The multimodality cell segmentation challenge: toward universal solutions

Cell segmentation is a critical step for quantitative single-cell analysis in microscopy images. Existing cell segmentation methods are often tailored to specific modalities or require manual interventions to specify hyper-parameters in different experimental settings. Here, we present a multimodality cell segmentation benchmark, comprising more than 1,500 labeled images derived from more than 50 diverse biological experiments. The top participants developed a Transformer-based deep-learning algorithm that not only exceeds existing methods but can also be applied to diverse microscopy images across imaging platforms and tissue types without manual parameter adjustments. This benchmark and the improved algorithm offer promising avenues for more accurate and versatile cell analysis in microscopy imaging.

Multi-scale dynamic imaging reveals that cooperative motility behaviors promote efficient predation in bacteria

Many species, such as fish schools or bird flocks, rely on collective motion to forage, prey, or escape predators. Likewise, Myxococcus xanthus forages and moves collectively to prey and feed on other bacterial species. These activities require two distinct motility machines enabling adventurous (A) and social (S) gliding, however when and how these mechanisms are used has remained elusive. Here, we address this long-standing question by applying multiscale semantic cell tracking during predation. We show that: (1) foragers and swarms can comprise A- and S-motile cells, with single cells exchanging frequently between these groups; (2) A-motility is critical to ensure the directional movement of both foragers and swarms; (3) the combined action of A- and S-motile cells within swarms leads to increased predation efficiencies. These results challenge the notion that A- and S-motilities are exclusive to foragers and swarms, and show that these machines act synergistically to enhance predation efficiency.

Qualitative and Quantitative Methods to Measure Antibacterial Activity Resulting from Bacterial Competition

In the environment, bacteria compete for niche occupancy and resources; they have, therefore, evolved a broad variety of antibacterial weapons to destroy competitors. Current laboratory techniques to evaluate antibacterial activity are usually labor intensive, low throughput, costly, and time consuming. Typical assays rely on the outgrowth of colonies of prey cells on selective solid media after competition. Here, we present fast, inexpensive, and complementary optimized protocols to qualitatively and quantitively measure antibacterial activity. The first method is based on the degradation of a cell-impermeable chromogenic substrate of the β-galactosidase, a cytoplasmic enzyme released during lysis of the attacked reporter strain. The second method relies on the lag time required for the attacked cells to reach a defined optical density after the competition, which is directly dependent on the initial number of surviving cells. Key features First method utilizes the release of β-galactosidase as a proxy for bacterial lysis. Second method is based on the growth timing of surviving cells. Combination of two methods discriminates between cell death and lysis, cell death without lysis, or survival to quasi-lysis. Methods optimized to various bacterial species such as Escherichia coli, Pseudomonas aeruginosa, and Myxococcus xanthus. Graphical overview.

Grafted chitosan thin films of various degrees of acetylation as a reusable platform for the investigation of biological interactions

Surface treatment by adhesive polymers is a promising solution to immobilize and study bacteria cells through microscopic assays and, for example, control their growth or determine their susceptibility to antibiotic treatment. The stability of such functional films in wet conditions is crucial, as the film degradation would compromise a persistent use of the coated devices. In this work, low roughness chitosan thin films of degrees of acetylation (DA) ranging from 0.5 % to 49 % were chemically grafted onto silicon and glass substrates and we have demonstrated how the physicochemical properties of the surfaces and the bacterial response were DA-dependent. A fully deacetylated chitosan film presented an anhydrous crystalline structure while the hydrated crystalline allomorph was the preferred structure at higher DA. Moreover, their hydrophilicity increased at higher DA, leading to higher film swelling. Low DA chitosan-grafted substrate favored bacterial growth away from the surface and could be envisioned as bacteriostatic surfaces. Contrarily, an optimum of Escherichia coli adhesion was found for substrates modified with chitosan of DA = 35 %: these surfaces are adapted for the study of bacterial growth and antibiotic testing, with the possibility of reusing the substrates without affecting the grafted film - ideal for limiting single-use devices.

Unorthodox regulation of the MglA Ras-like GTPase controlling polarity in Myxococcus xanthus

Motile cells have developed a large array of molecular machineries to actively change their direction of movement in response to spatial cues from their environment. In this process, small GTPases act as molecular switches and work in tandem with regulators and sensors of their guanine nucleotide status (GAP, GEF, GDI and effectors) to dynamically polarize the cell and regulate its motility. In this review, we focus on Myxococcus xanthus as a model organism to elucidate the function of an atypical small Ras GTPase system in the control of directed cell motility. M. xanthus cells direct their motility by reversing their direction of movement through a mechanism involving the redirection of the motility apparatus to the opposite cell pole. The reversal frequency of moving M. xanthus cells is controlled by modular and interconnected protein networks linking the chemosensory-like frizzy (Frz) pathway - that transmits environmental signals - to the downstream Ras-like Mgl polarity control system - that comprises the Ras-like MglA GTPase protein and its regulators. Here, we discuss how variations in the GTPase interactome landscape underlie single-cell decisions and consequently, multicellular patterns.

Unmasking of the von Willebrand A-domain surface adhesin CglB at bacterial focal adhesions mediates myxobacterial gliding motility

The predatory deltaproteobacterium Myxococcus xanthus uses a helically-trafficked motor at bacterial focal-adhesion (bFA) sites to power gliding motility. Using total internal reflection fluorescence and force microscopies, we identify the von Willebrand A domain-containing outer-membrane (OM) lipoprotein CglB as an essential substratum-coupling adhesin of the gliding transducer (Glt) machinery at bFAs. Biochemical and genetic analyses reveal that CglB localizes to the cell surface independently of the Glt apparatus; once there, it is recruited by the OM module of the gliding machinery, a heteroligomeric complex containing the integral OM β barrels GltA, GltB, and GltH, as well as the OM protein GltC and OM lipoprotein GltK. This Glt OM platform mediates the cell-surface accessibility and retention of CglB by the Glt apparatus. Together, these data suggest that the gliding complex promotes regulated surface exposure of CglB at bFAs, thus explaining the manner by which contractile forces exerted by inner-membrane motors are transduced across the cell envelope to the substratum.

FrzS acts as a polar beacon to recruit SgmX, a central activator of type IV pili during Myxococcus xanthus motility

In rod-shaped bacteria, type IV pili (Tfp) promote twitching motility by assembling and retracting at the cell pole. In Myxococcus xanthus, a bacterium that moves in highly coordinated cell groups, Tfp are activated by a polar activator protein, SgmX. However, while it is known that the Ras-like protein MglA is required for unipolar targeting, how SgmX accesses the cell pole to activate Tfp is unknown. Here, we demonstrate that a polar beacon protein, FrzS, recruits SgmX at the cell pole. We identified two main functional domains, including a Tfp-activating domain and a polar-binding domain. Within the latter, we show that the direct binding of MglA-GTP unveils a hidden motif that binds directly to the FrzS N-terminal response regulator (CheY). Structural analyses reveal that this binding occurs through a novel binding interface for response regulator domains. In conclusion, the findings unveil the protein interaction network leading to the spatial activation of Tfp at the cell pole. This tripartite system is at the root of complex collective behaviours in this predatory bacterium.

1H, 13C and 15N chemical shift assignments of the ZnR and GYF cytoplasmic domains of the GltJ protein from Myxococcus xanthus

Bacterial cell motility is essential for a range of physiological phenomena such as nutrient sensing, predation, biofilm formation and pathogenesis. One of the most intriguing motilities is bacterial gliding, which is defined as the ability of some bacteria to move across surfaces without an external appendage. In Myxococcus xanthus, gliding motility depends on the assembly of focal adhesion complexes (FAC) which include the Glt mutiprotein complex and allow directional movement of individual cells (A-motility). Within the Glt multiprotein complex, GltJ is one of the key proteins involved in FAC assembly. In this work we report complete backbone and side chain ¹H, ¹³C and ¹⁵N chemical shifts of the two cytoplasmic domains of GltJ, GltJ-ZnR (BMRB No. 51104) and GltJ-GYF (BMRB No. 51096). These data provide the first step toward the first high resolution structures of protein domains from the Glt machinery and the atomic level characterization of GltJ cytoplasmic activity during FAC assembly.

Dynamic proton-dependent motors power type IX secretion and gliding motility in Flavobacterium

Motile bacteria usually rely on external apparatus like flagella for swimming or pili for twitching. By contrast, gliding bacteria do not rely on obvious surface appendages to move on solid surfaces. Flavobacterium johnsoniae and other bacteria in the Bacteroidetes phylum use adhesins whose movement on the cell surface supports motility. In F. johnsoniae, secretion and helicoidal motion of the main adhesin SprB are intimately linked and depend on the type IX secretion system (T9SS). Both processes necessitate the proton motive force (PMF), which is thought to fuel a molecular motor that comprises the GldL and GldM cytoplasmic membrane proteins. Here, we show that F. johnsoniae gliding motility is powered by the pH gradient component of the PMF. We further delineate the interaction network between the GldLM transmembrane helices (TMHs) and show that conserved glutamate residues in GldL TMH2 are essential for gliding motility, although having distinct roles in SprB secretion and motion. We then demonstrate that the PMF and GldL trigger conformational changes in the GldM periplasmic domain. We finally show that multiple GldLM complexes are distributed in the membrane, suggesting that a network of motors may be present to move SprB along a helical path on the cell surface. Altogether, our results provide evidence that GldL and GldM assemble dynamic membrane channels that use the proton gradient to power both T9SS-dependent secretion of SprB and its motion at the cell surface.

Motile bacteria usually rely on external apparatus like flagella or pili, but gliding bacteria do not rely on obvious surface appendages for their movement. This study shows that bacteria in the phylum Bacteroidetes use proton-dependent motors to power protein secretion and gliding motility.

A Tad-like apparatus is required for contact-dependent prey killing in predatory social bacteria

Myxococcus xanthus, a soil bacterium, predates collectively using motility to invade prey colonies. Prey lysis is mostly thought to rely on secreted factors, cocktails of antibiotics and enzymes, and direct contact with Myxococcus cells. In this study, we show that on surfaces the coupling of A-motility and contact-dependent killing is the central predatory mechanism driving effective prey colony invasion and consumption. At the molecular level, contact-dependent killing involves a newly discovered type IV filament-like machinery (Kil) that both promotes motility arrest and prey cell plasmolysis. In this process, Kil proteins assemble at the predator-prey contact site, suggesting that they allow tight contact with prey cells for their intoxication. Kil-like systems form a new class of Tad-like machineries in predatory bacteria, suggesting a conserved function in predator-prey interactions. This study further reveals a novel cell-cell interaction function for bacterial pili-like assemblages.

Misic, a general deep learning-based method for the high-throughput cell segmentation of complex bacterial communities

Studies of bacterial communities, biofilms and microbiomes, are multiplying due to their impact on health and ecology. Live imaging of microbial communities requires new tools for the robust identification of bacterial cells in dense and often inter-species populations, sometimes over very large scales. Here, we developed MiSiC, a general deep-learning-based 2D segmentation method that automatically segments single bacteria in complex images of interacting bacterial communities with very little parameter adjustment, independent of the microscopy settings and imaging modality. Using a bacterial predator-prey interaction model, we demonstrate that MiSiC enables the analysis of interspecies interactions, resolving processes at subcellular scales and discriminating between species in millimeter size datasets. The simple implementation of MiSiC and the relatively low need in computing power make its use broadly accessible to fields interested in bacterial interactions and cell biology.

Complete Genome Assembly of Myxococcus xanthus Strain DZ2 Using Long High-Fidelity (HiFi) Reads Generated with PacBio Technology

Myxococcus xanthus is a Gram-negative social bacterium belonging to the order Myxococcales of the class Deltaproteobacteria. It is a facultative social predator found in soils across the globe and is thought to be crucial for the microbial ecosystem. Here, we report a complete high-quality reference genome of the M. xanthus strain DZ2.

Linking single-cell decisions to collective behaviours in social bacteria

Social bacteria display complex behaviours whereby thousands of cells collectively and dramatically change their form and function in response to nutrient availability and changing environmental conditions. In this review, we focus on Myxococcus xanthus motility, which supports spectacular transitions based on prey availability across its life cycle. A large body of work suggests that these behaviours require sensory capacity implemented at the single-cell level. Focusing on recent genetic work on a core cellular pathway required for single-cell directional decisions, we argue that signal integration, multi-modal sensing and memory are at the root of decision making leading to multicellular behaviours. Hence, Myxococcus may be a powerful biological system to elucidate how cellular building blocks cooperate to form sensory multicellular assemblages, a possible origin of cognitive mechanisms in biological systems. This article is part of the theme issue 'Basal cognition: conceptual tools and the view from the single cell'.

Modulation of bacterial multicellularity via spatio-specific polysaccharide secretion

The development of multicellularity is a key evolutionary transition allowing for differentiation of physiological functions across a cell population that confers survival benefits; among unicellular bacteria, this can lead to complex developmental behaviors and the formation of higher-order community structures. Herein, we demonstrate that in the social δ-proteobacterium Myxococcus xanthus, the secretion of a novel biosurfactant polysaccharide (BPS) is spatially modulated within communities, mediating swarm migration as well as the formation of multicellular swarm biofilms and fruiting bodies. BPS is a type IV pilus (T4P)-inhibited acidic polymer built of randomly acetylated β-linked tetrasaccharide repeats. Both BPS and exopolysaccharide (EPS) are produced by dedicated Wzx/Wzy-dependent polysaccharide-assembly pathways distinct from that responsible for spore-coat assembly. While EPS is preferentially produced at the lower-density swarm periphery, BPS production is favored in the higher-density swarm interior; this is consistent with the former being known to stimulate T4P retraction needed for community expansion and a function for the latter in promoting initial cell dispersal. Together, these data reveal the central role of secreted polysaccharides in the intricate behaviors coordinating bacterial multicellularity.

A study of the social bacterium Myxococcus xanthus reveals that the bacteria preferentially secrete specific polysaccharides within distinct zones of a swarm to facilitate spreading across a surface.

A divergent CheW confers plasticity to nucleoid-associated chemosensory arrays

Chemosensory systems are highly organized signaling pathways that allow bacteria to adapt to environmental changes. The Frz chemosensory system from M. xanthus possesses two CheW-like proteins, FrzA (the core CheW) and FrzB. We found that FrzB does not interact with FrzE (the cognate CheA) as it lacks the amino acid region responsible for this interaction. FrzB, instead, acts upstream of FrzCD in the regulation of M. xanthus chemotaxis behaviors and activates the Frz pathway by allowing the formation and distribution of multiple chemosensory clusters on the nucleoid. These results, together, show that the lack of the CheA-interacting region in FrzB confers new functions to this small protein.

Dynamic polarity control by a tunable protein oscillator in bacteria

In bacteria, cell polarization involves the controlled targeting of specific proteins to the poles, defining polar identity and function. How a specific protein is targeted to one pole and what are the processes that facilitate its dynamic relocalization to the opposite pole is still unclear. The Myxococcus xanthus polarization example illustrates how the dynamic and asymmetric localization of polar proteins enable a controlled and fast switch of polarity. In M. xanthus, the opposing polar distribution of the small GTPase MglA and its cognate activating protein MglB defines the direction of movement of the cell. During a reversal event, the switch of direction is triggered by the Frz chemosensory system, which controls polarity reversals through a so-called gated relaxation oscillator. In this review, we discuss how this genetic architecture can provoke sharp behavioral transitions depending on Frz activation levels, which is central to multicellular behaviors in this bacterium.

Live nanoscopic to mesoscopic topography reconstruction with an optical microscope for chemical and biological samples

Macroscopic properties of physical and biological processes like friction, wetting, and adhesion or cell migration are controlled by interfacial properties at the nanoscopic scale. In an attempt to bridge simultaneously investigations at different scales, we demonstrate here how optical microscopy in Wet-Surface Ellipsometric Enhanced Contrast (Wet-SEEC) mode offers imaging and measurement of thin films at solid/liquid interfaces in the range 1-500 nm with lateral optical resolution. A live, label-free and noninvasive methodology integrated with microfluidic devices allowed here characterization of polymers and proteins patterns together with corresponding phenotypes of living cells.

MotAB-like machinery drives the movement of MreB filaments during bacterial gliding motility

MreB is a bacterial actin that is important for cell shape and cell wall biosynthesis in many bacterial species. MreB also plays crucial roles in Myxococcus xanthus gliding motility, but the underlying mechanism remains unknown. Here we tracked the dynamics of single MreB particles in M. xanthus using single-particle tracking photoactivated localization microscopy. We found that a subpopulation of MreB particles moves rapidly along helical trajectories, similar to the movements of the MotAB-like gliding motors. The rapid MreB motion was stalled in the mutants that carried truncated gliding motors. Remarkably, M. xanthus MreB moves one to two orders of magnitude faster than its homologs that move along with the cell wall synthesis machinery in Bacillus subtilis and Escherichia coli, and this rapid movement was not affected by the inhibitors of cell wall biosynthesis. Our results show that in M. xanthus, MreB provides a scaffold for the gliding motors while the gliding machinery drives the movement of MreB filaments, analogous to the interdependent movements of myosin motors and actin in eukaryotic cells.

Colony analysis and deep learning uncover 5-hydroxyindole as an inhibitor of gliding motility and iridescence in Cellulophaga lytica

Iridescence is an original type of colouration that is relatively widespread in nature but has been either incompletely described or entirely neglected in prokaryotes. Recently, we reported a brilliant 'pointillistic' iridescence in agar-grown colony biofilms of Cellulophaga lytica and some other marine Flavobacteria that exhibit gliding motility. Bacterial iridescence is created by a unique self-organization of sub-communities of cells, but the mechanisms underlying such living photonic crystals are unknown. In this study, we used Petri dish assays to screen a large panel of potential activators or inhibitors of C. lytica's iridescence. Derivatives potentially interfering with quorum-sensing and other communication or biofilm formation processes were tested, as well as metabolic poisons or algal exoproducts. We identified an indole derivative, 5-hydroxyindole (5HI, 250 µM) which inhibited both gliding and iridescence at the colonial level. 5HI did not affect growth or cell respiration. At the microscopic level, phase-contrast imaging confirmed that 5HI inhibits the gliding motility of cells. Moreover, the lack of iridescence correlated with a perturbation of self-organization of the cell sub-communities in both the WT and a gliding-negative mutant. This effect was proved using recent advances in machine learning (deep neuronal networks). In addition to its effect on colony biofilms, 5HI was found to stimulate biofilm formation in microplates. Our data are compatible with possible roles of 5HI or marine analogues in the eco-biology of iridescent bacteria.

The nucleoid as a scaffold for the assembly of bacterial signaling complexes

The FrzCD chemoreceptor from the gliding bacterium Myxococcus xanthus forms cytoplasmic clusters that occupy a large central region of the cell body also occupied by the nucleoid. In this work, we show that FrzCD directly binds to the nucleoid with its N-terminal positively charged tail and recruits active signaling complexes at this location. The FrzCD binding to the nucleoid occur in a DNA-sequence independent manner and leads to the formation of multiple distributed clusters that explore constrained areas. This organization might be required for cooperative interactions between clustered receptors as observed in membrane-bound chemosensory arrays.

New insights into the function of a versatile class of membrane molecular motors from studies of Myxococcus xanthus surface (gliding) motility

Cell motility is a central function of living cells, as it empowers colonization of new environmental niches, cooperation, and development of multicellular organisms. This process is achieved by complex yet precise energy-consuming machineries in both eukaryotes and bacteria. Bacteria move on surfaces using extracellular appendages such as flagella and pili but also by a less-understood process called gliding motility. During this process, rod-shaped bacteria move smoothly along their long axis without any visible morphological changes besides occasional bending. For this reason, the molecular mechanism of gliding motility and its origin have long remained a complete mystery. An important breakthrough in the understanding of gliding motility came from single cell and genetic studies in the delta-proteobacterium Myxococcus xanthus. These early studies revealed, for the first time, the existence of bacterial Focal Adhesion complexes (FA). FAs are formed at the bacterial pole and rapidly move towards the opposite cell pole. Their attachment to the underlying surface is linked to cell propulsion, in a process similar to the rearward translocation of actomyosin complexes in Apicomplexans. The protein machinery that forms at FAs was shown to contain up to seventeen proteins predicted to localize in all layers of the bacterial cell envelope, the cytosolic face, the inner membrane (IM), the periplasmic space and the outer membrane (OM). Among these proteins, a proton-gated channel at the inner membrane was identified as the molecular motor. Thus, thrust generation requires the transduction of traction forces generated at the inner membrane through the cell envelope beyond the rigid barrier of the bacterial peptidoglycan.

Acylation of the Type 3 Secretion System Translocon Using a Dedicated Acyl Carrier Protein

Bacterial pathogens often deliver effectors into host cells using type 3 secretion systems (T3SS), the extremity of which forms a translocon that perforates the host plasma membrane. The T3SS encoded by Salmonella pathogenicity island 1 (SPI-1) is genetically associated with an acyl carrier protein, IacP, whose role has remained enigmatic. In this study, using tandem affinity purification, we identify a direct protein-protein interaction between IacP and the translocon protein SipB. We show, by mass spectrometry and radiolabelling, that SipB is acylated, which provides evidence for a modification of the translocon that has not been described before. A unique and conserved cysteine residue of SipB is identified as crucial for this modification. Although acylation of SipB was not essential to virulence, we show that this posttranslational modification promoted SipB insertion into host-cell membranes and pore-forming activity linked to the SPI-1 T3SS. Cooccurrence of acyl carrier and translocon proteins in several γ- and β-proteobacteria suggests that acylation of the translocon is conserved in these other pathogenic bacteria. These results also indicate that acyl carrier proteins, known for their involvement in metabolic pathways, have also evolved as cofactors of new bacterial protein lipidation pathways.

Acyl carrier proteins are small ubiquitous proteins involved in the synthesis of hydrocarbon based molecules. Notably, they are essential for the synthesis of fatty acids, which are the precursors of membrane phospholipids. They can also be involved in secondary metabolism, for example for the synthesis of molecules with antibacterial properties. Although acyl carrier proteins are widespread, the specific role of each individual protein seems comparatively poorly explored. In this study, we investigate the role of an acyl carrier protein genetically associated with a type 3 secretion system (T3SS). Many Gram-negative bacterial pathogens use T3SS to deliver effectors directly into the cytoplasm of eukaryotic host cells and to subvert host cellular pathways. For this purpose, the translocon, which is the terminal part of T3SS, forms a pore inserted into the host-cell membrane. Here we show that the acyl carrier protein associated with the T3SS has specialized to allow acylation of the translocon. The novel posttranslational modification of the translocon that we describe optimizes insertion into the host-cell membrane and pore-forming activity. This mechanism is likely to be conserved in other pathogenic bacteria given the conserved genetic association between T3SS and acyl carrier protein in several bacteria.

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.

A unique self-organization of bacterial sub-communities creates iridescence in Cellulophaga lytica colony biofilms

Iridescent color appearances are widespread in nature. They arise from the interaction of light with micron- and submicron-sized physical structures spatially arranged with periodic geometry and are usually associated with bright angle-dependent hues. Iridescence has been reported for many animals and marine organisms. However, iridescence has not been well studied in bacteria. Recently, we reported a brilliant "pointillistic" iridescence in colony biofilms of marine Flavobacteria that exhibit gliding motility. The mechanism of their iridescence is unknown. Here, using a multi-disciplinary approach, we show that the cause of iridescence is a unique periodicity of the cell population in the colony biofilm. Cells are arranged together to form hexagonal photonic crystals. Our model highlights a novel pattern of self-organization in a bacterial biofilm. "Pointillistic" bacterial iridescence can be considered a new light-dependent phenomenon for the field of microbiology.

Single-Cell Analysis of Growth and Cell Division of the Anaerobe Desulfovibrio vulgaris Hildenborough

Recent years have seen significant progress in understanding basic bacterial cell cycle properties such as cell growth and cell division. While characterization and regulation of bacterial cell cycle is quite well-documented in the case of fast growing aerobic model organisms, no data has been so far reported for anaerobic bacteria. This lack of information in anaerobic microorganisms can mainly be explained by the absence of molecular and cellular tools such as single cell microscopy and fluorescent probes usable for anaerobes and essential to study cellular events and/or subcellular localization of the actors involved in cell cycle. In this study, single-cell microscopy has been adapted to study for the first time, in real time, the cell cycle of a bacterial anaerobe, Desulfovibrio vulgaris Hildenborough (DvH). This single-cell analysis provides mechanistic insights into the cell division cycle of DvH, which seems to be governed by the recently discussed so-called incremental model that generates remarkably homogeneous cell sizes. Furthermore, cell division was reversibly blocked during oxygen exposure. This may constitute a strategy for anaerobic cells to cope with transient exposure to oxygen that they may encounter in their natural environment, thereby contributing to their aerotolerance. This study lays the foundation for the first molecular, single-cell assay that will address factors that cannot otherwise be resolved in bulk assays and that will allow visualization of a wide range of molecular mechanisms within living anaerobic cells.

Crises psychogènes non épileptiques : une maladie émotionnelle ?

Les crises psychogènes non épileptiques (CPNE) sont définies comme des changements brutaux et paroxystiques du comportement moteur, des sensations ou de la conscience qui ressemblent à des crises épileptiques. Ces crises sont en rapport avec des processus psychogènes complexes et non avec une décharge neuronale excessive. Les CPNE sont une pathologie fréquente, coûteuse et grave dont l’incidence équivaut à celle de la sclérose en plaques. Pourtant, cette pathologie reste peu et mal connue des psychiatres. Beaucoup d’entre nous pensent que ces crises ont disparu après Charcot. En effet, actuellement, ces patients sont adressés aux neurologues. Un quart des patients vus par un épileptologue souffrent de CPNE. L’étiopathogénie est multifactorielle, comprenant des facteurs pédisposants, des facteurs déclenchants et des facteurs de maintien. Deux mécanismes principaux apparaissent : une prédisposition neurobiologique et des facteurs traumatiques inducteurs de processus dissociatif. Les modèles récents placent la dysrégulation émotionnelle au centre de la problématique. L’imagerie fonctionnelle a mis en évidence des anomalies fonctionnelles entre les aires cérébrales impliquées dans les émotions et les aires motrices. Il existe aussi des perturbations des 3 composantes émotionnelles : sur le plan cognitif avec une alexithymie chez 85 % des patients et des biais attentionnels pour les stimuli négatifs ; sur le plan comportemental avec une réponse d’évitement et sur le plan physiologique avec des taux de cortisol basal augmentés et une diminution de l’arythmie sinusale. Nous exposerons les résultats de notre étude comparative prospective inédite qui compare la réponse électrodermale à des stimuli visuels entre un groupe de patientes CPNE et de témoins sains appariés. Nous avons mis en évidence des perturbations de la réponse neuro-végétative avec une hyporéceptivité aux stimuli plaisants et une corrélation négative entre la propension à la dissociation et l’amplitude de la réponse électrodermale. Ces résultats ouvrent des perspectives en termes de compréhension des mécanismes mais aussi en termes diagnostique et thérapeutique pour développer des outils psychothérapiques de rééducation émotionnelle chez les patients souffrant de CPNE.

The mysterious nature of bacterial surface (gliding) motility: A focal adhesion-based mechanism in Myxococcus xanthus

Motility of bacterial cells promotes a range of important physiological phenomena such as nutrient detection, harm avoidance, biofilm formation, and pathogenesis. While much research has been devoted to the mechanism of bacterial swimming in liquid via rotation of flagellar filaments, the mechanisms of bacterial translocation across solid surfaces are poorly understood, particularly when cells lack external appendages such as rotary flagella and/or retractile type IV pili. Under such limitations, diverse bacteria at the single-cell level are still able to "glide" across solid surfaces, exhibiting smooth translocation of the cell along its long axis. Though multiple gliding mechanisms have evolved in different bacterial classes, most remain poorly characterized. One exception is the gliding motility mechanism used by the Gram-negative social predatory bacterium Myxococcus xanthus. The available body of research suggests that M. xanthus gliding motility is mediated by trafficked multi-protein (Glt) cell envelope complexes, powered by proton-driven flagellar stator homologues (Agl). Through coupling to the substratum via polysaccharide slime, Agl-Glt assemblies can become fixed relative to the substratum, forming a focal adhesion site. Continued directional transport of slime-associated substratum-fixed Agl-Glt complexes would result in smooth forward movement of the cell. In this review, we have provided a comprehensive synthesis of the latest mechanistic and structural data for focal adhesion-mediated gliding motility in M. xanthus, with emphasis on the role of each Agl and Glt protein. Finally, we have also highlighted the possible connection between the motility complex and a new type of spore coat assembly system, suggesting that gliding and cell envelope synthetic complexes are evolutionarily linked.

Evolution and Design Governing Signal Precision and Amplification in a Bacterial Chemosensory Pathway

Understanding the principles underlying the plasticity of signal transduction networks is fundamental to decipher the functioning of living cells. In Myxococcus xanthus, a particular chemosensory system (Frz) coordinates the activity of two separate motility systems (the A- and S-motility systems), promoting multicellular development. This unusual structure asks how signal is transduced in a branched signal transduction pathway. Using combined evolution-guided and single cell approaches, we successfully uncoupled the regulations and showed that the A-motility regulation system branched-off an existing signaling system that initially only controlled S-motility. Pathway branching emerged in part following a gene duplication event and changes in the circuit structure increasing the signaling efficiency. In the evolved pathway, the Frz histidine kinase generates a steep biphasic response to increasing external stimulations, which is essential for signal partitioning to the motility systems. We further show that this behavior results from the action of two accessory response regulator proteins that act independently to filter and amplify signals from the upstream kinase. Thus, signal amplification loops may underlie the emergence of new connectivity in signal transduction pathways.

The small G-protein MglA connects to the MreB actin cytoskeleton at bacterial focal adhesions

In Myxococcus xanthus the gliding motility machinery is assembled at the leading cell pole to form focal adhesions, translocated rearward to propel the cell, and disassembled at the lagging pole. We show that MglA, a Ras-like small G-protein, is an integral part of this machinery. In this function, MglA stimulates the assembly of the motility complex by directly connecting it to the MreB actin cytoskeleton. Because the nucleotide state of MglA is regulated spatially and MglA only binds MreB in the guanosine triphosphate-bound form, the motility complexes are assembled at the leading pole and dispersed at the lagging pole where the guanosine triphosphatase activating protein MglB disrupts the MglA-MreB interaction. Thus, MglA acts as a nucleotide-dependent molecular switch to regulate the motility machinery spatially. The function of MreB in motility is independent of its function in peptidoglycan synthesis, representing a coopted function. Our findings highlight a new function for the MreB cytoskeleton and suggest that G-protein-cytoskeleton interactions are a universally conserved feature.

A versatile class of cell surface directional motors gives rise to gliding motility and sporulation in Myxococcus xanthus

The Myxococcus Agl-Nfs machinery, a type of bacterial transport system, is modular and is seen to also rotate a carbohydrate polymer directionally at the spore surface to assist spore coat assembly.

Eukaryotic cells utilize an arsenal of processive transport systems to deliver macromolecules to specific subcellular sites. In prokaryotes, such transport mechanisms have only been shown to mediate gliding motility, a form of microbial surface translocation. Here, we show that the motility function of the Myxococcus xanthus Agl-Glt machinery results from the recent specialization of a versatile class of bacterial transporters. Specifically, we demonstrate that the Agl motility motor is modular and dissociates from the rest of the gliding machinery (the Glt complex) to bind the newly expressed Nfs complex, a close Glt paralogue, during sporulation. Following this association, the Agl system transports Nfs proteins directionally around the spore surface. Since the main spore coat polymer is secreted at discrete sites around the spore surface, its transport by Agl-Nfs ensures its distribution around the spore. Thus, the Agl-Glt/Nfs machineries may constitute a novel class of directional bacterial surface transporters that can be diversified to specific tasks depending on the cognate cargo and machinery-specific accessories.

Many living cells use processive cytoskeletal motors to transport proteins and subcellular organelles to specific subcellular sites. In bacteria, this type of transport has yet to be identified and it is generally thought that random protein collisions underlie most biochemical processes. In recent years, our view of the bacterial cell was changed by the discovery of subcellular compartmentalization and a cytoskeleton, suggesting that processive motors might also operate in prokaryotes. We previously characterized a mechanism of intracellular transport that drives cell motility across solid surfaces in the gram-negative bacterium Myxococcus xanthus. Since the transport apparatus was also found in bacterial species that do not move on surfaces, we postulated that intracellular transport underlies other cellular processes in bacteria. Indeed, we show here that the Myxococcus motility motor can be adapted to transport sporulation-specific proteins around the nascent spore surface. Because the transported proteins are linked to the main spore coat, this motion assists the assembly of a protective spore coat. In conclusion, the Myxococcus motility/sporulation transport machinery defines an emerging class of versatile transport systems, suggesting that processive transport has been overlooked and may well orchestrate many processes in bacteria.

Super-resolution imaging of bacteria in a microfluidics device

Bacteria have evolved complex, highly-coordinated, multi-component cellular engines to achieve high degrees of efficiency, accuracy, adaptability, and redundancy. Super-resolution fluorescence microscopy methods are ideally suited to investigate the internal composition, architecture, and dynamics of molecular machines and large cellular complexes. These techniques require the long-term stability of samples, high signal-to-noise-ratios, low chromatic aberrations and surface flatness, conditions difficult to meet with traditional immobilization methods. We present a method in which cells are functionalized to a microfluidics device and fluorophores are injected and imaged sequentially. This method has several advantages, as it permits the long-term immobilization of cells and proper correction of drift, avoids chromatic aberrations caused by the use of different filter sets, and allows for the flat immobilization of cells on the surface. In addition, we show that different surface chemistries can be used to image bacteria at different time-scales, and we introduce an automated cell detection and image analysis procedure that can be used to obtain cell-to-cell, single-molecule localization and dynamic heterogeneity as well as average properties at the super-resolution level.

Direct live imaging of cell-cell protein transfer by transient outer membrane fusion in Myxococcus xanthus

In bacteria, multicellular behaviors are regulated by cell-cell signaling through the exchange of both diffusible and contact-dependent signals. In a multicellular context, Myxococcus cells can share outer membrane (OM) materials by an unknown mechanism involving the traAB genes and gliding motility. Using live imaging, we show for the first time that transient contacts between two cells are sufficient to transfer OM materials, proteins and lipids, at high efficiency. Transfer was associated with the formation of dynamic OM tubes, strongly suggesting that transfer results from the local fusion of the OMs of two transferring cells. Last, large amounts of OM materials were released in slime trails deposited by gliding cells. Since cells tend to follow trails laid by other cells, slime-driven OM material exchange may be an important stigmergic regulation of Myxococcus social behaviors. DOI:http://dx.doi.org/10.7554/eLife.00868.001.

Imaging type VI secretion-mediated bacterial killing

In the environment, bacteria compete with each other for nutrient availability or to extend their ecological niche. The type VI secretion system contributes to bacterial competition by the translocation of antibacterial effectors from predators into prey cells. The T6SS assembles a dynamic structure-the sheath-wrapped around a tube constituted of the Hcp protein. It has been proposed that by cycling between extended and contracted conformations the sheath acts as a crossbow to propel the Hcp tube toward the target cell. While the sheath dynamics have been studied in monocultures, the activity of the T6SS has not been recorded in presence of the prey. Here, time-lapse fluorescence microscopy of cocultures demonstrates that prey cells are killed upon contact with predator cells. Additional experiments provide evidence that sheath contraction correlates with nearby cell fading and that prey lysis occurs within minutes after sheath contraction. The results support a model in which T6SS dynamics are responsible for T6SS effectors translocation into recipient cells.

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.

Glitter-like iridescence within the bacteroidetes especially Cellulophaga spp.: optical properties and correlation with gliding motility

Iridescence results from structures that generate color. Iridescence of bacterial colonies has recently been described and illustrated. The glitter-like iridescence class, created especially for a few strains of Cellulophaga lytica, exhibits an intense iridescence under direct illumination. Such color appearance effects were previously associated with other bacteria from the Bacteroidetes phylum, but without clear elucidation and illustration. To this end, we compared various bacterial strains to which the iridescent trait was attributed. All Cellulophaga species and additional Bacteroidetes strains from marine and terrestrial environments were investigated. A selection of bacteria, mostly marine in origin, were found to be iridescent. Although a common pattern of reflected wavelengths was recorded for the species investigated, optical spectroscopy and physical measurements revealed a range of different glitter-like iridescence intensity and color profiles. Importantly, gliding motility was found to be a common feature of all iridescent colonies. Dynamic analyses of “glitter” formation at the edges of C. lytica colonies showed that iridescence was correlated with layer superposition. Both gliding motility, and unknown cell-to-cell communication processes, may be required for the establishment, in time and space, of the necessary periodic structures responsible for the iridescent appearance of Bacteroidetes.

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.

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.

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.

A bacterial Ras-like small GTP-binding protein and its cognate GAP establish a dynamic spatial polarity axis to control directed motility

Directional control of bacterial motility is regulated by dynamic polarity inversions driven by pole-to-pole oscillation of a Ras family small G-protein and its associated GTPase-activating protein.

Regulated cell polarity is central to many cellular processes. We investigated the mechanisms that govern the rapid switching of cell polarity (reversals) during motility of the bacterium Myxococcus xanthus. Cellular reversals are mediated by pole-to-pole oscillations of motility proteins and the frequency of the oscillations is under the control of the Frz chemosensory system. However, the molecular mechanism that creates dynamic polarity remained to be characterized. In this work, we establish that polarization is regulated by the GTP cycle of a Ras-like GTPase, MglA. We initially sought an MglA regulator and purified a protein, MglB, which was found to activate GTP hydrolysis by MglA. Using live fluorescence microscopy, we show that MglA and MglB localize at opposite poles and oscillate oppositely when cells reverse. In absence of MglB, MglA-YFP accumulates at the lagging cell end, leading to a strikingly aberrant reversal cycle. Spatial control of MglA is achieved through the GAP activity of MglB because an MglA mutant that cannot hydrolyze GTP accumulates at the lagging cell end, despite the presence of MglB. Genetic and cell biological studies show that the MglA-GTP cycle controls dynamic polarity and the reversal switch. The study supports a model wherein a chemosensory signal transduction system (Frz) activates reversals by relieving a spatial inhibition at the back pole of the cells: reversals are allowed by Frz-activated switching of MglB to the opposite pole, allowing MglA-GTP to accumulate at the back of the cells and create the polarity switch. In summary, our results provide insight into how bacteria regulate their polarity dynamically, revealing unsuspected conserved regulations with eukaryots.

Motile cells have evolved complex regulatory networks to respond to environmental cues and change their direction of movement appropriately. In this process, an arsenal of receptor-coupled small G-proteins acts as a cellular compass to dynamically polarize the leading edge and regulate the motility response. However, the precise mechanism of action of these G-proteins in controlling bacteria movement on solid surfaces has remained an enigma. We investigate this process in Gram negative Myxococcus xanthus cells. Surprisingly, we find that the Ras-like small G-protein MglA polarizes the cell by accumulating at the leading cell pole in its active GTP-bound form. This localization is dependent on MglB, a GTPase-activating protein that converts MglA to its inactive form specifically at the opposite, lagging cell pole. Furthermore, we show that a receptor-coupled signal transduction cascade can activate re-localization of MglA and MglB at opposite poles in a synchronous manner, resulting in inversion of the polarity axis and cell movement in the opposite direction. Thus, a simple, eukaryote-like signaling module also governs dynamic polarity mechanisms in bacteria, demonstrating broader conservation of these signaling systems than initially suspected.

A microscope automated fluidic system to study bacterial processes in real time

Most time lapse microscopy experiments studying bacterial processes ie growth, progression through the cell cycle and motility have been performed on thin nutrient agar pads. An important limitation of this approach is that dynamic perturbations of the experimental conditions cannot be easily performed. In eukaryotic cell biology, fluidic approaches have been largely used to study the impact of rapid environmental perturbations on live cells and in real time. However, all these approaches are not easily applicable to bacterial cells because the substrata are in all cases specific and also because microfluidics nanotechnology requires a complex lithography for the study of micrometer sized bacterial cells. In fact, in many cases agar is the experimental solid substratum on which bacteria can move or even grow. For these reasons, we designed a novel hybrid micro fluidic device that combines a thin agar pad and a custom flow chamber. By studying several examples, we show that this system allows real time analysis of a broad array of biological processes such as growth, development and motility. Thus, the flow chamber system will be an essential tool to study any process that take place on an agar surface at the single cell level.

Active and passive mechanisms of intracellular transport and localization in bacteria

Spatial complexity is a hallmark of living organisms. All cells adopt specific shapes and organize their contents in such a way that makes possible fundamental tasks such as growth, metabolism, replication, and division. Although many of these tasks in bacteria have been studied extensively, only recently have we begun to understand the influence of spatial organization on cell function. Clearly, bacteria are highly organized cells where proteins do not simply diffuse in a 'cytoplasmic soup' to exert function but can also be localized to specific subcellular sites. In this review, we discuss whether such order can be achieved solely by diffusive capture mechanisms or if active intracellular transport systems are required.

Genetic circuitry controlling motility behaviors of Myxococcus xanthus

M. xanthus has a complex multicellular lifestyle including swarming, predation and development. These behaviors depend on the ability of the cells to achieve directed motility across solid surfaces. M. xanthus cells have evolved two motility systems including Type-IV pili that act as grappling hooks and a controversial engine involving mucus secretion and fixed focal adhesion sites. The necessity for cells to coordinate the motility systems and to respond rapidly to environmental cues is reflected by a complex genetic network involving at least three complete sets of chemosensory systems and eukaryotic-like signaling proteins. In this review, we discuss recent advances suggesting that motor synchronization results from spatial oscillations of motility proteins. We further propose that these dynamics are modulated by the action of multiple upstream complementary signaling systems. M. xanthus is thus an exciting emerging model system to study the intricate processes of directed cell migration.

The elusive engine in Myxococcus xanthus gliding motility

Bacterial motility is essential for chemotaxis, virulence and complex social interactions leading to biofilm and fruiting body formation. Although bacterial swimming in liquids with a flagellum is well understood, little is known regarding bacterial movements across solid surfaces. Gliding motility, one such mode of locomotion, has remained largely mysterious because cells move smoothly along their long axis in the absence of any visible organelle. In this review, I discuss recent evidence that focal adhesion systems mediate gliding motility in the social bacterium Myxococcus xanthus and combine this evidence with previous work to suggest a new working hypothesis inspired from knowledge in apicomplexan parasites. I then propose experimental directions to test the model and compare it to other pre-existing models. Finally, evidence on gliding mechanisms of selected organisms are presented to ask whether some features of the model have precedents in other bacteria and whether this complex biological process could be explained by a single mechanism or involves multiple distinct mechanisms.

Two localization motifs mediate polar residence of FrzS during cell movement and reversals of Myxococcus xanthus

Myxococcus xanthus utilizes two motility systems for surface locomotion: A-motility and S-motility. S-motility is mediated by extension and retraction of type IV pili. Cells exhibiting S-motility periodically reverse by switching the assembly of type IV pili from the old leading pole to the new leading pole. These cellular reversals involve regulated pole-to-pole oscillations of the FrzS protein. We constructed and characterized in-frame deletion mutations in several FrzS domains to determine their roles in protein localization. We found that FrzS has distinct domains required for residence at the leading cell pole, pole-to-pole transport and lagging cell pole. Our results are consistent with a model whereby S-motility reversals are mediated by a protein translocation system that delivers motility proteins such as FrzS from the leading cell pole to the lagging cell pole.

An atypical receiver domain controls the dynamic polar localization of the Myxococcus xanthus social motility protein FrzS

The Myxococcus xanthus FrzS protein transits from pole-to-pole within the cell, accumulating at the pole that defines the direction of movement in social (S) motility. Here we show using atomic-resolution crystallography and NMR that the FrzS receiver domain (RD) displays the conserved switch Tyr102 in an unusual conformation, lacks the conserved Asp phosphorylation site, and fails to bind Mg(2+) or the phosphoryl analogue, Mg(2+) x BeF(3). Mutation of Asp55, closest to the canonical site of RD phosphorylation, showed no motility phenotype in vivo, demonstrating that phosphorylation at this site is not necessary for domain function. In contrast, the Tyr102Ala and His92Phe substitutions on the canonical output face of the FrzS RD abolished S-motility in vivo. Single-cell fluorescence microscopy measurements revealed a striking mislocalization of these mutant FrzS proteins to the trailing cell pole in vivo. The crystal structures of the mutants suggested that the observed conformation of Tyr102 in the wild-type FrzS RD is not sufficient for function. These results support the model that FrzS contains a novel 'pseudo-receiver domain' whose function requires recognition of the RD output face but not Asp phosphorylation.

Evidence that focal adhesion complexes power bacterial gliding motility

The bacterium Myxococcus xanthus has two motility systems: S motility, which is powered by type IV pilus retraction, and A motility, which is powered by unknown mechanism(s). We found that A motility involved transient adhesion complexes that remained at fixed positions relative to the substratum as cells moved forward. Complexes assembled at leading cell poles and dispersed at the rear of the cells. When cells reversed direction, the A-motility clusters relocalized to the new leading poles together with S-motility proteins. The Frz chemosensory system coordinated the two motility systems. The dynamics of protein cluster localization suggest that intracellular motors and force transmission by dynamic focal adhesions can power bacterial motility.

Regulated pole-to-pole oscillations of a bacterial gliding motility protein

Little is known about directed motility of bacteria that move by type IV pilus-mediated (twitching) motility. Here, we found that during periodic cell reversals of Myxoccocus xanthus, type IV pili were disassembled at one pole and reassembled at the other pole. Accompanying these reversals, FrzS, a protein required for directed motility, moved in an oscillatory pattern between the cell poles. The frequency of the oscillations was controlled by the Frz chemosensory system, which is essential for directed motility. Pole-to-pole migration of FrzS appeared to involve movement along a filament running the length of the cell. FrzS dynamics may thus regulate cell polarity during directed motility.