Motor-driven intracellular transport powers bacterial gliding motility

Phosphorylation-dependent localization of the response regulator FrzZ signals cell reversals in Myxococcus xanthus

The life cycle of Myxococcus xanthus includes co-ordinated group movement and fruiting body formation, and requires directed motility and controlled cell reversals. Reversals are achieved by inverting cell polarity and re-organizing many motility proteins. The Frz chemosensory pathway regulates the frequency of cell reversals. While it has been established that phosphotransfer from the kinase FrzE to the response regulator FrzZ is required, it is unknown how phosphorylated FrzZ, the putative output of the pathway, targets the cell polarity axis. In this study, we used Phos-tag SDS-PAGE to determine the cellular level of phospho-FrzZ under different growth conditions and in Frz signalling mutants. We detected consistent FrzZ phosphorylation, albeit with a short half-life, in cells grown on plates, but not from liquid culture. The available pool of phospho-FrzZ correlated with reversal frequencies, with higher levels found in hyper-reversing mutants. Phosphorylation was not detected in hypo-reversing mutants. Fluorescence microscopy revealed that FrzZ is recruited to the leading cell pole upon phosphorylation and switches to the opposite pole during reversals. These results are consistent with the hypothesis that the Frz pathway modulates reversal frequency through a localized response regulator that targets cell polarity regulators at the leading cell pole.

A novel locus essential for spreading of Cytophaga hutchinsonii colonies on agar

Cytophaga hutchinsonii is an aerobic cellulolytic gliding bacterium. The mechanism of its cell motility over surfaces without flagella and type IV pili is not known. In this study, mariner-based transposon mutagenesis was used to identify a new locus CHU_1797 essential for colony spreading on both hard and soft agar surfaces through gliding. CHU_1797 encodes a putative outer membrane protein of 348 amino acids with unknown function, and proteins which have high sequence similarity to CHU_1797 were widespread in the members of the phylum Bacteroidetes. The disruption of CHU_1797 suppressed spreading toward glucose on an agar surface, but had no significant effect on cellulose degradation for cells already in contact with cellulose. SEM observation showed that the mutant cells also regularly arranged on the surface of cellulose fiber similar with that of the wild type strain. These results indicated that the colony spreading ability on agar surfaces was not required for cellulose degradation by C. hutchinsonii. This was the first study focused on the relationship between cell motility and cellulose degradation of C. hutchinsonii.

Flagella stator homologs function as motors for myxobacterial gliding motility by moving in helical trajectories

Many bacterial species use gliding motility in natural habitats because external flagella function poorly on hard surfaces. However, the mechanism(s) of gliding remain elusive because surface motility structures are not apparent. Here, we characterized the dynamics of the Myxococcus xanthus gliding motor protein AglR, a homolog of the Escherichia coli flagella stator protein MotA. We observed that AglR decorated a helical structure, and the AglR helices rotated when cells were suspended in liquid or when cells moved on agar surfaces. With photoactivatable localization microscopy, we found that single molecules of AglR, unlike MotA/MotB, can move laterally within the membrane in helical trajectories. AglR slowed down transiently at gliding surfaces, accumulating in clusters. Our work shows that the untethered gliding motors of M. xanthus, by moving within the membrane, can transform helical motion into linear driving forces that push against the surface.

Multidimensional view of the bacterial cytoskeleton

The perspective of the cytoskeleton as a feature unique to eukaryotic organisms was overturned when homologs of the eukaryotic cytoskeletal elements were identified in prokaryotes and implicated in major cell functions, including growth, morphogenesis, cell division, DNA partitioning, and cell motility. FtsZ and MreB were the first identified homologs of tubulin and actin, respectively, followed by the discovery of crescentin as an intermediate filament-like protein. In addition, new elements were identified which have no apparent eukaryotic counterparts, such as the deviant Walker A-type ATPases, bactofilins, and several novel elements recently identified in streptomycetes, highlighting the unsuspected complexity of cytostructural components in bacteria. In vivo multidimensional fluorescence microscopy has demonstrated the dynamics of the bacterial intracellular world, and yet we are only starting to understand the role of cytoskeletal elements. Elucidating structure-function relationships remains challenging, because core cytoskeletal protein motifs show remarkable plasticity, with one element often performing various functions and one function being performed by several types of elements. Structural imaging techniques, such as cryo-electron tomography in combination with advanced light microscopy, are providing the missing links and enabling scientists to answer many outstanding questions regarding prokaryotic cellular architecture. Here we review the recent advances made toward understanding the different roles of cytoskeletal proteins in bacteria, with particular emphasis on modern imaging approaches.

Characterization of myxobacterial A-motility: insights from microcinematographic observations

Myxococcus xanthus, a predatory soil bacterium, has long been used as a model organism to study bacterial gliding motility. Research has revealed that two fundamentally distinct motor systems power gliding in this bacterium: repeated extensions and retractions of pili mediate social or (S-) motility, whereas the motor powering adventurous or (A-) motility has not yet been identified with certainty. Several different hypotheses to explain A-motility have been suggested and differ with respect to the involved motor structures as well as the mechanics of motility. As some of the more recent models invoke helically arranged structures and processes that require rotations of the cell, we decided to re-examine myxobacterial motility using microcinematographic techniques. This re-examination was also prompted by the lack of direct experimental data on the rotation of M. xanthus during gliding. Microcinematographic observations of deformed cells and cells containing large stationary intracellular structures reveal clearly that M. xanthus gliding does not require cell rotation.

Single cell microfluidic studies of bacterial motility

A large number of bacterial species move smoothly on solid surfaces in the absence of extracellular -organelles. In the deltaproteobacterium Myxococcus xanthus, this surface motion, termed gliding motility, involves a novel macromolecular machinery Agl-Glt. During the motility process, the Agl-Glt system, an integral envelope protein complex, is assembled on the ventral side of the cell. Doing so, the complex couples surface adhesion to the activity of the Agl motility motor. On the cytosolic side, the Agl-Glt system is linked to the bacterial actin cytoskeleton MreB. It is proposed that motility is produced when surface immobilized Agl-Glt complexes produce traction on a rigid track, possibly the MreB cables. Testing this hypothesis directly requires both microfluidic techniques to perturb the motility process with inhibitors (i.e., A22, CCCP) and state-of-the-art microscopy techniques (i.e., TIRF and AFM). These approaches require a microscopy chamber where the cells glide in liquid on a non-agar substrate. Here, we describe a straightforward coating procedure to construct a chitosan-functionalized microfluidic chamber that fulfills these requirements. This set up circumvents all the disadvantages of traditional agar-based assays, providing new grounds for high-resolution experiments. We also describe simple image processing to maximize the quality of data representation. In theory, our procedure could be used for any bacterial system that adheres to chitosan.

Evolution of intracellular compartmentalization

Cells compartmentalize their biochemical functions in a variety of ways, notably by creating physical barriers that separate a compartment via membranes or proteins. Eukaryotes have a wide diversity of membrane-based compartments, many that are lineage- or tissue-specific. In recent years, it has become increasingly evident that membrane-based compartmentalization of the cytosolic space is observed in multiple prokaryotic lineages, giving rise to several types of distinct prokaryotic organelles. Endosymbionts, previously believed to be a hallmark of eukaryotes, have been described in several bacteria. Protein-based compartments, frequent in bacteria, are also found in eukaryotes. In the present review, we focus on selected intracellular compartments from each of these three categories, membrane-based, endosymbiotic and protein-based, in both prokaryotes and eukaryotes. We review their diversity and the current theories and controversies regarding the evolutionary origins. Furthermore, we discuss the evolutionary processes acting on the genetic basis of intracellular compartments and how those differ across the domains of life. We conclude that the distinction between eukaryotes and prokaryotes no longer lies in the existence of a compartmentalized cell plan, but rather in its complexity.

Analysis of energy sources for Mycoplasma penetrans gliding motility

Mycoplasma penetrans, a potential human pathogen found mainly in HIV-infected individuals, uses a tip structure for both adherence and gliding motility. To improve our understanding of the molecular mechanism of M. penetrans gliding motility, we used chemical inhibitors of energy sources associated with motility of other organisms to determine which of these is used by M. penetrans and also tested whether gliding speed responded to temperature and pH. Mycoplasma penetrans gliding motility was not eliminated in the presence of a proton motive force inhibitor, a sodium motive force inhibitor, or an agent that depletes cellular ATP. At near-neutral pH, gliding speed increased as temperature increased. The absence of a clear chemical energy source for gliding motility and a positive correlation between speed and temperature suggest that energy derived from heat provides the major source of power for the gliding motor of M. penetrans.

Chemosensory signaling controls motility and subcellular polarity in Myxococcus xanthus

Myxococcus xanthus is a model system for the study of dynamic protein localization and cell polarity in bacteria. M. xanthus cells are motile on solid surfaces enabled by two forms of motility. Motility is controlled by the Che-like Frz pathway, which is essential for fruiting body formation and differentiation. The Frz signal is mediated by a GTPase/GAP protein pair that establishes cell polarity and directs the motility systems. Pilus driven motility at the leading pole of the cell requires dynamic localization of two ATPases and the coordinated production of EPS synthesis. Gliding motility requires dynamic movement of large protein complexes, but the mechanism by which this system generates propulsive force is still an active area of investigation.

Myxobacterial tools for social interactions

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.

A response regulator interfaces between the Frz chemosensory system and the MglA/MglB GTPase/GAP module to regulate polarity in Myxococcus xanthus

How cells establish and dynamically change polarity are general questions in cell biology. Cells of the rod-shaped bacterium Myxococcus xanthus move on surfaces with defined leading and lagging cell poles. Occasionally, cells undergo reversals, which correspond to an inversion of the leading-lagging pole polarity axis. Reversals are induced by the Frz chemosensory system and depend on relocalization of motility proteins between the poles. The Ras-like GTPase MglA localizes to and defines the leading cell pole in the GTP-bound form. MglB, the cognate MglA GTPase activating protein, localizes to and defines the lagging pole. During reversals, MglA-GTP and MglB switch poles and, therefore, dynamically localized motility proteins switch poles. We identified the RomR response regulator, which localizes in a bipolar asymmetric pattern with a large cluster at the lagging pole, as important for motility and reversals. We show that RomR interacts directly with MglA and MglB in vitro. Furthermore, RomR, MglA, and MglB affect the localization of each other in all pair-wise directions, suggesting that RomR stimulates motility by promoting correct localization of MglA and MglB in MglA/RomR and MglB/RomR complexes at opposite poles. Moreover, localization analyses suggest that the two RomR complexes mutually exclude each other from their respective poles. We further show that RomR interfaces with FrzZ, the output response regulator of the Frz chemosensory system, to regulate reversals. Thus, RomR serves at the functional interface to connect a classic bacterial signalling module (Frz) to a classic eukaryotic polarity module (MglA/MglB). This modular design is paralleled by the phylogenetic distribution of the proteins, suggesting an evolutionary scheme in which RomR was incorporated into the MglA/MglB module to regulate cell polarity followed by the addition of the Frz system to dynamically regulate cell polarity.

Most cells are spatially organized with proteins localizing to specific regions. The ability of cells to polarize facilitates many processes including motility. Myxococcus xanthus cells move in the direction of their long axis and occasionally change direction of movement by undergoing reversals. Similarly to eukaryotic cells, the leading pole of M. xanthus cells is defined by a Ras-like GTPase and the lagging pole by its partner GAP MglB. We show that MglA and MglB localization depends on the RomR protein. RomR recruits MglA to a pole and MglB GAP activity at the lagging pole results in MglA/RomR localizing asymmetrically to the leading pole. Conversely, RomR together with MglB forms a complex that localizes to the lagging pole, and this asymmetry is set up by MglA/RomR at the leading pole. Thus, MglA/RomR and MglB/RomR localize to opposite poles because they exclude each other from the same pole. RomR also interfaces with the Frz chemosensory system that induces reversals. Thus, RomR links the MglA/MglB/RomR polarity module to the Frz signaling module that triggers the inversion of polarity. Phylogenomics suggests an evolutionary scheme in which the MglA/MglB module incorporated RomR early to impart cell polarity while the Frz module was appropriated later on to direct polarity reversals.

A dynamic response regulator protein modulates G-protein-dependent polarity in the bacterium Myxococcus xanthus

Migrating cells employ sophisticated signal transduction systems to respond to their environment and polarize towards attractant sources. Bacterial cells also regulate their polarity dynamically to reverse their direction of movement. In Myxococcus xanthus, a GTP-bound Ras-like G-protein, MglA, activates the motility machineries at the leading cell pole. Reversals are provoked by pole-to-pole switching of MglA, which is under the control of a chemosensory-like signal transduction cascade (Frz). It was previously known that the asymmetric localization of MglA at one cell pole is regulated by MglB, a GTPase Activating Protein (GAP). In this process, MglB specifically localizes at the opposite lagging cell pole and blocks MglA localization at that pole. However, how MglA is targeted to the leading pole and how Frz activity switches the localizations of MglA and MglB synchronously remained unknown. Here, we show that MglA requires RomR, a previously known response regulator protein, to localize to the leading cell pole efficiently. Specifically, RomR-MglA and RomR-MglB complexes are formed and act complementarily to establish the polarity axis, segregating MglA and MglB to opposite cell poles. Finally, we present evidence that Frz signaling may regulate MglA localization through RomR, suggesting that RomR constitutes a link between the Frz-signaling and MglAB polarity modules. Thus, in Myxococcus xanthus, a response regulator protein governs the localization of a small G-protein, adding further insight to the polarization mechanism and suggesting that motility regulation evolved by recruiting and combining existing signaling modules of diverse origins.

Migrating cells have evolved a molecular compass to rapidly respond to environmental signals. During chemotaxis, small G-proteins and their regulators are activated and determine a leading cell edge towards attractant molecules. Bacteria also move across surfaces in a directed manner. The rod-shaped bacterium Myxococcus xanthus can switch its direction of movement in a process where the cell poles exchange roles (reversal), allowing complex multicellular behaviors. In Myxococcus, a small G-protein, MglA, determines the leading cell pole. In this study, we investigated how MglA localizes to the pole and found that its localization depends on the dual complementary action of RomR and MglB. In this process, RomR targets MglA to the pole while, in turn, MglB prevents its accumulation at the back of the cell. Moreover, RomR potentially links MglA to the Frz signal transduction pathway, a chemosensory system controlling the reversal frequency. The results provide a new molecular basis to understand motility regulation in a bacterium, which may have arisen from co-optation and branching of prokaryotic and eukaryotic-like signaling modules.

Wet-surface-enhanced ellipsometric contrast microscopy identifies slime as a major adhesion factor during bacterial surface motility

In biology, the extracellular matrix (ECM) promotes both cell adhesion and specific recognition, which is essential for central developmental processes in both eukaryotes and prokaryotes. However, live studies of the dynamic interactions between cells and the ECM, for example during motility, have been greatly impaired by imaging limitations: mostly the ability to observe the ECM at high resolution in absence of specific staining by live microscopy. To solve this problem, we developed a unique technique, wet-surface enhanced ellipsometry contrast (Wet-SEEC), which magnifies the contrast of transparent organic materials deposited on a substrate (called Wet-surf) with exquisite sensitivity. We show that Wet-SEEC allows both the observation of unprocessed nanofilms as low as 0.2 nm thick and their accurate 3D topographic reconstructions, directly by standard light microscopy. We next used Wet-SEEC to image slime secretion, a poorly defined property of many prokaryotic and eukaryotic organisms that move across solid surfaces in absence of obvious extracellular appendages (gliding). Using combined Wet-SEEC and fluorescent-staining experiments, we observed slime deposition by gliding Myxococcus xanthus cells at unprecedented resolution. Altogether, the results revealed that in this bacterium, slime associates preferentially with the outermost components of the motility machinery and promotes its adhesion to the substrate on the ventral side of the cell. Strikingly, analogous roles have been proposed for the extracellular proteoglycans of gliding diatoms and apicomplexa, suggesting that slime deposition is a general means for gliding organisms to adhere and move over surfaces.

Flavobacterium johnsoniae RemA is a mobile cell surface lectin involved in gliding

Cells of Flavobacterium johnsoniae move rapidly over surfaces by a process known as gliding motility. Gld proteins are thought to comprise the motor that propels the cell surface adhesin SprB. Cells with mutations in sprB are partially defective in motility and are also resistant to some bacteriophages. Transposon mutagenesis of a strain carrying a deletion spanning sprB identified eight mutants that were resistant to additional phages and exhibited reduced motility. Four of the mutants had transposon insertions in remA, which encodes a cell surface protein that has a lectin domain and appears to interact with polysaccharides. Three other genes identified in this screen (remC, wza, and wzc) encode proteins predicted to be involved in polysaccharide synthesis and secretion. Myc-tagged versions of RemA localized to the cell surface and were propelled rapidly along the cell at speeds of 1 to 2 μm/s. Deletion of gldN and gldO, which encode components of a bacteroidete protein secretion system, blocked the transport of RemA to the cell surface. Overexpression of RemA resulted in the formation of cell aggregates that were dispersed by the addition of galactose or rhamnose. Cells lacking RemC, Wza, and Wzc failed to aggregate. Cells of a remC mutant and cells of a remA mutant, neither of which formed aggregates in isolation, aggregated when they were mixed together, suggesting that polysaccharides secreted by one cell may interact with RemA on another cell. Fluorescently labeled lectin Ricinus communis agglutinin I detected polysaccharides secreted by F. johnsoniae. The polysaccharides bound to cells expressing RemA and were rapidly propelled on the cell surface. RemA appears to be a mobile cell surface adhesin, and secreted polysaccharides may interact with the lectin domain of RemA and enhance motility.

On the mysterious propulsion of Synechococcus

We propose a model for the self-propulsion of the marine bacterium Synechococcus utilizing a continuous looped helical track analogous to that found in Myxobacteria [1]. In our model cargo-carrying protein motors, driven by proton-motive force, move along a continuous looped helical track. The movement of the cargo creates surface distortions in the form of small amplitude traveling ridges along the S-layer above the helical track. The resulting fluid motion adjacent to the helical ribbon provides the propulsive thrust. A variation on the helical rotor model of [1] allows the motors to be anchored to the peptidoglycan layer, where they drive rotation of the track creating traveling helical waves along the S-layer. We derive expressions relating the swimming speed to the amplitude, wavelength, and velocity of the surface waves induced by the helical rotor, and show that they fall in reasonable ranges to explain the velocity and rotation rate of swimming Synechococcus.

Image analysis in fluorescence microscopy: bacterial dynamics as a case study

Fluorescence microscopy is the primary tool for studying complex processes inside individual living cells. Technical advances in both molecular biology and microscopy have made it possible to image cells from many genetic and environmental backgrounds. These images contain a vast amount of information, which is often hidden behind various sources of noise, convoluted with other information and stochastic in nature. Accessing the desired biological information therefore requires new tools of computational image analysis and modeling. Here, we review some of the recent advances in computational analysis of images obtained from fluorescence microscopy, focusing on bacterial systems. We emphasize techniques that are readily available to molecular and cell biologists but also point out examples where problem-specific image analyses are necessary. Thus, image analysis is not only a toolkit to be applied to new images but also an integral part of the design and implementation of a microscopy experiment.

The orphan histidine protein kinase SgmT is a c-di-GMP receptor and regulates composition of the extracellular matrix together with the orphan DNA binding response regulator DigR in Myxococcus xanthus

In Myxococcus xanthus the extracellular matrix is essential for type IV pili-dependent motility and starvation-induced fruiting body formation. Proteins of two-component systems including the orphan DNA binding response regulator DigR are essential in regulating the composition of the extracellular matrix. We identify the orphan hybrid histidine kinase SgmT as the partner kinase of DigR. In addition to kinase and receiver domains, SgmT consists of an N-terminal GAF domain and a C-terminal GGDEF domain. The GAF domain is the primary sensor domain. The GGDEF domain binds the second messenger bis-(3′-5′)-cyclic-dimeric-GMP (c-di-GMP) and functions as a c-di-GMP receptor to spatially sequester SgmT. We identify the DigR binding site in the promoter of the fibA gene, which encodes an abundant extracellular matrix metalloprotease. Whole-genome expression profiling experiments in combination with the identified DigR binding site allowed the identification of the DigR regulon and suggests that SgmT/DigR regulates the expression of genes for secreted proteins and enzymes involved in secondary metabolite synthesis. We suggest that SgmT/DigR regulates extracellular matrix composition and that SgmT activity is regulated by two sensor domains with ligand binding to the GAF domain resulting in SgmT activation and c-di-GMP binding to the GGDEF domain resulting in spatial sequestration of SgmT.

Understanding parasite transmission through imaging approaches

Unicellular parasites are of high medical relevance as they cause such devastating diseases as malaria or sleeping sickness. Besides the search for improved treatments, research on these parasites is valuable as they constitute interesting model cells to study basic processes of life. They can also serve as valuable reality checks for our presumed understanding of biological processes that emerge from the study of human or yeast cells, as our common ancestor with many parasites is much older than the one with yeast. But working with parasites can be tricky and time-consuming, if not outright impossible. Here, we focus on examples from imaging studies investigating the transmission of the malaria parasite. Achieving an understanding of the processes important for malaria transmission necessitates different imaging approaches and new molecular and material technologies. The discussed techniques will include in vivo imaging of pathogens in living animals, screening methodologies, and new materials as surrogate 3D environments.

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.

Uncovering the mystery of gliding motility in the myxobacteria

Bacterial gliding motility is the smooth movement of cells on solid surfaces unaided by flagella or pili. Many diverse groups of bacteria exhibit gliding, but the mechanism of gliding motility has remained a mystery since it was first observed more than a century ago. Recent studies on the motility of Myxococcus xanthus, a soil myxobacterium, suggest a likely mechanism for gliding in this organism. About forty M. xanthus genes were shown to be involved in gliding motility, and some of their protein products were labeled and localized within cells. These studies suggest that gliding motility in M. xanthus involves large multiprotein structural complexes, regulatory proteins, and cytoskeletal filaments. In this review, we summarize recent experiments that provide the basis for this emerging view of M. xanthus motility. We also discuss alternative models for gliding.

Emergence and modular evolution of a novel motility machinery in bacteria

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.

Mutations in Flavobacterium johnsoniae sprE result in defects in gliding motility and protein secretion

Cells of the gliding bacterium Flavobacterium johnsoniae move rapidly over surfaces. Transposon mutagenesis was used to identify sprE, which is involved in gliding. Mutations in sprE resulted in the formation of nonspreading colonies on agar. sprE mutant cells in wet mounts were almost completely deficient in attachment to and movement on glass, but a small percentage of cells exhibited slight movements, indicating that the motility machinery was not completely disrupted. SprE is a predicted lipoprotein with a tetratricopeptide repeat domain. SprE is similar in sequence to Porphyromonas gingivalis PorW, which is required for secretion of gingipain protease virulence factors. Disruption of F. johnsoniae sprE resulted in decreased extracellular chitinase activity and decreased secretion of the cell surface motility protein SprB. Reduced secretion of cell surface components of the gliding machinery, such as SprB, may account for the defects in gliding. Orthologs of sprE are found in many gliding and nongliding members of the phylum Bacteroidetes, suggesting that similar protein secretion systems are common among members of this large and diverse group of bacteria.

Endemic social diversity within natural kin groups of a cooperative bacterium

The spatial structure of genetic diversity underlying social variation is a critical determinant of how cooperation and conflict evolve. Here we investigated whether natural social groups of the cooperative soil bacterium Myxococcus xanthus harbor internal genetic and phenotypic variation and thus the potential for social conflict between interacting cells. Ten M. xanthus fruiting bodies isolated from soil were surveyed for variation in multiple social phenotypes and genetic loci, and patterns of diversity within and across fruiting body groups were examined. Eight of the 10 fruiting bodies were found to be internally diverse, with four exhibiting significant variation in social swarming phenotypes and five harboring large variation in the number of spores produced by member clones in pure culture. However, genetic variation within fruiting bodies was much lower than across fruiting bodies, suggesting that migration across even spatially proximate groups is limited relative to mutational generation of persisting endemic diversity. Our results simultaneously highlight the potential for social conflict within Myxococcus social groups and the possibility of social coevolution among diverse related lineages that are clustered in space and cotransmitted across generations.

Close encounters: contact-dependent interactions in bacteria

Bacterial cells interact extensively within and between species. These interactions can be divided into those that rely on diffusible factors and those that depend on direct cell-to-cell contacts. An example of a contact-dependent interaction is the transfer of lipoproteins between Myxococcus xanthus cells that leads to transient stimulation of motility in certain motility mutants. In this issue of Molecular Microbiology, Wei et al. (2011) provide mechanistic insights into this contact-dependent transfer of lipoproteins. Briefly, a heterologous protein fused to a type II (lipoprotein) signal sequence that targets the protein to the outer membrane is required and sufficient for transfer. Moreover, evidence is provided that transfer may depend on specific contacts between donor and recipient cells. The data demonstrate that lipoprotein transfer in M. xanthus is not restricted to a few odd motility proteins but could be a wide-spread phenomenon in M. xanthus and possibly other bacteria. Recent years have been fruitful in identifying contact-dependent interactions between bacterial cells. These interactions can be grouped into those that involve delivery of cargo to a recipient and those that seem to be involved in cell-to-cell signalling. Several contact-dependent interactions involve widely conserved proteins, suggesting that cell contact-dependent processes may be widespread among bacteria.