Myxococcus xanthus is a model organism for studying bacterial social behaviors due to its ability to form complex multi-cellular structures. Knowledge of M. xanthus surface gliding motility and the mechanisms that coordinated it are critically important to our understanding of collective cell behaviors. Although the mechanism of gliding motility is still under investigation, recent experiments suggest that there are two possible mechanisms underlying force production for cell motility: the focal adhesion mechanism and the helical rotor mechanism, which differ in the biophysics of the cell–substrate interactions. Whereas the focal adhesion model predicts an elastic coupling, the helical rotor model predicts a viscous coupling. Using a combination of computational modeling, imaging, and force microscopy, we find evidence for elastic coupling in support of the focal adhesion model. Using a biophysical model of the M. xanthus cell, we investigated how the mechanical interactions between cells are affected by interactions with the substrate. Comparison of modeling results with experimental data for cell-cell collision events pointed to a strong, elastic attachment between the cell and substrate. These results are robust to variations in the mechanical and geometrical parameters of the model. We then directly measured the motor-substrate coupling by monitoring the motion of optically trapped beads and find that motor velocity decreases exponentially with opposing load. At high loads, motor velocity approaches zero velocity asymptotically and motors remain bound to beads indicating a strong, elastic attachment.
Studies of collective bacterial motility on solid surfaces are essential for understanding self-organization of biofilms. The Gram-negative bacterium Myxococcus xanthus has long been used as a model organism for studying surface motility but its mechanism of gliding is still under investigation. Recent experiments point to two potential mechanisms for gliding motility that differ qualitatively in the details of their cell-substrate interactions. To investigate the biophysical nature of this interaction (viscous vs. elastic coupling), we developed a synergistically multidisciplinary approach combining computational modeling, time-lapse microscopy, and biophysical optical trap experiments. First we studied the mechanical cell interaction behavior in isolated cell pair collisions in a computational model and compared the results with experimental cell behavior. The results indicated a strong adhesive attachment between cell and substrate which is further confirmed by applying opposing loads on beads attached to cell surface in an optical trap. Thus our results conclusively showed strong adhesive attachments between cell and substrate, providing support for an elastic rather than viscous coupling between cell and substrate similar to phenomena observed in focal adhesions from eukaryotic cells.
The formation of spore-filled fruiting bodies by myxobacteria is a fascinating case of multicellular self-organization by bacteria. The organization of Myxococcus xanthus into fruiting bodies has long been studied not only as an important example of collective motion of bacteria, but also as a simplified model for developmental morphogenesis. Sporulation within the nascent fruiting body requires signaling between moving cells in order that the rod-shaped self-propelled cells differentiate into spores at the appropriate time. Probing the three-dimensional structure of myxobacteria fruiting bodies has previously presented a challenge due to limitations of different imaging methods. A new technique using Infrared Optical Coherence Tomography (OCT) revealed previously unknown details of the internal structure of M. xanthus fruiting bodies consisting of interconnected pockets of relative high and low spore density regions. To make sense of the experimentally observed structure, modeling and computer simulations were used to test a hypothesized mechanism that could produce high-density pockets of spores. The mechanism consists of self-propelled cells aligning with each other and signaling by end-to-end contact to coordinate the process of differentiation resulting in a pattern of clusters observed in the experiment. The integration of novel OCT experimental techniques with computational simulations can provide new insight into the mechanisms that can give rise to the pattern formation seen in other biological systems such as dictyostelids, social amoeba known to form multicellular aggregates observed as slugs under starvation conditions.
Understanding bacteria self-organization is an active area of research with broad implications in both microbiology and developmental biology. Myxococcus xanthus undergoes multicellular aggregation and differentiation under starvation and is widely used as a model organism for studying bacteria self-organization. In this paper, we present the findings of an innovative non-invasive experimental technique that reveals a heterogeneous structure of the fruiting body not seen in earlier studies. Insight into the biological mechanism for these observed patterns is gained by integrating experiments with biologically relevant computational simulations. The simulations show that a novel mechanism requiring cell alignment, signaling and steric interactions can explain the pockets of spore clusters observed experimentally in the fruiting bodies of M. xanthus.
Gliding motility is observed in a large variety of phylogenetically unrelated bacteria. Gliding provides a means for microbes to travel in environments with a low water content, such as might be found in biofilms, microbial mats, and soil. Gliding is defined as the movement of a cell on a surface in the direction of the long axis of the cell. Because this definition is operational and not mechanistic, the underlying molecular motor(s) may be quite different in diverse microbes. In fact, studies on the gliding bacterium Myxococcus xanthus suggest that two independent gliding machineries, encoded by two multigene systems, operate in this microorganism. One machinery, which allows individual cells to glide on a surface, independent of whether the cells are moving alone or in groups, requires the function of the genes of the A-motility system. More than 37 A-motility genes are known to be required for this form of movement. Depending on an additional phenotype, these genes are divided into two subclasses, the agl and cgl genes. Videomicroscopic studies on gliding movement, as well as ultrastructural observations of two myxobacteria, suggest that the A-system motor may consist of multiple single motor elements that are arrayed along the entire cell body. Each motor element is proposed to be localized to the periplasmic space and to be anchored to the peptidoglycan layer. The force to glide which may be generated here is coupled to adhesion sites that move freely in the outer membrane. These adhesion sites provide a specific contact with the substratum. Based on single-cell observations, similar models have been proposed to operate in the unrelated gliding bacteria Flavobacterium johnsoniae (formerly Cytophaga johnsonae), Cytophaga strain U67, and Flexibacter polymorphus (a filamentous glider). Although this model has not been verified experimentally, M. xanthus seems to be the ideal organism with which to test it, given the genetic tools available. The second gliding motor in M. xanthus controls cell movement in groups (S-motility system). It is dependent on functional type IV pili and is operative only when cells are in close proximity to each other. Type IV pili are known to be involved in another mode of bacterial surface translocation, called twitching motility. S-motility may well represent a variation of twitching motility in M. xanthus. However, twitching differs from gliding since it involves cell movements that are jerky and abrupt and that lack the organization and smoothness observed in gliding. Components of this motor are encoded by genes of the S-system, which appear to be homologs of genes involved in the biosynthesis, assembly, and function of type IV pili in Pseudomonas aeruginosa and Neisseria gonorrhoeae. How type IV pili generate force in S-motility is currently unknown, but it is to be expected that ongoing physiological, genetic, and biochemical studies in M. xanthus, in conjunction with studies on twitching in P. aeruginosa and N. gonorrhoeae, will provide important insights into this microbial motor. The two motility systems of M. xanthus are affected to different degrees by the MglA protein, which shows similarity to a small GTPase. Bacterial chemotaxis-like sensory transduction systems control gliding motility in M. xanthus. The frz genes appear to regulate gliding movement of individual cells and movement by the S-motility system, suggesting that the two motors found in this bacterium can be regulated to result in coordinated multicellular movements. In contrast, the dif genes affect only S-system-dependent swarming.
Bacteria glide across solid surfaces by mechanisms that have remained largely mysterious despite decades of research. In the deltaproteobacterium Myxococcus xanthus, this locomotion allows the formation stress-resistant fruiting bodies where sporulation takes place. However, despite the large number of genes identified as important for gliding, no specific machinery has been identified so far, hampering in-depth investigations. Based on the premise that components of the gliding machinery must have co-evolved and encode both envelope-spanning proteins and a molecular motor, we re-annotated known gliding motility genes and examined their taxonomic distribution, genomic localization, and phylogeny. We successfully delineated three functionally related genetic clusters, which we proved experimentally carry genes encoding the basal gliding machinery in M. xanthus, using genetic and localization techniques. For the first time, this study identifies structural gliding motility genes in the Myxobacteria and opens new perspectives to study the motility mechanism. Furthermore, phylogenomics provide insight into how this machinery emerged from an ancestral conserved core of genes of unknown function that evolved to gliding by the recruitment of functional modules in Myxococcales. Surprisingly, this motility machinery appears to be highly related to a sporulation system, underscoring unsuspected common mechanisms in these apparently distinct morphogenic phenomena.
Motility over solid surfaces (gliding) is an important bacterial mechanism that allows complex social behaviours and pathogenesis. Conflicting models have been suggested to explain this locomotion in the deltaproteobacterium Myxococcus xanthus: propulsion by polymer secretion at the rear of the cells as opposed to energized nano-machines distributed along the cell body. However, in absence of characterized molecular machinery, the exact mechanism of gliding could not be resolved despite several decades of research. In this study, using a combination of experimental and computational approaches, we showed for the first time that the motility machinery is composed of large macromolecular assemblies periodically distributed along the cell envelope. Furthermore, the data suggest that the motility machinery derived from an ancient gene cluster also found in several non-gliding bacterial lineages. Intriguingly, we find that most of the components of the gliding machinery are closely related to a sporulation system, suggesting unsuspected links between these two apparently distinct biological processes. Our findings now pave the way for the first molecular studies of a long mysterious motility mechanism.
The dsp mutant of Myxococcus xanthus lacks extracellular fibrils and as a result is unable to undergo cohesion, group motility, or development (J. W. Arnold and L. J. Shimkets, J. Bacteriol. 170:5765-5770, 1983; J. W. Arnold and L. J. Shimkets, J. Bacteriol. 170:5771-5777, 1983; R. M. Behmlander and M. Dworkin, J. Bacteriol. 173:7810-7821, 1991; L. J. Shimkets, J. Bacteriol. 166:837-841, 1986; L. J. Shimkets, J. Bacteriol. 166:842-848, 1986). However, cohesion and development can be phenotypically restored by the addition of isolated fibrils (R. M. Behmlander, Ph.D. thesis, University of Minnesota, Minneapolis, 1994; B.-Y. Chang and M. Dworkin, J. Bacteriol. 176:7190-7196, 1994). As part of our attempts to examine the interaction of fibrils and cells of M. xanthus, we have isolated a series of secondary mutants of M. xanthus dsp in which cohesion, unlike that of the parent strain, could not be rescued by the addition of isolated fibrils. Cells of M. xanthus dsp were mutagenized either by ethyl methanesulfonate or by Tn5 insertions. Mutagenized cultures were enriched by selection of those cells that could not be rescued, i.e., that failed to cohere in the presence of isolated fibrils. Seven mutants of M. xanthus dsp, designated fbd mutants, were isolated from 6,983 colonies; these represent putative fibril receptor-minus mutants. The fbd mutants, like the parent dsp mutant, still lacked fibrils, but displayed a number of unexpected properties. They regained group motility and the ability to aggregate but not the ability to form mature fruiting bodies. In addition, they partially regained the ability to form myxospores. The fbd mutant was backcrossed into the dsp mutant by Mx4 transduction. Three independently isolated transconjugants showed essentially the same properties as the fbd mutants--loss of fibril rescue of cohesion, partial restoration of myxospore morphogenesis, and restoration of group motility. These results suggest that the physical presence of fibrils is not necessary for group motility, myxospore formation, or the early aggregative stage of development. We propose, however, that the perception of fibril binding is required for normal social behavior and development. The dsp fbd mutants (from here on referred to as fbd mutants) open the possibility of isolating and characterizing a putative fibril receptor gene.
As prokaryotic models for multicellular development, Stigmatellaaurantiaca and Myxococcus xanthus share many similarities in terms of social behaviors, such as gliding motility. Our current understanding of myxobacterial grouped-cell motilities comes mainly from the research on M. xanthus, which shows that filamentous type IV pili (TFP), composed of type IV pilin (also called PilA protein) subunits, are the key apparatus for social motility (S-motility). However, little is known about the pilin protein in S. aurantiaca. We cloned and sequenced four genes (pilASa1~4) from S. aurantiaca DSM17044 that are homologous to pilAMx (pilA gene in M. xanthus DK1622). The homology and similarities among PilASa proteins and other myxobacterial homologues were systematically analyzed. To determine their potential biological functions, the four pilASa genes were expressed in M. xanthus DK10410 (ΔpilAMx), which did not restore S-motility on soft agar or EPS production to host cells. After further analysis of the motile behaviors in a methylcellulose solution, the M. xanthus strains were categorized into three types. YL6101, carrying pilASa1, and YL6104, carrying pilASa4, produced stable but unretractable surface pili; YL6102, carrying pilASa2, produced stable surface pili and exhibited reduced TFP-dependent motility in methylcellulose; YL6103, carrying pilASa3, produced unstable surface pili. Based on these findings, we propose that pilASa2 might be responsible for the type IV pilin production involved in group motility in S. aurantiaca DSM17044. After examining the developmental processes, it was suggested that the expression of PilASa4 protein might have positive effects on the fruiting body formation of M. xanthus DK10410 cells. Moreover, the formation of fruiting body in M. xanthus cells with stable exogenous TFPSa were compensated by mixing them with S. aurantiaca DSM17044 cells. Our results shed some light on the features and functions of type IV pilin homologues in S. aurantiaca.
Chemosensory systems (CSS) are complex regulatory pathways capable of perceiving external signals and translating them into different cellular behaviors such as motility and development. In the δ-proteobacterium Myxococcus xanthus, chemosensing allows groups of cells to orient themselves and aggregate into specialized multicellular biofilms termed fruiting bodies. M. xanthus contains eight predicted CSS and 21 chemoreceptors. In this work, we systematically deleted genes encoding components of each CSS and chemoreceptors and determined their effects on M. xanthus social behaviors. Then, to understand how the 21 chemoreceptors are distributed among the eight CSS, we examined their phylogenetic distribution, genomic organization and subcellular localization. We found that, in vivo, receptors belonging to the same phylogenetic group colocalize and interact with CSS components of the respective phylogenetic group. Finally, we identified a large chemosensory module formed by three interconnected CSS and multiple chemoreceptors and showed that complex behaviors such as cell group motility and biofilm formation require regulatory apparatus composed of multiple interconnected Che-like systems.
Myxococcus xanthus is a social bacterium that exhibits a complex life cycle including biofilm formation, microbial predation and the formation of multicellular fruiting bodies. Genomic analyses indicate that M. xanthus produces an unusual number of chemosensory proteins: eight chemosensory systems (CSS) and 21 chemoreceptors, 13 of which are orphans located outside operons. In this paper we used genetic, phylogenetic and cell biology techniques to analyze the organization of the chemoreceptors and their functions in the regulation of M. xanthus social behaviors. Results indicate the existence of one large and three small chemosensory modules that occupy different positions within cells. This organization is consistent with in vivo protein interaction assays. Our analyses revealed the presence of a complex network of regulators that might integrate different stimuli to modulate bacterial social behaviors. Such networks might be conserved in other bacterial species with a life cycle of similar complexity and whose genome carries multiple CSS encoding operons.
Swarming, a collective motion of many thousands of cells, produces colonies that rapidly spread over surfaces. In this paper, we introduce a cell-based model to study how interactions between neighboring cells facilitate swarming. We chose to study Myxococcus xanthus, a species of myxobacteria, because it swarms rapidly and has well-defined cell–cell interactions mediated by type IV pili and by slime trails. The aim of this paper is to test whether the cell contact interactions, which are inherent in pili-based S motility and slime-based A motility, are sufficient to explain the observed expansion of wild-type swarms. The simulations yield a constant rate of swarm expansion, which has been observed experimentally. Also, the model is able to quantify the contributions of S motility and A motility to swarming. Some pathogenic bacteria spread over infected tissue by swarming. The model described here may shed some light on their colonization process.
Many bacteria are able to spread rapidly over the surface using a strategy called swarming. When the cells cover a surface at high density and compete with each other for nutrients, swarming permits them to maintain rapid growth at the swarm edge. Swarming with flagella has been investigated for many years, and much has been learned about its regulation. Nevertheless, its choreography, which is somewhat related to the counterflow of pedestrians on a city sidewalk, has remained elusive. It is the bacterial equivalent of dancing toward the exit in a crowd of moving bodies that usually are in close contact. Myxococcus xanthus expands its swarms at 1.6 μm/min, about a third the speed of individual cells gliding over the same surface. Each cell has pilus engines at its front end and slime secretion engines at its rear. Using the known mechanics of these engines and the ways they are coordinated, we have developed a cell-based model to study the role of social interactions in bacterial swarming. The model is able to quantify the contributions of individual motility engines to swarming. It also shows that microscopic social interactions help to form the ordered collective motion observed in swarms.
Nutrient or niche-based competition among bacteria is a widespread phenomenon in natural environment. Such inter-species interactions are often mediated by secreted soluble factors and/or direct cell–cell contact. As ubiquitous soil bacteria, Myxococcus species are able to produce a variety of bioactive secondary metabolites to inhibit the growth of other competing bacterial species. Meanwhile, Myxococcus sp. also exhibit sophisticated predatory behavior, an extreme form of competition that is often stimulated by close contact with prey cells and largely depends on the availability of solid surfaces. Myxococcus sp. can also be isolated from aquatic environments. However, studies focusing on the interaction between Myxococcus and other bacteria in such environments are still limited. In this study, using the well-studied M. xanthus DK1622 and E. coli as model interspecies interaction pair, we demonstrated that in a aqueous environment, Myxococcus xanthus was able to kill Escherichia coli in a cell contact-dependent manner, and that the observed contact dependent killing required the formation of co-aggregates between M. xanthus and E. coli cells. Further analysis revealed that exopolysaccharide (EPS), type IV pilus (TFP) and lipopolysaccharide (LPS) mutants of M. xanthus displayed various degrees of attenuation in E. coli killing, and it correlated well with the mutants' reduction in EPS production. In addition, M. xanthus showed differential binding ability to different bacteria, and bacterial strains unable to co-aggregate with M. xanthus can escape the killing, suggesting the specific nature of co-aggregation and the targeted killing of interacting bacteria. In conclusion, our results demonstrated EPS mediated, contact-dependent killing of E. coli by M. xanthus, a strategy that might facilitate the survival of this ubiquitous bacterium in aquatic environments.
M. xanthus; contact-dependent interaction; co-aggregation; aqueous environment
Many bacteria exhibit multicellular behaviour, with individuals within a colony coordinating their actions for communal benefit. One example of complex multicellular phenotypes is myxobacterial fruiting body formation, where thousands of cells aggregate into large three-dimensional structures, within which sporulation occurs. Here we describe a novel theoretical model, which uses Monte Carlo dynamics to simulate and explain multicellular development. The model captures multiple behaviours observed during fruiting, including the spontaneous formation of aggregation centres and the formation and dissolution of fruiting bodies. We show that a small number of physical properties in the model is sufficient to explain the most frequently documented population-level behaviours observed during development in Myxococcus xanthus.
Understanding how relatively simple, single cell bacteria can communicate and coordinate their actions is important for explaining how complex multicellular behaviour can emerge without a central controller. Myxobacteria are particularly interesting in this respect because cells undergo multiple phases of coordinated behaviour during their life-cycle. One of the most fascinating and complex phases is the formation of fruiting bodies—large multicellular aggregates of cells formed in response to starvation. In this article we use evidence from the latest experimental data to construct a computational model explaining how cells can form fruiting bodies. Both in our model and in nature, cells move together in dense swarms, which collide to form aggregation centres. In particular, we show that it is possible for aggregates to form spontaneously where previous models require artificially induced aggregates to start the fruiting process.
Myxococcus xanthus, a Gram-negative soil bacterium, undergoes multicellular development when nutrients become limiting. Aggregation, which is part of the developmental process, requires the surface motility of this organism. One component of M. xanthus motility, the social (S) gliding motility, enables the movement of cells in close physical proximity. Previous studies demonstrated that the cell surface-associated exopolysaccharide (EPS) is essential for S motility and that the Dif proteins form a chemotaxis-like pathway that regulates EPS production in M. xanthus. DifA, a homologue of methyl-accepting chemotaxis proteins (MCPs) in the Dif system, is required for EPS production, S motility and development. In this study, a spontaneous extragenic suppressor of a difA deletion was isolated in order to identify additional regulators of EPS production. The suppressor mutation was found to be a single base pair insertion in cheW7 at the che7 chemotaxis gene cluster. Further examination indicated that mutations in cheW7 may lead to the interaction of Mcp7 with DifC (CheW-like) and DifE (CheA-like) to reconstruct a functional pathway to regulate EPS production in the absence of DifA. In addition, the cheW7 mutation was found to partially suppress a pilA mutation in EPS production in a difA+ background. Further deletion of difA from the pilA cheW7 double mutant resulted in a triple mutant that produced wild-type levels of EPS, implying that DifA (MCP-like) and Mcp7 compete for interactions with DifC and DifE in the modulation of EPS production.
Myxococcus xanthus, a Gram-negative soil bacterium, undergoes multicellular development when nutrients become limiting. Aggregation, which is part of the developmental process, requires the surface motility of this organism. One component of M. xanthus motility, the social (S) gliding motility, enables the movement of cells in close physical proximity. Previous studies demonstrated that the cell-surface associated exopolysaccharide (EPS) is essential for S motility and the Dif proteins form a chemotaxis-like pathway that regulates EPS production in M. xanthus. DifA, a homologue of methyl-accepting chemotaxis proteins (MCPs) in the Dif system, is required for EPS production, S motility and development. In this study, a spontaneous extragenic suppressor of a difA deletion was isolated in order to identify additional regulators of EPS production. The suppressor mutation was found to be a single base-pair insertion in cheW7 at the che7 chemotaxis gene cluster. Further examination indicated that mutations in cheW7 may lead to the interaction of Mcp7 with DifC (CheW-like) and DifE (CheA-like) to reconstruct a functional pathway to regulate EPS production in the absence of DifA. In addition, the cheW7 mutation was found to partially suppress a pilA mutation in EPS production in a difA+ background. Further deletion of difA from the pilA cheW7 double mutant resulted in a triple mutant that produced wild-type levels of EPS, implying that DifA (MCP-like) and Mcp7 compete for interactions with DifC and DifE in the modulation of EPS production.
Myxococcus xanthus is a predatory bacterium that exhibits complex social behavior. The most pronounced behavior is the aggregation of cells into raised fruiting body structures in which cells differentiate into stress-resistant spores. In the laboratory, monocultures of M. xanthus at a very high density will reproducibly induce hundreds of randomly localized fruiting bodies when exposed to low nutrient availability and a solid surface. In this report, we analyze how M. xanthus fruiting body development proceeds in a coculture with suitable prey. Our analysis indicates that when prey bacteria are provided as a nutrient source, fruiting body aggregation is more organized, such that fruiting bodies form specifically after a step-down or loss of prey availability, whereas a step-up in prey availability inhibits fruiting body formation. This localization of aggregates occurs independently of the basal nutrient levels tested, indicating that starvation is not required for this process. Analysis of early developmental signaling relA and asgD mutants indicates that they are capable of forming fruiting body aggregates in the presence of prey, demonstrating that the stringent response and A-signal production are surprisingly not required for the initiation of fruiting behavior. However, these strains are still defective in differentiating to spores. We conclude that fruiting body formation does not occur exclusively in response to starvation and propose an alternative model in which multicellular development is driven by the interactions between M. xanthus cells and their cognate prey.
Myxobacteria build their species-specific fruiting bodies by cell movement and then differentiate spores in specific places within that multicellular structure. New steps in the developmental aggregation of Myxococcus xanthus were discovered through a frame-by-frame analysis of a motion picture. The formation and fate of 18 aggregates were captured in the time-lapse movie. Still photographs of 600 other aggregates were also analyzed. M. xanthus has two engines that propel the gliding of its rod-shaped cells: slime-secreting jets at the rear and retractile pili at the front. The earliest aggregates are stationary masses of cells that look like three-dimensional traffic jams. We propose a model in which both engines stall as the cells' forward progress is blocked by other cells in the traffic jam. We also propose that these blockades are eventually circumvented by the cell's capacity to turn, which is facilitated by the push of slime secretion at the rear of each cell and by the flexibility of the myxobacterial cell wall. Turning by many cells would transform a traffic jam into an elliptical mound, in which the cells are streaming in closed orbits. Pairs of adjacent mounds are observed to coalesce into single larger mounds, probably reflecting the fusion of orbits in the adjacent mounds. Although fruiting bodies are relatively large structures that contain 105 cells, no long-range interactions between cells were evident. For aggregation, M. xanthus appears to use local interactions between its cells.
Myxobacteria exhibit complex social traits during which large populations of cells coordinate their behaviors. An iconic example is their response to starvation: thousands of cells move by gliding motility to build a fruiting body in which vegetative cells differentiate into spores. Here we review mechanisms that the model species Myxococcus xanthus uses for cell—cell interactions, with a focus on developmental signaling and social gliding motility. We also discuss a newly discovered cell—cell interaction whereby myxobacteria exchange their outer membrane (OM) proteins and lipids. The mechanism of OM transfer requires physical contact between aligned cells on a hard surface and is apparently mediated by OM fusion. The TraA and TraB proteins are required in both donor and recipient cells for transfer, suggesting bidirectional exchange, and TraA is thought to serve as a cell surface adhesin. OM exchange results in phenotypic changes that can alter gliding motility and development and is proposed to represent a novel microbial interacting platform to coordinate multicellular activities.
Cell—cell signaling; Fruiting body; Type IV pilus; Membrane fusion; Protein transfer; Surface receptor
Myxococcus xanthus cells self-organize into periodic bands of traveling waves, termed ripples, during multicellular fruiting body development and predation on other bacteria. To investigate the mechanistic basis of rippling behavior and its physiological role during predation by this Gram-negative soil bacterium, we have used an approach that combines mathematical modeling with experimental observations. Specifically, we developed an agent-based model (ABM) to simulate rippling behavior that employs a new signaling mechanism to trigger cellular reversals. The ABM has demonstrated that three ingredients are sufficient to generate rippling behavior: (i) side-to-side signaling between two cells that causes one of the cells to reverse, (ii) a minimal refractory time period after each reversal during which cells cannot reverse again, and (iii) physical interactions that cause the cells to locally align. To explain why rippling behavior appears as a consequence of the presence of prey, we postulate that prey-associated macromolecules indirectly induce ripples by stimulating side-to-side contact-mediated signaling. In parallel to the simulations, M. xanthus predatory rippling behavior was experimentally observed and analyzed using time-lapse microscopy. A formalized relationship between the wavelength, reversal time, and cell velocity has been predicted by the simulations and confirmed by the experimental data. Furthermore, the results suggest that the physiological role of rippling behavior during M. xanthus predation is to increase the rate of spreading over prey cells due to increased side-to-side contact-mediated signaling and to allow predatory cells to remain on the prey longer as a result of more periodic cell motility.
Myxococcus xanthus cells collectively move on solid surfaces and reorganize their colonies in response to environmental cues. Under some conditions, cells exhibit an intriguing form of collective motility by self-organizing into bands of travelling alternating-density waves termed ripples. These waves are distinct from the waves originating from Turing instability in diffusion-reaction systems, as these counter-traveling waves do not annihilate but appear to pass through each other. Here we developed a new mathematical model of rippling behavior based on a recently observed contact signaling mechanism – cells that make side-to-side contacts can signal one another to reverse. We hypothesize that this signaling is enhanced by the presence of prey-associated macromolecules and compare modeling predictions with experimentally observed waves generated on E. coli prey cells. The model predicts a modified relationship between the wavelength and individual predatory cell motility parameters and provides a physiological role for rippling during predation. We show that ripples allow predatory cells to increase the rate of their spreading to quickly envelope the prey, and subsequently to decrease their random drift to remain in the prey region for longer. These and other predictions are confirmed by the experimental observations.
The gene encoding the GroEL chaperonin is duplicated in nearly 30% of bacterial genomes; and although duplicated groEL genes have been comprehensively determined to have distinct physiological functions in different species, the mechanisms involved have not been characterized to date. Myxococcus xanthus DK1622 has two copies of the groEL gene, each of which can be deleted without affecting cell viability; however, the deletion of either gene does result in distinct defects in the cellular heat-shock response, predation, and development. In this study, we show that, from the expression levels of different groELs, the distinct functions of groEL1 and groEL2 in predation and development are probably the result of the substrate selectivity of the paralogous GroEL chaperonins, whereas the lethal effect of heat shock due to the deletion of groEL1 is caused by a decrease in the total groEL expression level. Following a bioinformatics analysis of the composition characteristics of GroELs from different bacteria, we performed region-swapping assays in M. xanthus, demonstrating that the differences in the apical and the C-terminal equatorial regions determine the substrate specificity of the two GroELs. Site-directed mutagenesis experiments indicated that the GGM repeat sequence at the C-terminus of GroEL1 plays an important role in functional divergence. Divergent functions of duplicated GroELs, which have similar patterns of variation in different bacterial species, have thus evolved mainly via alteration of the apical and the C-terminal equatorial regions. We identified the specific substrates of strain DK1622's GroEL1 and GroEL2 using immunoprecipitation and mass spectrometry techniques. Although 68 proteins bound to both GroEL1 and GroEL2, 83 and 46 proteins bound exclusively to GroEL1 or GroEL2, respectively. The GroEL-specific substrates exhibited distinct molecular sizes and secondary structures, providing an encouraging indication for GroEL evolution for functional divergence.
GroEL is a type I chaperonin, involved in protein folding, assembly, and transport. It is a major group of heat-shock proteins that are over-expressed at high temperatures and has fundamental roles in growth and survival at non-permissive temperatures. Because of its importance in many cellular processes, the groEL gene is ubiquitously distributed in bacteria. Most bacterial species possess a single groEL gene, while others (close to 30% of sequenced bacterial genomes) have two or more groEL copies. Many studies have described the functional divergence of duplicated groEL genes in different bacterial species, but the involved mechanisms have not yet been characterized. Myxobacteria are characterized by their unique multicellular behaviors. Myxococcus xanthus DK1622, the model strain of myxobacteria, possesses a large genome (9.14 Mb), containing many gene duplications, including two copies of the groEL gene. Gene duplications and their functional divergence are suggested for complex cellular behaviors, which, however, have not yet been testified. In this paper, using combined proteomic and genetic approaches, we explored how the duplicated groEL genes of M. xanthus DK1622 evolved to fit the functional divergence for social behaviors.
Myxobacteria are very important due to their unique characteristics, such as multicellular social behavior and the production of diverse and novel bioactive secondary metabolites. However, the lack of autonomously replicating plasmids has hindered genetic manipulation of myxobacteria for decades. To determine whether indigenous plasmids are present, we screened about 150 myxobacterial strains, and a circular plasmid designated pMF1 was isolated from Myxococcus fulvus 124B02. Sequence analysis showed that this plasmid was 18,634 bp long and had a G+C content of 68.7%. Twenty-three open reading frames were found in the plasmid, and 14 of them were not homologous to any known sequence. Plasmids containing the gene designated pMF1.14, which encodes a large unknown protein, were shown to transform Myxococcus xanthus DZ1 and DK1622 at high frequencies (∼105 CFU/μg DNA), suggesting that the locus is responsible for the autonomous replication of pMF1. Shuttle vectors were constructed for both M. xanthus and Escherichia coli. The pilA gene, which is essential for pilus formation and social motility in M. xanthus, was cloned into the shuttle vectors and introduced into the pilA-deficient mutant DK10410. The transformants subsequently exhibited the ability to form pili and social motility. Autonomously replicating plasmid pMF1 provides a new tool for genetic manipulation in Myxococcus.
Myxococcus xanthus is a gram-negative soil bacterium which exhibits a complex life cycle and social behavior. In this study, two developmental mutants of M. xanthus were isolated through Tn5 transposon mutagenesis. The mutants were found to be defective in cellular aggregation as well as in sporulation. Further phenotypic characterization indicated that the mutants were defective in social motility but normal in directed cell movements. Both mutations were cloned by a transposon-tagging method. Sequence analysis indicated that both insertions occurred in the same gene, which encodes a homolog of DnaK. Unlike the dnaK genes in other bacteria, this M. xanthus homolog appears not to be regulated by temperature or heat shock and is constitutively expressed during vegetative growth and under starvation. The defects of the mutants indicate that this DnaK homolog is important for the social motility and development of M. xanthus.
In bacteria with multiple sets of chemotaxis genes, the deletion of homologous genes or even different genes in the same operon can result in disparate phenotypes. Myxococcus xanthus is a bacterium with multiple sets of chemotaxis genes and/or homologues. It was shown previously that difA and difE, encoding homologues of the methyl-accepting chemoreceptor protein (MCP) and the CheA kinase, respectively, are required for M. xanthus social gliding (S) motility and development. Both difA and difE mutants were also defective in the biogenesis of the cell surface appendages known as extracellular matrix fibrils. In this study, we investigated the roles of the CheW homologue encoded by difC, a gene at the same locus as difA and difE. We showed that difC mutations resulted in defects in M. xanthus developmental aggregation, sporulation, and S motility. We demonstrated that difC is indispensable for wild-type cellular cohesion and fibril biogenesis but not for pilus production. We further illustrated the ectopic complementation of a difC in-frame deletion by a wild-type difC. The identical phenotypes of difA, difC, and difE mutants are consistent and supportive of the hypothesis that the Dif chemotaxis homologues constitute a chemotaxis-like signal transduction pathway that regulates M. xanthus fibril biogenesis and S motility.
Dsp mutants of Myxococcus xanthus have a complex phenotype with abnormal cell cohesion, social motility, and development. All three defects are the result of a single mutation in the dsp locus, a region of DNA about 14 kilobases long. Cohesion appears to play a central role in social motility, since nonsocial mutants exhibit weak agglutination or, in the case of Dsp cells, no agglutination (L. J. Shimkets, J. Bacteriol. 166:837-841, 1986). However, Dsp cells can be agglutinated by cohesive strains of M. xanthus. This provided the opportunity to examine the role of cohesion during development by comparing the developmental phenotype of Dsp cells with that of Dsp cells mixed with cohesive strains. Dsp mutants were unable to complete any of the developmental behaviors: aggregation, fruiting body formation, developmental autolysis, and sporulation. Contact with cohesive strains seemed to restore some developmental characteristics to the Dsp cells. When allowed to develop with wild-type cells, Dsp cells accumulated in fruiting bodies and underwent developmental autolysis, but did not form a significant portion of the spore population. Igl mutants, which may be similar to the previously described frizzy mutants, are cohesive strains that are unable to form fruiting bodies. Mixing Igl cells with Dsp cells under developmental conditions resulted in fruiting body formation, although the Dsp cells were unable to form significant levels of myxospores. In spite of their inability to sporulate under developmental conditions, Dsp mutants did not appear to be defective in the sporulation process. In fact, they formed normal levels of myxospores in response to the chemical inducer glycerol.
Bdellovibrio bacteriovorus invade Gram-negative bacteria in a predatory process requiring Type IV pili (T4P) at a single invasive pole, and also glide on surfaces to locate prey. Ras-like G-protein MglA, working with MglB and RomR in the deltaproteobacterium Myxococcus xanthus, regulates adventurous gliding and T4P-mediated social motility at both M. xanthus cell poles. Our bioinformatic analyses suggested that the GTPase activating protein (GAP)-encoding gene mglB was lost in Bdellovibrio, but critical residues for MglABd GTP-binding are conserved. Deletion of mglABd abolished prey-invasion, but not gliding, and reduced T4P formation. MglABd interacted with a previously uncharacterised tetratricopeptide repeat (TPR) domain protein Bd2492, which we show localises at the single invasive pole and is required for predation. Bd2492 and RomR also interacted with cyclic-di-GMP-binding receptor CdgA, required for rapid prey-invasion. Bd2492, RomRBd and CdgA localize to the invasive pole and may facilitate MglA-docking. Bd2492 was encoded from an operon encoding a TamAB-like secretion system. The TamA protein and RomR were found, by gene deletion tests, to be essential for viability in both predatory and non-predatory modes. Control proteins, which regulate bipolar T4P-mediated social motility in swarming groups of deltaproteobacteria, have adapted in evolution to regulate the anti-social process of unipolar prey-invasion in the “lone-hunter” Bdellovibrio. Thus GTP-binding proteins and cyclic-di-GMP inputs combine at a regulatory hub, turning on prey-invasion and allowing invasion and killing of bacterial pathogens and consequent predatory growth of Bdellovibrio.
Bacterial cell polarity control is important for maintaining asymmetry of polar components such as flagella and pili. Bdellovibrio bacteriovorus is a predatory deltaproteobacterium which attaches to, and invades, other bacteria using Type IV pili (T4P) extruded from the specialised, invasive, non-flagellar pole of the cell. It was not known how that invasive pole is specified and regulated. Here we discover that a regulatory protein-hub, including Ras-GTPase-like protein MglA and cyclic-di-GMP receptor-protein CdgA, control prey-invasion. In the deltaproteobacterium, Myxococcus xanthus, MglA, with MglB and RomR, was found by others to regulate switching of T4P in social ‘swarming’ surface motility by swapping the pole at which T4P are found. In contrast, in B. bacteriovorus MglA regulates the process of prey-invasion and RomR, which is required for surface motility regulation in Myxococcus, is essential for growth and viability in Bdellovibrio. During evolution, B. bacteriovorus has lost mglB, possibly as T4P-pole-switching is not required; pili are only required at the invasive pole. A previously unidentified tetratricopeptide repeat (TPR) protein interacts with MglA and is essential for prey-invasion. This regulatory protein hub allows prey-invasion, likely integrating cyclic-di-GMP signals, pilus assembly and TamAB secretion in B. bacteriovorus.
Myxobacteria are social microbes that exhibit complex multicellular behaviors. By use of fluorescent reporters, we show that Myxococcus xanthus isolates produce long narrow filaments that are enclosed by the outer membrane (OM) and contain proteins. We show that these OM tube (OMT) structures are produced at surprisingly high levels when cells are placed in liquid medium or buffer without agitation. OMTs can be long and easily exceed multiple cell lengths. When viewed by transmission electron microscopy, their morphology varies between tubes and chain-like structures. Intermediate-like structures are also found, suggesting that OMTs may transition between these two morphotypes. In support of this, video epifluorescence microscopy found that OMTs in solution dynamically twist and jiggle. On hard surfaces, myxobacteria glide, and upon cell-cell contact, they can efficiently exchange their OM proteins and lipids by a TraAB-dependent mechanism. Although the structure of OMTs hints at a possible role as conduits for exchange, evidence is presented to the contrary. For example, abundant OMT production occurs in traA or traB mutants and when cells are grown in liquid medium, yet transfer cannot occur under these conditions. Thus, genetic and environmental conditions that promote OMT production are incongruent with OM exchange.
Single mutations in the mglA gene in Myxococcus xanthus render cells incapable of gliding. The mglA strains are unique in that all other nonmotile strains of M. xanthus isolated are the result of at least two independent mutations in separate motility system genes. Translational fusions of trpE, or of lacZ, to mglA were constructed, and the resulting fusion polypeptides were used to generate antibodies. Antibodies specific to MglA protein were purified. Antibody-tagged MglA was found localized to the cytoplasm of M. xanthus cells both by fractionation of cell extracts and by electron microscopy of thin sections of whole cells. Four of the five mglA missense mutants tested failed to produce detectable levels of the MglA antigen in whole cell extracts. Nonmotile double mutants (A-S-), which have one mutation in a gene of system A and one mutation in a gene of system S, have the same phenotype as null mglA mutants but produce wild-type levels of MglA protein. MglA protein is conserved in all strains of myxobacteria tested. The amino acid sequence of MglA protein includes three sequence motifs characteristic of GDP/GTP-binding proteins. On the basis of its genetic properties, intracellular location, and amino acid sequence, it is argued that MglA protein is a regulator in the sequence of functions leading to cell movement.
Myxococcus xanthus belongs to the delta class of the proteobacteria and is notable for its complex life-style with social behaviors and relatively large genome. Although previous observations have suggested the existence of horizontal gene transfer in M. xanthus, its ability to take up exogenous DNA via natural transformation has not been experimentally demonstrated. In this study, we achieved natural transformation in M. xanthus using the autonomously replicating myxobacterial plasmid pZJY41 as donor DNA. M. xanthus exopolysaccharide (EPS) was shown to be an extracellular barrier for transformation. Cells deficient in EPS production, e.g., mutant strains carrying ΔdifA or ΔepsA, became naturally transformable. Among the inner barriers to transformation were restriction-modification systems in M. xanthus, which could be partially overcome by methylating DNA in vitro using cell extracts of M. xanthus prior to transformation. In addition, the incubation time of DNA with cells and the presence of divalent magnesium ion affected transformation frequency of M. xanthus. Furthermore, we also observed a potential involvement of the type IV pilus system in the DNA uptake machinery of M. xanthus. The natural transformation was totally eliminated in the ΔpilQ/epsA and Δtgl/epsA mutants, and null mutation of pilB or pilC in an ΔepsA background diminished the transformation rate. Our study, to the best of our knowledge, provides the first example of a naturally transformable species among deltaproteobacteria.