. nodosus-mediated disease process.

Fatty acids from membrane lipids become incorporated into lipid bodies during Myxococcus xanthus differentiation

Myxococcus xanthus responds to amino acid limitation by producing fruiting bodies containing dormant spores. During development, cells produce triacylglycerides in lipid bodies that become consumed during spore maturation. As the cells are starved to induce development, the production of triglycerides represents a counterintuitive metabolic switch. In this paper, lipid bodies were quantified in wild-type strain DK1622 and 33 developmental mutants at the cellular level by measuring the cross sectional area of the cell stained with the lipophilic dye Nile red. We provide five lines of evidence that triacylglycerides are derived from membrane phospholipids as cells shorten in length and then differentiate into myxospores. First, in wild type cells, lipid bodies appear early in development and their size increases concurrent with an 87% decline in membrane surface area. Second, developmental mutants blocked at different stages of shortening and differentiation accumulated lipid bodies proportionate with their cell length with a Pearson's correlation coefficient of 0.76. Third, peripheral rods, developing cells that do not produce lipid bodies, fail to shorten. Fourth, genes for fatty acid synthesis are down-regulated while genes for fatty acid degradation are up regulated. Finally, direct movement of fatty acids from membrane lipids in growing cells to lipid bodies in developing cells was observed by pulse labeling cells with palmitate. Recycling of lipids released by Programmed Cell Death appears not to be necessary for lipid body production as a fadL mutant was defective in fatty acid uptake but proficient in lipid body production. The lipid body regulon involves many developmental genes that are not specifically involved in fatty acid synthesis or degradation. MazF RNA interferase and its target, enhancer-binding protein Nla6, appear to negatively regulate cell shortening and TAG accumulation whereas most cell-cell signals activate these processes.

Unlocking the secrets of multi-flagellated propulsion: drawing insights from Tritrichomonas foetus

In this work, a high-speed imaging platform and a resistive force theory (RFT) based model were applied to investigate multi-flagellated propulsion, using Tritrichomonas foetus as an example. We discovered that T. foetus has distinct flagellar beating motions for linear swimming and turning, similar to the 'run and tumble' strategies observed in bacteria and Chlamydomonas. Quantitative analysis of the motion of each flagellum was achieved by determining the average flagella beat motion for both linear swimming and turning, and using the velocity of the flagella as inputs into the RFT model. The experimental approach was used to calculate the curvature along the length of the flagella throughout each stroke. It was found that the curvatures of the anterior flagella do not decrease monotonically along their lengths, confirming the ciliary waveform of these flagella. Further, the stiffness of the flagella was experimentally measured using nanoindentation, allowing for calculation of the flexural rigidity of T. foetus's flagella, 1.55×10(-21) N m(2). Finally, using the RFT model, it was discovered that the propulsive force of T. foetus was similar to that of sperm and Chlamydomonas, indicating that multi-flagellated propulsion does not necessarily contribute to greater thrust generation, and may have evolved for greater manoeuvrability or sensing. The results from this study have demonstrated the highly coordinated nature of multi-flagellated propulsion and have provided significant insights into the biology of T. foetus.

Diverse populations of lake water bacteria exhibit chemotaxis towards inorganic nutrients

Chemotaxis allows microorganisms to rapidly respond to different environmental stimuli; however, understanding of this process is limited by conventional assays, which typically focus on the response of single axenic cultures to given compounds. In this study, we used a modified capillary assay coupled with flow cytometry and 16S rRNA gene amplicon pyrosequencing to enumerate and identify populations within a lake water microbial community that exhibited chemotaxis towards ammonium, nitrate and phosphate. All compounds elicited chemotactic responses from populations within the lake water, with members of Sphingobacteriales exhibiting the strongest responses to nitrate and phosphate, and representatives of the Variovorax, Actinobacteria ACK-M1 and Methylophilaceae exhibiting the strongest responses to ammonium. Our results suggest that chemotaxis towards inorganic substrates may influence the rates of biogeochemical processes.

The Myxococcus xanthus spore cuticula protein C is a fragment of FibA, an extracellular metalloprotease produced exclusively in aggregated cells

Myxococcus xanthus is a soil bacterium with a complex life cycle involving distinct cell fates, including production of environmentally resistant spores to withstand periods of nutrient limitation. Spores are surrounded by an apparently self-assembling cuticula containing at least Proteins S and C; the gene encoding Protein C is unknown. During analyses of cell heterogeneity in M. xanthus, we observed that Protein C accumulated exclusively in cells found in aggregates. Using mass spectrometry analysis of Protein C either isolated from spore cuticula or immunoprecipitated from aggregated cells, we demonstrate that Protein C is actually a proteolytic fragment of the previously identified but functionally elusive zinc metalloprotease, FibA. Subpopulation specific FibA accumulation is not due to transcriptional regulation suggesting post-transcriptional regulation mechanisms mediate its heterogeneous accumulation patterns.

From individual cell motility to collective behaviors: insights from a prokaryote, Myxococcus xanthus

In bird flocks, fish schools, and many other living organisms, regrouping among individuals of the same kin is frequently an advantageous strategy to survive, forage, and face predators. However, these behaviors are costly because the community must develop regulatory mechanisms to coordinate and adapt its response to rapid environmental changes. In principle, these regulatory mechanisms, involving communication between individuals, may also apply to cellular systems which must respond collectively during multicellular development. Dissecting the mechanisms at work requires amenable experimental systems, for example, developing bacteria. Myxococcus xanthus, a Gram-negative delatproteobacterium, is able to coordinate its motility in space and time to swarm, predate, and grow millimeter-size spore-filled fruiting bodies. A thorough understanding of the regulatory mechanisms first requires studying how individual cells move across solid surfaces and control their direction of movement, which was recently boosted by new cell biology techniques. In this review, we describe current molecular knowledge of the motility mechanism and its regulation as a lead-in to discuss how multicellular cooperation may have emerged from several layers of regulation: chemotaxis, cell-cell signaling, and the extracellular matrix. We suggest that Myxococcus is a powerful system to investigate collective principles that may also be relevant to other cellular systems.

Coordinated, long-range, solid substrate movement of the purple photosynthetic bacterium Rhodobacter capsulatus

The long-range movement of Rhodobacter capsulatus cells in the glass-agar interstitial region of borosilicate Petri plates was found to be due to a subset of the cells inoculated into plates. The macroscopic appearance of plates indicated that a small group of cells moved in a coordinated manner to form a visible satellite cluster of cells. Satellite clusters were initially separated from the point of inoculation by the absence of visible cell density, but after 20 to 24 hours this space was colonized by cells apparently shed from a group of cells moving away from the point of inoculation. Cell movements consisted of flagellum-independent and flagellum-dependent motility contributions. Flagellum-independent movement occurred at an early stage, such that satellite clusters formed after 12 to 24 hours. Subsequently, after 24 to 32 hours, a flagellum-dependent dispersal of cells became visible, extending laterally outward from a line of flagellum-independent motility. These modes of taxis were found in several environmental isolates and in a variety of mutants, including a strain deficient in the production of the R. capsulatus acyl-homoserine lactone quorum-sensing signal. Although there was great variability in the direction of movement in illuminated plates, cells were predisposed to move toward broad spectrum white light. This predisposition was increased by the use of square plates, and a statistical analysis indicated that R. capsulatus is capable of genuine phototaxis. Therefore, the variability in the direction of cell movement was attributed to optical effects on light waves passing through the plate material and agar medium.

Statistical image analysis reveals features affecting fates of Myxococcus xanthus developmental aggregates

Starving Myxococcus xanthus bacteria use their motility systems to self-organize into multicellular fruiting bodies, large mounds in which cells differentiate into metabolically inert spores. Despite the identification of the genetic pathways required for aggregation and the use of microcinematography to observe aggregation dynamics in WT and mutant strains, a mechanistic understanding of aggregation is still incomplete. For example, it is not clear why some of the initial aggregates mature into fruiting bodies, whereas others disperse, merge, or split into two. Here, we develop high-throughput image quantification and statistical analysis methods to gain insight into M. xanthus developmental aggregation dynamics. A quantitative metric of features characterizing each aggregate is used to deduce the properties of the aggregates that are correlated with each fate. The analysis shows that small aggregate size but not neighbor-related parameters correlate with aggregate dispersal. Furthermore, close proximity is necessary but not sufficient for aggregate merging. Finally, splitting occurs for those aggregates that are unusually large and elongated. These observations place severe constraints on the underlying aggregation mechanisms and present strong evidence against the role of long-range morphogenic gradients or biased cell exchange in the dispersal, merging, or splitting of transient aggregates. This approach can be expanded and adapted to study self-organization in other cellular systems.

Gliding motility revisited: how do the myxobacteria move without flagella?

In bacteria, motility is important for a wide variety of biological functions such as virulence, fruiting body formation, and biofilm formation. While most bacteria move by using specialized appendages, usually external or periplasmic flagella, some bacteria use other mechanisms for their movements that are less well characterized. These mechanisms do not always exhibit obvious motility structures. Myxococcus xanthus is a motile bacterium that does not produce flagella but glides slowly over solid surfaces. How M. xanthus moves has remained a puzzle that has challenged microbiologists for over 50 years. Fortunately, recent advances in the analysis of motility mutants, bioinformatics, and protein localization have revealed likely mechanisms for the two M. xanthus motility systems. These results are summarized in this review.

Phosphorylation and dephosphorylation among Dif chemosensory proteins essential for exopolysaccharide regulation in Myxococcus xanthus

Myxococcus xanthus social gliding motility, which is powered by type IV pili, requires the presence of exopolysaccharides (EPS) on the cell surface. The Dif chemosensory system is essential for the regulation of EPS production. It was demonstrated previously that DifA (methyl-accepting chemotaxis protein [MCP]-like), DifC (CheW-like), and DifE (CheA-like) stimulate whereas DifD (CheY-like) and DifG (CheC-like) inhibit EPS production. DifD was found not to function downstream of DifE in EPS regulation, as a difD difE double mutant phenocopied the difE single mutant. It has been proposed that DifA, DifC, and DifE form a ternary signaling complex that positively regulates EPS production through the kinase activity of DifE. DifD was proposed as a phosphate sink of phosphorylated DifE (DifE approximately P), while DifG would augment the function of DifD as a phosphatase of phosphorylated DifD (DifD approximately P). Here we report in vitro phosphorylation studies with all the Dif chemosensory proteins that were expressed and purified from Escherichia coli. DifE was demonstrated to be an autokinase. Consistent with the formation of a DifA-DifC-DifE complex, DifA and DifC together, but not individually, were found to influence DifE autophosphorylation. DifD, which did not inhibit DifE autophosphorylation directly, was found to accept phosphate from autophosphorylated DifE. While DifD approximately P has an unusually long half-life for dephosphorylation in vitro, DifG efficiently dephosphorylated DifD approximately P as a phosphatase. These results support a model where DifE complexes with DifA and DifC to regulate EPS production through phosphorylation of a downstream target, while DifD and DifG function synergistically to divert phosphates away from DifE approximately P.

Dynamics of a microorganism moving by chemotaxis in its own secretion

The Brownian dynamics of a single microorganism coupled by chemotaxis to a diffusing concentration field that is secreted by the microorganism itself is studied by computer simulations in spatial dimensions d=1,2,3 . Both cases of a chemoattractant and a chemorepellent are discussed. For a chemoattractant, we find a transient dynamical arrest until the microorganism diffuses for long times. For a chemorepellent, there is a transient ballistic motion in all dimensions and a long-time diffusion. These results are interpreted with the help of a theoretical analysis.

Deciphering the hunting strategy of a bacterial wolfpack

Myxococcus xanthus is a common soil bacterium with an intricate multicellular lifestyle that continues to challenge the way in which we conceptualize the capabilities of prokaryotic organisms. Myxococcus xanthus is the preferred laboratory representative from the Myxobacteria, a family of organisms distinguished by their ability to form highly structured biofilms that include tentacle-like packs of surface-gliding cell groups, synchronized rippling waves of oscillating cells and massive spore-filled aggregates that protrude upwards from the substratum to form fruiting bodies. But most of the Myxobacteria are also predators that thrive on the degradation of macromolecules released through the lysis of other microbial cells. The aim of this review is to examine our understanding of the predatory life cycle of M. xanthus. We will examine the multicellular structures formed during contact with prey, and the molecular mechanisms utilized by M. xanthus to detect and destroy prey cells. We will also examine our understanding of microbial predator-prey relationships and the prospects for how bacterial predation mechanisms can be exploited to generate new antimicrobial technologies.

Chemotaxis as an emergent property of a swarm

We have characterized and quantified a form of bacterial chemotaxis that manifests only as an emergent property by measuring symmetry breaking in a swarm of Myxococcus xanthus exposed to a two-dimensional nutrient gradient from within an agar substrate. M. xanthus chemotaxis requires cell-cell contact and coordinated motility, as individual motile cells exhibit only nonvectorial movement in the presence of a nutrient gradient. Genes that specifically affect M. xanthus chemotaxis include at least 10 of the 53 that express enhancer binding proteins of the NtrC-like class, an indication that this behavior is controlled through transcription, most likely by a complex signal transduction network.

Independence and interdependence of Dif and Frz chemosensory pathways in Myxococcus xanthus chemotaxis

Dif and Frz, two Myxococcus xanthus chemosensory pathways, are required in phosphatidylethanolamine (PE) chemotaxis for excitation and adaptation respectively. DifA and FrzCD, the homologues of methyl-accepting chemoreceptors in the two pathways, were examined for methylation in the context of chemotaxis and inter-pathway interactions. Evidence indicates that DifA may not undergo methylation, but signals transmitting through DifA do modulate FrzCD methylation. Results also revealed that M. xanthus possesses Dif-dependent and Dif-independent PE-sensing mechanisms. Previous studies showed that FrzCD methylation is decreased by negative chemostimuli but increased by attractants such as PE. Results here demonstrate that the Dif-dependent sensory mechanism suppresses the increase in FrzCD methylation in attractant response and elevates FrzCD methylation upon negative stimulation. In other words, FrzCD methylation is governed by opposing forces from Dif-dependent and Dif-independent sensing mechanisms. We propose that the Dif-independent but Frz-dependent PE sensing leads to increases in FrzCD methylation and subsequent adaptation, while the Dif-dependent PE signalling suppresses or diminishes the increase in FrzCD methylation to decelerate or delay adaptation. We contend that these antagonistic interactions are crucial for effective chemotaxis in this gliding bacterium to ensure that adaptation does not occur too quickly relative to the slow speed of M. xanthus movement.

The receiver domain of FrzE, a CheA-CheY fusion protein, regulates the CheA histidine kinase activity and downstream signalling to the A- and S-motility systems of Myxococcus xanthus

The Frz chemosensory system is a two-component signal transduction pathway that controls cell reversals and directional movements for the two motility systems in Myxococcus xanthus. To trigger cell reversals, FrzE, a hybrid CheA-CheY fusion protein, autophosphorylates the kinase domain at His-49, and phosphoryl groups are transferred to aspartate residues (Asp-52 and Asp-220) in the two receiver domains of FrzZ, a dual CheY-like protein that serves as the pathway output. The role of the receiver domain of FrzE was unknown. In this paper, we characterize the FrzE protein in vitro and show that the receiver domain of FrzE negatively regulates the autophosphorylation activity of the kinase domain of FrzE. Unexpectedly, it does not appear to play a direct role in phospho-relay as in most other histidine kinase receiver domain hybrid systems. The regulatory role of the FrzE receiver domain suggests that it may interact with or be phosphorylated by an unknown protein. We also show the dynamics of motility system-specific marker proteins in FrzE mutants as cells move forward and reverse. Our studies indicate that the two motility systems are functionally co-ordinated and that any system-specific branching of the pathway most likely occurs downstream of FrzE.

Pseudomonas aeruginosa twitching motility-mediated chemotaxis towards phospholipids and fatty acids: specificity and metabolic requirements

Pseudomonas aeruginosa demonstrates type IV pilus-mediated directional twitching motility up a gradient of phosphatidylethanolamine (PE). Only one of four extracellular phospholipases C of P. aeruginosa (i.e., PlcB), while not required for twitching motility per se, is required for twitching-mediated migration up a gradient of PE or phosphatidylcholine. Whether other lipid metabolism genes are associated with this behavior was assessed by analysis of transcription during twitching up a PE gradient in comparison to transcription during twitching in the absence of any externally applied phospholipid. Data support the hypothesis that PE is further degraded and that the long-chain fatty acid (LCFA) moieties of PE are completely metabolized via beta-oxidation and the glyoxylate shunt. It was discovered that P. aeruginosa exhibits twitching-mediated chemotaxis toward unsaturated LCFAs (e.g., oleic acid), but not saturated LCFAs (e.g., stearic acid) of corresponding lengths. Analysis of mutants that are deficient in glyoxylate shunt enzymes, specifically isocitrate lyase (DeltaaceA) and malate synthase (DeltaaceB), suggested that the complete metabolism of LCFAs through this pathway was required for the migration of P. aeruginosa up a gradient of PE or unsaturated LCFAs. At this point, our data suggested that this process should be classified as energy taxis. However, further evaluation of the ability of the DeltaaceA and DeltaaceB mutants to migrate up a gradient of PE or unsaturated LCFAs in the presence of an alternative energy source clearly indicated that metabolism of LCFAs for energy is not required for chemotaxis toward these compounds.

Self-produced extracellular stimuli modulate the Pseudomonas aeruginosa swarming motility behaviour

Pseudomonas aeruginosa presents three types of motilities: swimming, twitching and swarming. The latter is characterized by rapid and coordinated group movement over a semisolid surface resulting from morphological differentiation and intercellular interactions. A striking feature of P. aeruginosa swarming motility is the formation of migrating tendrils producing colonies with complex fractal-like patterns. Previous studies have shown that normal swarming motility is intimately related to the production of extracellular surface-active molecules: rhamnolipids (RLs), composed of monorhamnolipids (mono-RLs) and dirhamnolipids (di-RLs), and 3-(3-hydroxyalkanoyloxy) alkanoic acids (HAAs). Here, we report that (i) di-RLs attract active swarming cells while HAAs behave as strong repellents, (ii) di-RLs promote and HAAs inhibit tendril formation and migration, (iii) di-RLs and HAAs display different diffusion kinetics on a surface as di-RLs spread faster than HAAs in agar, (iv) di-RLs and HAAs have no effect on swimming cells, suggesting that swarming cells are different from swimming cells not only in morphology but also at the regulatory level and (v) mono-RLs act as wetting agents. We propose a model explaining how HAAs and di-RLs together modulate the behaviour of swarming migrating cells by acting as self-produced negative and positive chemotactic-like stimuli.

Chemotaxis mediated by NarX-FrzCD chimeras and nonadapting repellent responses in Myxococcus xanthus

Myxococcus xanthus requires gliding motility for swarming and fruiting body formation. It uses the Frz chemosensory pathway to regulate cell reversals. FrzCD is a cytoplasmic chemoreceptor required for sensing effectors for this pathway. NarX is a transmembrane sensor for nitrate from Escherichia coli. In this study, two NarX-FrzCD chimeras were constructed to investigate M. xanthus chemotaxis: NazD(F) contains the N-terminal sensory module of NarX fused to the C-terminal signalling domain of FrzCD; NazD(R) is similar except that it contains a G51R mutation in the NarX domain known to reverse the signalling output of a NarX-Tar chimera to nitrate. We report that while nitrate had no effect on the wild type, it decreased the reversal frequency of M. xanthus expressing NazD(F) and increased that of M. xanthus expressing NazD(R). These results show that directional motility in M. xanthus can be regulated independently of cellular metabolism and physiology. Surprisingly, the NazD(R) strain failed to adapt to nitrate in temporal assays as did the wild type to known repellents. The lack of temporal adaptation to negative stimuli appears to be a general feature in M. xanthus chemotaxis. Thus, the appearance of biased movements by M. xanthus in repellent gradients is likely due to the inhibition of net translocation by repellents.

FrzZ, a dual CheY-like response regulator, functions as an output for the Frz chemosensory pathway of Myxococcus xanthus

Myxococcus xanthus utilizes two distinct motility systems for movement (gliding) on solid surfaces: adventurous motility (A-motility) and social motility (S-motility). Both systems are regulated by the Frz signal transduction pathway, which controls cell reversals required for directed motility and fruiting body formation. The Frz chemosensory system, unlike the Escherichia coli chemotaxis system, contains proteins with multiple response regulator domains: FrzE, a CheA-CheY hybrid protein, and FrzZ, a CheY-CheY hybrid protein. Previously, the CheY domain of FrzE was hypothesized to act as the response regulator output of the Frz system. In this study, using a genetic suppressor screen, we identified FrzZ and showed FrzZ is epistatic to FrzE, demonstrating that FrzZ is the principal output component of the pathway. We constructed M. xanthus point mutations in the phosphoaccepting aspartate residues of FrzZ and demonstrated the respective roles of these residues in group and single cell motility. We also performed in vitro assays and showed rapid phosphotransfer between the CheA domain of FrzE and each of the CheY domains of FrzZ. These experiments showed that FrzZ plays a direct role as an output of the Frz chemosensory pathway and that both CheY domains of FrzZ are functional.

Spatial organization of Myxococcus xanthus during fruiting body formation

Microcinematography was used to examine fruiting body development of Myxococcus xanthus. Wild-type cells progress through three distinct phases: a quiescent phase with some motility but little aggregation (0 to 8 h), a period of vigorous motility leading to raised fruiting bodies (8 to 16 h), and a period of maturation during which sporulation is initiated (16 to 48 h). Fruiting bodies are extended vertically in a series of tiers, each involving the addition of a cell monolayer on top of the uppermost layer. A pilA (MXAN_5783) mutant produced less extracellular matrix material and thus allowed closer examination of tiered aggregate formation. A csgA (MXAN_1294) mutant exhibited no quiescent phase, aberrant aggregation in phase 2, and disintegration of the fruiting bodies in the third phase.

Extracellular fatty acids facilitate flagella-independent translocation by Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is widespread in natural environments such as soil, sewage and plant rhizospheres. Surfactants frequently function in modulating bacterial surface translocation. In this study, rpfB and rpfF orthologues were identified from S. maltophilia strain WR-C, which was isolated from the clogged zone of a septic system. These genes play a role in the biosynthesis of eight extracellular compounds that facilitated flagella-independent translocation by the wild-type or a flagella-defective mutant. This type of surface translocation has not been reported previously for this organism. These eight compounds include cis-delta 2-11-methyl-dodecenoic acid and seven structural derivatives. Two are saturated fatty acids; the others are unsaturated fatty acids with double bonds at position 2. These fatty acids vary in chain length from 12 to 14 carbons and in the position of the branched methyl group. Our results demonstrated that independently cis-delta 2-11-methyl-dodecenoic acid and 11-methyl-dodecanoic acid promoted flagella-independent translocation by S. maltophilia strain WR-C by acting as wetting agents.

The autotransporter esterase EstA of Pseudomonas aeruginosa is required for rhamnolipid production, cell motility, and biofilm formation

Pseudomonas aeruginosa PAO1 produces the biodetergent rhamnolipid and secretes it into the extracellular environment. The role of rhamnolipids in the life cycle and pathogenicity of P. aeruginosa has not been completely understood, but they are known to affect outer membrane composition, cell motility, and biofilm formation. This report is focused on the influence of the outer membrane-bound esterase EstA of P. aeruginosa PAO1 on rhamnolipid production. EstA is an autotransporter protein which exposes its catalytically active esterase domain on the cell surface. Here we report that the overexpression of EstA in the wild-type background of P. aeruginosa PAO1 results in an increased production of rhamnolipids whereas an estA deletion mutant produced only marginal amounts of rhamnolipids. Also the known rhamnolipid-dependent cellular motility and biofilm formation were affected. Although only a dependence of swarming motility on rhamnolipids has been known so far, the other kinds of motility displayed by P. aeruginosa PAO1, swimming and twitching, were also affected by an estA mutation. In order to demonstrate that EstA enzyme activity is responsible for these effects, inactive variant EstA* was constructed by replacement of the active serine by alanine. None of the mutant phenotypes could be complemented by expression of EstA*, demonstrating that the phenotypes affected by the estA mutation depend on the enzymatically active protein.

FibA and PilA act cooperatively during fruiting body formation of Myxococcus xanthus

The extracellular matrix (ECM) of Myxococcus xanthus is essential for social (S-) motility and fruiting body formation. An ECM-bound protein, FibA, is homologous to M4 zinc metalloproteases and is important for stimulation by a phosphatidylethanolamine (PE) chemoattractant and for formation of discrete aggregation foci. In this work, we demonstrate that a correlation exists between a reduced ability to respond to PE and the observed defects in fruiting body morphogenesis. Furthermore, the fibA aggregation defect is accentuated by the absence of either PilA, the structural subunit of type IV pili, or DifD, a chemosensory response regulator. The inability to form fruiting bodies is not due to a loss of S-motility, but rather the loss of PilA and pili as pilT fibA mutants form fruiting bodies. The FibA active site residue E342 is important for fruiting body morphogenesis in the absence of PilA. Mutants exhibiting defects in fruiting body morphogenesis also produce fewer viable spores. It is proposed that FibA and PilA act as extracellular sensors for developmental signals.

Phospholipid directed motility of surface-motile bacteria

Myxococcus xanthus is a surface-motile bacterium that has adapted at least one chemosensory system to allow directed movement towards the slowly diffusible lipid phosphatidylethanolamine (PE). The Dif chemosensory pathway is remarkable because it has at least three inputs coupled to outputs that control extracellular matrix (ECM) production and lipid chemotaxis. The methyl-accepting chemotaxis protein, DifA, has two different sensor inputs that have been localized by mutagenesis. The Dif chemosensory pathway employs a novel protein that slows adaptation. Lipid chemotaxis may play important roles in the M. xanthus life cycle where prey-specific and development-specific attractants have been identified. Lipid chemotaxis may also be an important mechanism for locating nutrients by lung pathogens such as Pseudomonas aeruginosa.

Novel lipids in Myxococcus xanthus and their role in chemotaxis

Organisms that colonize solid surfaces, like Myxococcus xanthus, use novel signalling systems to organize multicellular behaviour. Phosphatidylethanolamine (PE) containing the fatty acid 16:1omega5 (Delta11) elicits a chemotactic response. The phenomenon was examined by observing the effects of PE species with varying fatty acid pairings. Wild-type M. xanthus contains 17 different PE species under vegetative conditions and 19 at the midpoint of development; 13 of the 17 have an unsaturated fatty acid at the sn-1 position, a novelty among Proteobacteria. Myxococcus xanthus has two glycerol-3-phosphate acyltransferase (PlsB) homologues which add the sn-1 fatty acid. Each produces PE with 16:1 at the sn-1 position and supports growth and fruiting body development. Deletion of plsB1 (MXAN3288) results in more dramatic changes in PE species distribution than deletion of plsB2 (MXAN1675). PlsB2 has a putative N-terminal eukaryotic fatty acid reductase domain and may support both ether lipid synthesis and PE synthesis. Disruption of a single sn-2 acyltransferase homologue (PlsC, of which M. xanthus contains five) results in minor changes in membrane PE. Derivatization of purified PE extracts with dimethyldisulfide was used to determine the position of the double bonds in unsaturated fatty acids. The results suggest that Delta5 and Delta11 desaturases may create the double bonds after synthesis of the fatty acid. Phosphatidylethanolamine enriched for 16:1 at the sn-1 position stimulates chemotaxis more strongly than PE with 16:1 enriched at the sn-2 position. It appears that the deployment of a rare fatty acid (16:1omega5) at an unusual position (sn-1) has facilitated the evolution of a novel cell signal.

Mutations affecting predation ability of the soil bacterium Myxococcus xanthus

Myxococcus xanthus genetic mutants with characterized phenotypes were analysed for the ability to prey on susceptible bacteria. Quantification of predatory ability was scored by a newly developed method under conditions in which prey bacteria provided the only source of nutrients. These results were corroborated by data derived using a previously published protocol that measures predation in the presence of limited external nutrients. First, early developmental regulatory mutants were examined, because their likely functions in assessing the local nutrient status were predicted to be also important for predation. The results showed that predation efficiency is reduced by 64-80 % for mutants of three A-signalling components, AsgA, AsgC and AsgE, but not for AsgB. This suggests that an Asg regulon function that is separate from A-signal production is needed for predation. Besides the Asg components, mutations in the early developmental genes sdeK and csgA were also consistently observed to reduce predatory efficacy by 36 and 33 %, respectively. In contrast, later developmental components, such as DevRS, 4406 and PhoP4, did not appear to play significant roles in predation. The predatory abilities of mutants defective for motility were also tested. The data showed that adventurous, but not social, motility is required for predation in the assay. Also, mutants for components in the chemotaxis-like Frz system were found to be reduced in predation efficiency by between 62 and 85 %. In sum, it was demonstrated here that defects in development and development-related processes affect the ability of M. xanthus to prey on other bacteria.

The Dif chemosensory pathway is directly involved in phosphatidylethanolamine sensory transduction inMyxococcus xanthus

Myxococcus xanthus cells glide on solid surfaces and are chemotactically stimulated by certain phosphatidylethanolamine species. The dif gene cluster consists of six genes, difABCDEG, five of which encode proteins homologous to known chemotaxis proteins. DifA and DifE are required for the biosynthesis of fibrils, an extracellular matrix comprised of polysaccharide and protein. Chemotactic stimulation by 1,2-O-Bis[11-(Z)-hexadecenoyl]-sn-glycero-3-phosphatidylethanolamine (16:1 PE) and dilauroyl PE (12:0 PE) requires fibrils. Although previous work has shown that difA and difE mutants are not stimulated by 12:0 PE, these results do not distinguish between a dependence on fibrils or a direct role in chemosensory transduction. Here we provide evidence that the Dif chemosensory pathway directly mediates PE sensory transduction. First, stimulation by and adaptation to 16:1 PE requires all of the dif genes, including difBDG, which are not essential for fibril biogenesis. Second, a specific residue within the first putative methylation domain of DifA is required for stimulation by 16:1 PE but not fibril biogenesis. Transmembrane signalling through a chimeric NarX-DifA chemoreceptor is required for fibril formation but not for stimulation by or adaptation to 16:1 PE. Third, difD and difE are required for stimulation by dioleoyl PE (18:1 PE) although the response does not require fibrils. Taken together these results argue that the Dif pathway mediates both matrix formation and lipid chemotaxis.

PilT is required for PI(3,4,5)P3-mediated crosstalk between Neisseria gonorrhoeae and epithelial cells

The retractile type IV pilus participates in a number of fundamental bacterial processes, including motility, DNA transformation, fruiting body formation and attachment to host cells. Retraction of the N. gonorrhoeae type IV pilus requires a functional pilT. Retraction generates substantial force on its substrate (> 100 pN per retraction event), and it has been speculated that epithelial cells sense and respond to these forces during infection. We provide evidence that piliated, Opa non-expressing Neisseria gonorrhoeae activates the stress-responsive PI-3 kinase/Akt (PKB) pathway in human epithelial cells, and activation is enhanced by a functional pilT. PI-3 kinase inhibitors wortmannin and LY294002 reduce cell entry by 81% and 50%, respectively, illustrating the importance of this cascade in bacterial invasion. PI-3 kinase and its direct downstream effectors [PI(3,4,5)P3] and Akt are concentrated in the cell cortex beneath adherent bacteria, particularly at the periphery of the bacterial microcolonies. Furthermore, [PI(3,4,5)P3] is translocated to the outer leaflet of the plasma membrane. Finally, we show that [PI(3,4,5)P3] stimulates microcolony formation and upregulates pilT expression in vitro. We conclude that N. gonorrhoeae activation of PI-3 kinase triggers the host cell to produce a lipid second messenger that influences bacterial behaviour.

Resource level affects relative performance of the two motility systems of Myxococcus xanthus

The adventurous (A) and social (S) motility systems of the microbial predator Myxococcus xanthus show differential swarming performance on distinct surface types. Under standard laboratory conditions, A-motility performs well on hard agar but poorly on soft agar, whereas the inverse pattern is shown by S-motility. These properties may allow M. xanthus to swarm effectively across a greater diversity of natural surfaces than would be possible with one motility system alone. Nonetheless, the range of ecological conditions under which dual motility enhances effective swarming across distinct surfaces and how ecological parameters affect the complementarity of A-motility and S-motility remain unclear. Here we have examined the role of nutrient concentration in determining swarming patterns driven by dual motility on distinct agar surfaces, as well as the relative contributions of A-motility and S-motility to these patterns. Swarm expansion rates of dually motile (A+S+), solely A-motile (A+S-), and solely S-motile (A-S+) strains were compared on hard and soft agar across a wide range of casitone concentrations. At low casitone concentrations (0-0.1%), swarming on soft agar driven by S-motility is very poor, and is significantly slower than swarming on hard agar driven by A-motility. This reverses at high casitone concentration (1-3.2%) such that swarming on soft agar is much faster than swarming on hard agar. This pattern greatly constrained the ability of M. xanthus to encounter patches of prey bacteria on a soft agar surface when nutrient levels between the patches were low. The swarming patterns of a strain that is unable to produce extracellular fibrils indicate that these appendages are responsible for the elevated swarming of S-motility at high resource levels. Together, these data suggest that large contributions by S-motility to predatory swarming in natural soils may be limited to soft, wet, high-nutrient conditions that may be uncommon. Several likely benefits of S-motility to the M. xanthus life cycle are discussed, including synergistic interactions with A-motility across a wide variety of conditions.

Divergent regulatory pathways control A and S motility in Myxococcus xanthus through FrzE, a CheA-CheY fusion protein

Myxococcus xanthus moves on solid surfaces by using two gliding motility systems, A motility for individual-cell movement and S motility for coordinated group movements. The frz genes encode chemotaxis homologues that control the cellular reversal frequency of both motility systems. One of the components of the core Frz signal transduction pathway, FrzE, is homologous to both CheA and CheY from the enteric bacteria and is therefore a novel CheA-CheY fusion protein. In this study, we investigated the role of this fusion protein, in particular, the CheY domain (FrzECheY). FrzECheY retains all of the highly conserved residues of the CheY superfamily of response regulators, including Asp709, analogous to phosphoaccepting Asp57 of Escherichia coli CheY. While in-frame deletion of the entire frzE gene caused both motility systems to show a hyporeversal phenotype, in-frame deletion of the FrzECheY domain resulted in divergent phenotypes for the two motility systems: hyperreversals of the A-motility system and hyporeversals of the S-motility system. To further investigate the role of FrzECheY in A and S motility, point mutations were constructed such that the putative phosphoaccepting residue, Asp709, was changed from D to A (and was therefore never subject to phosphorylation) or E (possibly mimicking constitutive phosphorylation). The D709A mutant showed hyperreversals for both motilities, while the D709E mutant showed hyperreversals for A motility and hyporeversal for S motility. These results show that the FrzECheY domain plays a critical signaling role in coordinating A and S motility. On the basis of the phenotypic analyses of the frzE mutants generated in this study, a model is proposed for the divergent signal transduction through FrzE in controlling and coordinating A and S motility in M. xanthus.

A novel extracellular phospholipase C of Pseudomonas aeruginosa is required for phospholipid chemotaxis

Pseudomonas aeruginosa and other bacterial pathogens express one or more homologous extracellular phospholipases C (PLC) that are secreted through the inner membrane via the twin arginine translocase (TAT) pathway. Analysis of TAT mutants of P. aeruginosa uncovered a previously unidentified extracellular PLC that is secreted via the Sec pathway (PlcB). Whereas all presently known PLCs of P. aeruginosa (PlcH, PlcN and PlcB) hydrolyse phosphatidylcholine (PC), only PlcB is active on phosphatidylethanolamine (PE). plcB candidates were identified based on deductions made from bioinformatics data and extant DNA microarray data. Among these candidates, a gene (PA0026) required for the expression of an extracellular PE-PLC was identified. The protein encoded by PA0026 has limited, but significant similarity, over a short region (approximately 60aa of 328), to a class of zinc-dependent prokaryotic PLCs. A conserved His residue of PlcB (His216) that is required for coordinate binding of zinc in this class of PLCs was mutated. Analysis of this mutant established that the protein encoded by PA0026 is PlcB. Three in-dependent recently published reports indicate that homoserine lactone-mediated quorum sensing regulates the expression of PA0026 (i.e. plcB). PlcB, but not PlcH or PlcN, is required for directed twitching motility up a gradient of certain kinds of phospholipids. This response shows specificity for the fatty acid moiety of the phospholipid.

Interplay of chemotaxis and chemokinesis mechanisms in bacterial dynamics

Motivated by observations of the dynamics of Myxococcus xanthus, we present a self-interacting random walk model that describes the competition between chemokinesis and chemotaxis. Cells are constrained to move in one dimension, but release a chemical chemoattractant at a steady state. The bacteria sense the chemical that they produce. The probability of direction reversals is modeled as a function of both the absolute level of chemoattractant sensed directly under each cell as well as the gradient sensed across the length of the cell. If the chemical does not degrade or diffuse rapidly, the one-dimensional trajectory depends on the entire past history of the trajectory. We derive the corresponding Fokker-Planck equations, use an iterative mean-field approach that we solve numerically for short times, and perform extensive Monte Carlo simulations of the model. Cell positional distributions and the associated moments are computed in this feedback system. Average drift and mean squared displacements are found. Crossover behaviors among different diffusion regimes are found.

Biofilm formation by Pseudomonas aeruginosa wild type, flagella and type IV pili mutants

Biofilm formation by Gfp-tagged Pseudomonas aeruginosa PAO1 wild type, flagella and type IV pili mutants in flow chambers irrigated with citrate minimal medium was characterized by the use of confocal laser scanning microscopy and comstat image analysis. Flagella and type IV pili were not necessary for P. aeruginosa initial attachment or biofilm formation, but the cell appendages had roles in biofilm development, as wild type, flagella and type IV pili mutants formed biofilms with different structures. Dynamics and selection during biofilm formation were investigated by tagging the wild type and flagella/type IV mutants with Yfp and Cfp and performing time-lapse confocal laser scanning microscopy in mixed colour biofilms. The initial microcolony formation occurred by clonal growth, after which wild-type P. aeruginosa bacteria spread over the substratum by means of twitching motility. The wild-type biofilms were dynamic compositions with extensive motility, competition and selection occurring during development. Bacterial migration prevented the formation of larger microcolonial structures in the wild-type biofilms. The results are discussed in relation to the current model for P. aeruginosa biofilm development.