Contact- and Protein Transfer-Dependent Stimulation of Assembly of the Gliding Motility Machinery in Myxococcus xanthus

A deterministic, c-di-GMP-dependent program ensures the generation of phenotypically similar, symmetric daughter cells during cytokinesis

Phenotypic heterogeneity in bacteria can result from stochastic processes or deterministic programs. The deterministic programs often involve the versatile second messenger c-di-GMP, and give rise to daughter cells with different c-di-GMP levels by deploying c-di-GMP metabolizing enzymes asymmetrically during cell division. By contrast, less is known about how phenotypic heterogeneity is kept to a minimum. Here, we identify a deterministic c-di-GMP-dependent program that is hardwired into the cell cycle of Myxococcus xanthus to minimize phenotypic heterogeneity and guarantee the formation of phenotypically similar daughter cells during division. Cells lacking the diguanylate cyclase DmxA have an aberrant motility behaviour. DmxA is recruited to the cell division site and its activity is switched on during cytokinesis, resulting in a transient increase in the c-di-GMP concentration. During cytokinesis, this c-di-GMP burst ensures the symmetric incorporation and allocation of structural motility proteins and motility regulators at the new cell poles of the two daughters, thereby generating phenotypically similar daughters with correct motility behaviours. Thus, our findings suggest a general c-di-GMP-dependent mechanism for minimizing phenotypic heterogeneity, and demonstrate that bacteria can ensure the formation of dissimilar or similar daughter cells by deploying c-di-GMP metabolizing enzymes to distinct subcellular locations.

A molecular switch controls assembly of bacterial focal adhesions

Cell motility universally relies on spatial regulation of focal adhesion complexes (FAs) connecting the substrate to cellular motors. In bacterial FAs, the Adventurous gliding motility machinery (Agl-Glt) assembles at the leading cell pole following a Mutual gliding-motility protein (MglA)-guanosine 5'-triphosphate (GTP) gradient along the cell axis. Here, we show that GltJ, a machinery membrane protein, contains cytosolic motifs binding MglA-GTP and AglZ and recruiting the MreB cytoskeleton to initiate movement toward the lagging cell pole. In addition, MglA-GTP binding triggers a conformational shift in an adjacent GltJ zinc-finger domain, facilitating MglB recruitment near the lagging pole. This prompts GTP hydrolysis by MglA, leading to complex disassembly. The GltJ switch thus serves as a sensor for the MglA-GTP gradient, controlling FA activity spatially.

A lytic transglycosylase connects bacterial focal adhesion complexes to the peptidoglycan cell wall

The Gram-negative bacterium Myxococcus xanthus glides on solid surfaces. Dynamic bacterial focal adhesion complexes (bFACs) convert proton motive force from the inner membrane into mechanical propulsion on the cell surface. It is unclear how the mechanical force transmits across the rigid peptidoglycan (PG) cell wall. Here we show that AgmT, a highly abundant lytic PG transglycosylase homologous to Escherichia coli MltG, couples bFACs to PG. Coprecipitation assay and single-particle microscopy reveal that the gliding motors fail to connect to PG and thus are unable to assemble into bFACs in the absence of an active AgmT. Heterologous expression of E. coli MltG restores the connection between PG and bFACs and thus rescues gliding motility in the M. xanthus cells that lack AgmT. Our results indicate that bFACs anchor to AgmT-modified PG to transmit mechanical force across the PG cell wall.

Molecular Dynamics of Outer Membrane-Embedded Polysaccharide Secretion Porins Reveals Closed Resting-State Surface Gates Targetable by Virtual Fragment Screening for Drug Hotspot Identification

Recent advances in iterative neural network analyses (e.g., AlphaFold2 and RoseTTA fold) have been revolutionary for protein 3D structure prediction, especially for difficult-to-manipulate α-helical/β-barrel integral membrane proteins. These model structures are calculated based on the coevolution of amino acids within the protein of interest and similarities to existing protein structures; the local effects of the membrane on folding and stability of the calculated model structures are not considered. We recently reported the discovery, 3D modeling, and characterization of 18-β-stranded outer-membrane (OM) WzpX, WzpS, and WzpB β-barrel secretion porins for the exopolysaccharide (EPS), major spore coat polysaccharide (MASC), and biosurfactant polysaccharide (BPS) pathways (respectively) in the Gram-negative social predatory bacterium Myxococcus xanthus DZ2. However, information was not obtained regarding the dynamic behavior of surface-gating WzpX/S/B loop domains or on potential treatments to inactivate these porins. Herein, we developed a molecular dynamics (MD) protocol to study the core stability and loop dynamism of neural network-based integral membrane protein structure models embedded in an asymmetric OM bilayer, using the M. xanthus WzpX, WzpS, and WzpB proteins as test candidates. This was accomplished through integration of the CHARMM-graphical user interface (GUI) and Molecular Operating Environment (MOE) workflows to allow for a rapid simulation system setup and facilitate data analysis. In addition to serving as a method of model structure validation, our molecular dynamics simulations revealed a minimal movement of extracellular WzpX/S/B loops in the absence of an external stimulus as well as druggable cavities between the loops. Virtual screening of a commercial fragment library against these cavities revealed putative fragment-binding hotspots on the cell-surface face of each β-barrel, along with key interacting residues, and identified promising hits for the design of potential binders capable of plugging the β-barrels and inhibiting polysaccharide secretion.

The mechanism for polar localization of the type IVa pilus machine in Myxococcus xanthus

IMPORTANCE

Type IVa pili (T4aP) are widespread bacterial cell surface structures with important functions in motility, surface adhesion, biofilm formation, and virulence. Different bacteria have adapted different piliation patterns. To address how these patterns are established, we focused on the bipolar localization of the T4aP machine in the model organism Myxococcus xanthus by studying the localization of the PilQ secretin, the first component of this machine that assembles at the poles. Based on experiments using a combination of fluorescence microscopy, biochemistry, and computational structural analysis, we propose that PilQ, and specifically its AMIN domains, binds septal and polar peptidoglycan, thereby enabling polar Tgl localization, which then stimulates PilQ multimerization in the outer membrane. We also propose that the presence and absence of AMIN domains in T4aP secretins contribute to the different piliation patterns across bacteria.

Widespread density dependence of bacterial growth under acid stress

Many microbial phenotypes are density-dependent, including group-level phenotypes emerging from cooperation. However, surveys for the presence of a particular form of density dependence across diverse species are rare, as are direct tests for the Allee effect, i.e., positive density dependence of fitness. Here, we test for density-dependent growth under acid stress in five diverse bacterial species and find the Allee effect in all. Yet social protection from acid stress appears to have evolved by multiple mechanisms. In Myxococcus xanthus, a strong Allee effect is mediated by pH-regulated secretion of a diffusible molecule by high-density populations. In other species, growth from low density under acid stress was not enhanced by high-density supernatant. In M. xanthus, high cell density may promote predation on other microbes that metabolically acidify their environment, and acid-mediated density dependence may impact the evolution of fruiting-body development. More broadly, high density may protect most bacterial species against acid stress.

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.

Evidence for a Widespread Third System for Bacterial Polysaccharide Export across the Outer Membrane Comprising a Composite OPX/β-Barrel Translocon

In Gram-negative bacteria, secreted polysaccharides have multiple critical functions. In Wzx/Wzy- and ABC transporter-dependent pathways, an outer membrane (OM) polysaccharide export (OPX) type translocon exports the polysaccharide across the OM. The paradigm OPX protein Wza of Escherichia coli is an octamer in which the eight C-terminal domains form an α-helical OM pore and the eight copies of the three N-terminal domains (D1 to D3) form a periplasmic cavity. In synthase-dependent pathways, the OM translocon is a 16- to 18-stranded β-barrel protein. In Myxococcus xanthus, the secreted polysaccharide EPS (exopolysaccharide) is synthesized in a Wzx/Wzy-dependent pathway. Here, using experiments, phylogenomics, and computational structural biology, we identify and characterize EpsX as an OM 18-stranded β-barrel protein important for EPS synthesis and identify AlgE, a β-barrel translocon of a synthase-dependent pathway, as its closest structural homolog. We also find that EpsY, the OPX protein of the EPS pathway, consists only of the periplasmic D1 and D2 domains and completely lacks the domain for spanning the OM (herein termed a ^D1D2OPX protein). In vivo, EpsX and EpsY mutually stabilize each other and interact in in vivo pulldown experiments supporting their direct interaction. Based on these observations, we propose that EpsY and EpsX make up and represent a third type of translocon for polysaccharide export across the OM. Specifically, in this composite translocon, EpsX functions as the OM-spanning β-barrel translocon together with the periplasmic ^D1D2OPX protein EpsY. Based on computational genomics, similar composite systems are widespread in Gram-negative bacteria. IMPORTANCE Bacteria secrete a wide variety of polysaccharides that have critical functions in, e.g., fitness, surface colonization, and biofilm formation and in beneficial and pathogenic human-, animal-, and plant-microbe interactions. In Gram-negative bacteria, export of these chemically diverse polysaccharides across the outer membrane depends on two known translocons, i.e., an outer membrane OPX protein in Wzx/Wzy- and ABC transporter-dependent pathways and an outer membrane 16- to 18-stranded β-barrel protein in synthase-dependent pathways. Here, using a combination of experiments in Myxococcus xanthus, phylogenomics, and computational structural biology, we provide evidence supporting that a third type of translocon can export polysaccharides across the outer membrane. Specifically, in this translocon, an outer membrane-spanning β-barrel protein functions together with an entirely periplasmic OPX protein that completely lacks the domain for spanning the OM. Computational genomics support that similar composite systems are widespread in Gram-negative bacteria.

Production of 3′,3′-cGAMP by a Bdellovibrio bacteriovorus promiscuous GGDEF enzyme, Bd0367, regulates exit from prey by gliding motility

Bacterial second messengers are important for regulating diverse bacterial lifestyles. Cyclic di-GMP (c-di-GMP) is produced by diguanylate cyclase enzymes, named GGDEF proteins, which are widespread across bacteria. Recently, hybrid promiscuous (Hypr) GGDEF proteins have been described in some bacteria, which produce both c-di-GMP and a more recently identified bacterial second messenger, 3′,3′-cyclic-GMP-AMP (cGAMP). One of these proteins was found in the predatory Bdellovibrio bacteriovorus, Bd0367. The bd0367 GGDEF gene deletion strain was found to enter prey cells, but was incapable of leaving exhausted prey remnants via gliding motility on a solid surface once predator cell division was complete. However, it was unclear which signal regulated this process. We show that cGAMP signalling is active within B. bacteriovorus and that, in addition to producing c-di-GMP and some c-di-AMP, Bd0367 is a primary producer of cGAMP in vivo. Site-directed mutagenesis of serine 214 to an aspartate rendered Bd0367 into primarily a c-di-GMP synthase. B. bacteriovorus strain bd0367S214D phenocopies the bd0367 deletion strain by being unable to glide on a solid surface, leading to an inability of new progeny to exit from prey cells post-replication. Thus, this process is regulated by cGAMP. Deletion of bd0367 was also found to be incompatible with wild-type flagellar biogenesis, as a result of an acquired mutation in flagellin chaperone gene homologue fliS, implicating c-di-GMP in regulation of swimming motility. Thus the single Bd0367 enzyme produces two secondary messengers by action of the same GGDEF domain, the first reported example of a synthase that regulates multiple second messengers in vivo. Unlike roles of these signalling molecules in other bacteria, these signal to two separate motility systems, gliding and flagellar, which are essential for completion of the bacterial predation cycle and prey exit by B. bacteriovorus.

Secondary messengers are important signalling molecules in bacteria and a recently discovered one, called cGAMP, has recently been shown to be made by some enzymes which had previously been known to produce another secondary messenger, c-di-GMP. One of these “hybrid promiscuous” enzymes (Bd0367) is found in Bdellovibrio bacteriovorus, a bacterium that preys upon other bacteria, burrowing inside them and consuming them from within. Previous gene deletion work had shown that Bd0367 was essential in signalling for Bdellovibrio to leave the remains of its prey cell by gliding motility after predation was complete and it was thought that this was due to c-di-GMP signalling. However, here, we show that this gliding motility is actually regulated by cGAMP signalling and that c-di-GMP signalling is involved in swimming motility. A single enzyme produces two different molecules, signalling to two discrete motility systems, both of which are required for successful completion of the bacterium’s predatory lifestyle in prey on solid surfaces or in liquids.

Analysis of Myxococcus xanthus Vegetative Biofilms With Microtiter Plates

The bacterium Myxococcus xanthus forms both developmental and vegetative types of biofilms. While the former has been studied on both agar plates and submerged surfaces, the latter has been investigated predominantly on agar surfaces as swarming colonies. Here we describe the development of a microplate-based assay for the submerged biofilms of M. xanthus under vegetative conditions. We examined the impacts of inoculation, aeration, and temperature to optimize the conditions for the assay. Aeration was observed to be critical for the effective development of submerged biofilms by M. xanthus, an obligate aerobic bacterium. In addition, temperature plays an important role in the development of M. xanthus submerged biofilms. It is well established that the formation of submerged biofilms by many bacteria requires both exopolysaccharide (EPS) and the type IV pilus (T4P). EPS constitutes part of the biofilm matrix that maintains and organizes bacterial biofilms while the T4P facilitates surface attachment as adhesins. For validation, we used our biofilm assay to examine a multitude of M. xanthus strains with various EPS and T4P phenotypes. The results indicate that the levels of EPS, but not of piliation, positively correlate with submerged biofilm formation in M. xanthus.

Roadmap on emerging concepts in the physical biology of bacterial biofilms: from surface sensing to community formation

Bacterial biofilms are communities of bacteria that exist as aggregates that can adhere to surfaces or be free-standing. This complex, social mode of cellular organization is fundamental to the physiology of microbes and often exhibits surprising behavior. Bacterial biofilms are more than the sum of their parts: single-cell behavior has a complex relation to collective community behavior, in a manner perhaps cognate to the complex relation between atomic physics and condensed matter physics. Biofilm microbiology is a relatively young field by biology standards, but it has already attracted intense attention from physicists. Sometimes, this attention takes the form of seeing biofilms as inspiration for new physics. In this roadmap, we highlight the work of those who have taken the opposite strategy: we highlight the work of physicists and physical scientists who use physics to engage fundamental concepts in bacterial biofilm microbiology, including adhesion, sensing, motility, signaling, memory, energy flow, community formation and cooperativity. These contributions are juxtaposed with microbiologists who have made recent important discoveries on bacterial biofilms using state-of-the-art physical methods. The contributions to this roadmap exemplify how well physics and biology can be combined to achieve a new synthesis, rather than just a division of labor.

Three PilZ Domain Proteins, PlpA, PixA, and PixB, Have Distinct Functions in Regulation of Motility and Development in Myxococcus xanthus

In bacteria, the nucleotide-based second messenger bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) binds to effectors to generate outputs in response to changes in the environment. In Myxococcus xanthus, c-di-GMP regulates type IV pilus-dependent motility and the starvation-induced developmental program that results in formation of spore-filled fruiting bodies; however, little is known about the effectors that bind c-di-GMP. Here, we systematically inactivated all 24 genes encoding PilZ domain-containing proteins, which are among the most common c-di-GMP effectors. We confirm that the stand-alone PilZ domain protein PlpA is important for regulation of motility independently of the Frz chemosensory system and that Pkn1, which is composed of a Ser/Thr kinase domain and a PilZ domain, is specifically important for development. Moreover, we identify two PilZ domain proteins that have distinct functions in regulating motility and development. PixB, which is composed of two PilZ domains and an acetyltransferase domain, binds c-di-GMP in vitro and regulates type IV pilus-dependent and gliding motility in a Frz-dependent manner as well as development. The acetyltransferase domain is required and sufficient for function during growth, while all three domains and c-di-GMP binding are essential for PixB function during development. PixA is a response regulator composed of a PilZ domain and a receiver domain, binds c-di-GMP in vitro, and regulates motility independently of the Frz system, likely by setting up the polarity of the two motility systems. Our results support a model whereby PlpA, PixA, and PixB act in independent pathways and have distinct functions in regulation of motility. IMPORTANCE c-di-GMP signaling controls bacterial motility in many bacterial species by binding to downstream effector proteins. Here, we identify two PilZ domain-containing proteins in Myxococcus xanthus that bind c-di-GMP. We show that PixB, which contains two PilZ domains and an acetyltransferase domain, acts in a manner that depends on the Frz chemosensory system to regulate motility via the acetyltransferase domain, while the intact protein and c-di-GMP binding are essential for PixB to support development. In contrast, PixA acts in a Frz-independent manner to regulate motility. Taking our results together with previous observations, we conclude that PilZ domain proteins and c-di-GMP act in multiple independent pathways to regulate motility and development in M. xanthus.

Characterization of the Exopolysaccharide Biosynthesis Pathway in Myxococcus xanthus

The secreted polysaccharide referred to as exopolysaccharide (EPS) has important functions in the social life cycle of M. xanthus; however, little is known about how EPS is synthesized. Here, we characterized the EPS biosynthetic machinery and showed that it makes up a Wzx/Wzy-dependent pathway for polysaccharide biosynthesis. Mutants lacking a component of this pathway had reduced type IV pilus-dependent motility and a conditional defect in development. These analyses also suggest that EPS and/or the EPS biosynthetic machinery is important for type IV pilus formation.

Myxococcus xanthus arranges into two morphologically distinct biofilms depending on its nutritional status, i.e., coordinately spreading colonies in the presence of nutrients and spore-filled fruiting bodies in the absence of nutrients. A secreted polysaccharide, referred to as exopolysaccharide (EPS), is a structural component of both biofilms and is also important for type IV pilus-dependent motility and fruiting body formation. Here, we characterize the biosynthetic machinery responsible for EPS biosynthesis using bioinformatics, genetics, heterologous expression, and biochemical experiments. We show that this machinery constitutes a Wzx/Wzy-dependent pathway dedicated to EPS biosynthesis. Our data support that EpsZ (MXAN_7415) is the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for the initiation of the repeat unit synthesis. Heterologous expression experiments support that EpsZ has galactose-1-P transferase activity. Moreover, MXAN_7416, renamed Wzx_EPS, and MXAN_7442, renamed Wzy_EPS, are the Wzx flippase and Wzy polymerase responsible for translocation and polymerization of the EPS repeat unit, respectively. In this pathway, EpsV (MXAN_7421) also is the polysaccharide copolymerase and EpsY (MXAN_7417) the outer membrane polysaccharide export (OPX) protein. Mutants with single in-frame deletions in the five corresponding genes had defects in type IV pilus-dependent motility and a conditional defect in fruiting body formation. Furthermore, all five mutants were deficient in type IV pilus formation, and genetic analyses suggest that EPS and/or the EPS biosynthetic machinery stimulates type IV pilus extension. Additionally, we identify a polysaccharide biosynthesis gene cluster, which together with an orphan gene encoding an OPX protein make up a complete Wzx/Wzy-dependent pathway for synthesis of an unknown polysaccharide.

IMPORTANCE The secreted polysaccharide referred to as exopolysaccharide (EPS) has important functions in the social life cycle of M. xanthus; however, little is known about how EPS is synthesized. Here, we characterized the EPS biosynthetic machinery and showed that it makes up a Wzx/Wzy-dependent pathway for polysaccharide biosynthesis. Mutants lacking a component of this pathway had reduced type IV pilus-dependent motility and a conditional defect in development. These analyses also suggest that EPS and/or the EPS biosynthetic machinery is important for type IV pilus formation.

Kin recognition and outer membrane exchange (OME) in myxobacteria

Myxobacteria conduct complex social traits that requires populations to be highly related and devoid of exploiters. To enrich for clonal cells in populations, they employ kin discrimination mechanisms. One key system involves a polymorphic cell surface receptor, TraA, which recognizes self by homotypic interactions with neighboring myxobacterial cells. Recent studies revealed that TraA and its partner TraB are fluid outer membrane proteins that coalesce into foci upon recognition of kin. The formation of foci leads to transient membrane fusion junctions and the bidirectional exchange of outer membrane components that facilitates cooperative behaviors. Additionally, expansive suites of polymorphic lipoprotein toxins are exchanged, which act as self-identity barcodes that exquisitely discriminate against nonself to assemble homogenous populations.

Protein-protein interaction network controlling establishment and maintenance of switchable cell polarity

Cell polarity underlies key processes in all cells, including growth, differentiation and division. In the bacterium Myxococcus xanthus, front-rear polarity is crucial for motility. Notably, this polarity can be inverted, independent of the cell-cycle, by chemotactic signaling. However, a precise understanding of the protein network that establishes polarity and allows for its inversion has remained elusive. Here, we use a combination of quantitative experiments and data-driven theory to unravel the complex interplay between the three key components of the M. xanthus polarity module. By studying each of these components in isolation and their effects as we systematically reconstruct the system, we deduce the network of effective interactions between the polarity proteins. RomR lies at the root of this network, promoting polar localization of the other components, while polarity arises from interconnected negative and positive feedbacks mediated by the small GTPase MglA and its cognate GAP MglB, respectively. We rationalize this network topology as operating as a spatial toggle switch, providing stable polarity for persistent cell movement whilst remaining responsive to chemotactic signaling and thus capable of polarity inversions. Our results have implications not only for the understanding of polarity and motility in M. xanthus but also, more broadly, for dynamic cell polarity.

The asymmetric localization of cellular components (polarity) is at the core of many important cellular functions including growth, division, differentiation and motility. However, important questions still remain regarding the design principles underlying polarity networks and how their activity can be controlled in space and time. We use the rod-shaped bacterium Myxococcus xanthus as a model to study polarity and its regulation. Like many bacteria, in M. xanthus a well-defined front-rear polarity axis enables efficient translocation. This polarity axis is defined by asymmetric polar localization of a switch-like GTPase and its cognate regulators, and can be reversed in response to signaling cues. Here we use a combination of quantitative experiments and data-driven theory to deduce the network of interactions among the M. xanthus polarity proteins and to show how the combination of positive- and negative-feedback interactions give rise to asymmetric polar protein localization. We rationalize this network topology as operating as a spatial toggle switch, providing stable polarity for persistent cell movement whilst remaining responsive to chemotactic signaling and capable of polarity inversions. Our results have broader implications for our understanding of dynamic cell polarity and GTPase regulation in both bacteria and eukaryotic cells.

DivIVA Controls Progeny Morphology and Diverse ParA Proteins Regulate Cell Division or Gliding Motility in Bdellovibrio bacteriovorus

The predatory bacterium B. bacteriovorus grows and divides inside the periplasm of Gram-negative bacteria, forming a structure known as a bdelloplast. Cell division of predators inside the dead prey cell is not by binary fission but instead by synchronous division of a single elongated filamentous cell into odd or even numbers of progeny cells. Bdellovibrio replication and cell division processes are dependent on the finite level of nutrients available from inside the prey bacterium. The filamentous growth and division process of the predator maximizes the number of progeny produced by the finite nutrients in a way that binary fission could not. To learn more about such an unusual growth profile, we studied the role of DivIVA in the growing Bdellovibrio cell. This protein is well known for its link to polar cell growth and spore formation in Gram-positive bacteria, but little is known about its function in a predatory growth context. We show that DivIVA is expressed in the growing B. bacteriovorus cell and controls cell morphology during filamentous cell division, but not the number of progeny produced. Bacterial Two Hybrid (BTH) analysis shows DivIVA may interact with proteins that respond to metabolic indicators of amino-acid biosynthesis or changes in redox state. Such changes may be relevant signals to the predator, indicating the consumption of prey nutrients within the sealed bdelloplast environment. ParA, a chromosome segregation protein, also contributes to bacterial septation in many species. The B. bacteriovorus genome contains three ParA homologs; we identify a canonical ParAB pair required for predatory cell division and show a BTH interaction between a gene product encoded from the same operon as DivIVA with the canonical ParA. The remaining ParA proteins are both expressed in Bdellovibrio but are not required for predator cell division. Instead, one of these ParA proteins coordinates gliding motility, changing the frequency at which the cells reverse direction. Our work will prime further studies into how one bacterium can co-ordinate its cell division with the destruction of another bacterium that it dwells within.

Identification of the Wzx flippase, Wzy polymerase and sugar-modifying enzymes for spore coat polysaccharide biosynthesis in Myxococcus xanthus

The rod-shaped cells of Myxococcus xanthus, a Gram-negative deltaproteobacterium, differentiate to environmentally resistant spores upon starvation or chemical stress. The environmental resistance depends on a spore coat polysaccharide that is synthesised by the ExoA-I proteins, some of which are part of a Wzx/Wzy-dependent pathway for polysaccharide synthesis and export; however, key components of this pathway have remained unidentified. Here, we identify and characterise two additional loci encoding proteins with homology to enzymes involved in polysaccharide synthesis and export, as well as sugar modification and show that six of the proteins encoded by these loci are essential for the formation of environmentally resistant spores. Our data support that MXAN_3260, renamed ExoM and MXAN_3026, renamed ExoJ, are the Wzx flippase and Wzy polymerase, respectively, responsible for translocation and polymerisation of the repeat unit of the spore coat polysaccharide. Moreover, we provide evidence that three glycosyltransferases (MXAN_3027/ExoK, MXAN_3262/ExoO and MXAN_3263/ExoP) and a polysaccharide deacetylase (MXAN_3259/ExoL) are important for formation of the intact spore coat, while ExoE is the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for initiating repeat unit synthesis, likely by transferring N-acetylgalactosamine-1-P to undecaprenyl-phosphate. Together, our data generate a more complete model of the Exo pathway for spore coat polysaccharide biosynthesis and export.

Three-Dimensional Observations of an Aperiodic Oscillatory Gliding Behavior in Myxococcus xanthus Using Confocal Interference Reflection Microscopy

3D imaging of live bacteria with optical microscopy techniques is a challenge due to the small size of bacterial cells, meaning that previous studies have been limited to observing motility behavior in 2D. We introduce the application of confocal multiwavelength interference reflection microscopy to bacteria, which enables visualization of 3D motility behaviors in a single 2D image. Using the model organism Myxococcus xanthus, we identified novel motility behaviors that are not explained by current motility models, where gliding bacteria exhibit aperiodic changes in their adhesion to an underlying solid surface. We concluded that the 3D behavior was not linked to canonical motility mechanisms and that IRM could be applied to study a range of microbiological specimens with minimal adaptation to a commercial microscope.

The deltaproteobacterium Myxococcus xanthus is a model for bacterial motility and has provided unprecedented insights into bacterial swarming behaviors. Fluorescence microscopy techniques have been invaluable in defining the mechanisms that are involved in gliding motility, but these have almost entirely been limited to two-dimensional (2D) studies, and there is currently no understanding of gliding motility in a three-dimensional (3D) context. We present here the first use of confocal interference reflection microscopy (IRM) to study gliding bacteria, revealing aperiodic oscillatory behavior with changes in the position of the basal membrane relative to the substrate on the order of 90 nm in vitro. First, we use a model planoconvex lens specimen to show how topological information can be obtained from the wavelength-dependent interference pattern in IRM. We then use IRM to observe gliding M. xanthus bacteria and show that cells undergo previously unobserved changes in their adhesion profile as they glide. We compare the wild type with mutants that have reduced motility, which also exhibit the same changes in the adhesion profile during gliding. We find that the general gliding behavior is independent of the proton motive force-generating complex AglRQS and suggest that the novel behavior that we present here may be a result of recoil and force transmission along the length of the cell body following firing of the type IV pili.

IMPORTANCE 3D imaging of live bacteria with optical microscopy techniques is a challenge due to the small size of bacterial cells, meaning that previous studies have been limited to observing motility behavior in 2D. We introduce the application of confocal multiwavelength interference reflection microscopy to bacteria, which enables visualization of 3D motility behaviors in a single 2D image. Using the model organism Myxococcus xanthus, we identified novel motility behaviors that are not explained by current motility models, where gliding bacteria exhibit aperiodic changes in their adhesion to an underlying solid surface. We concluded that the 3D behavior was not linked to canonical motility mechanisms and that IRM could be applied to study a range of microbiological specimens with minimal adaptation to a commercial microscope.

Identification of the lipopolysaccharide O-antigen biosynthesis priming enzyme and the O-antigen ligase in Myxococcus xanthus: critical role of LPS O-antigen in motility and development

Myxococcus xanthus is a model bacterium to study social behavior. At the cellular level, the different social behaviors of M. xanthus involve extensive cell-cell contacts. Here, we used bioinformatics, genetics, heterologous expression and biochemical experiments to identify and characterize the key enzymes in M. xanthus implicated in O-antigen and lipopolysaccharide (LPS) biosynthesis and examined the role of LPS O-antigen in M. xanthus social behaviors. We identified WbaP_Mx (MXAN_2922) as the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for priming O-antigen synthesis. In heterologous expression experiments, WbaP_Mx complemented a Salmonella enterica mutant lacking the endogenous WbaP that primes O-antigen synthesis, indicating that WbaP_Mx transfers galactose-1-P to undecaprenyl-phosphate. We also identified WaaL_Mx (MXAN_2919), as the O-antigen ligase that joins O-antigen to lipid A-core. Our data also support the previous suggestion that Wzm_Mx (MXAN_4622) and Wzt_Mx (MXAN_4623) form the Wzm/Wzt ABC transporter. We show that mutations that block different steps in LPS O-antigen synthesis can cause pleiotropic phenotypes. Also, using a wbaP_Mx deletion mutant, we revisited the role of LPS O-antigen and demonstrate that it is important for gliding motility, conditionally important for type IV pili-dependent motility and required to complete the developmental program leading to the formation of spore-filled fruiting bodies.

Bacterial Gliding Motility: Rolling Out a Consensus Model

Some bacteria glide mysteriously on surfaces without using flagella, pili, or other external appendages. Recent studies suggest how gliding motors in the inner membrane may transduce force to the cell surface.

A TonB-dependent transporter is required for secretion of protease PopC across the bacterial outer membrane

TonB-dependent transporters (TBDTs) are ubiquitous outer membrane β-barrel proteins that import nutrients and bacteriocins across the outer membrane in a proton motive force-dependent manner, by directly connecting to the ExbB/ExbD/TonB system in the inner membrane. Here, we show that the TBDT Oar in Myxococcus xanthus is required for secretion of a protein, protease PopC, to the extracellular milieu. PopC accumulates in the periplasm before secretion across the outer membrane, and the proton motive force has a role in secretion to the extracellular milieu. Reconstitution experiments in Escherichia coli demonstrate that secretion of PopC across the outer membrane not only depends on Oar but also on the ExbB/ExbD/TonB system. Our results indicate that TBDTs and the ExbB/ExbD/TonB system may have roles not only in import processes but also in secretion of proteins.

TonB-dependent transporters (TBDTs) are outer membrane proteins that import nutrients and bacteriocins in bacteria. Here, Gómez-Santos et al. show that a TBDT is required for secretion of a protease in Myxococcus xanthus, suggesting that some TBDTs may be involved in protein secretion.

A Highly Polymorphic Receptor Governs Many Distinct Self-Recognition Types within the Myxococcales Order

Many biological species distinguish self from nonself by using different mechanisms. Higher animals recognize close kin via complex processes that often involve the five senses, cognition, and learning, whereas some microbes achieve self-recognition simply through the activity of a single genetic locus. Here we describe a single locus, traA, in myxobacteria that governs cell-cell recognition within natural populations. We found that traA is widespread across the order Myxococcales. TraA is highly polymorphic among diverse myxobacterial isolates, and such polymorphisms determine selectivity in self-recognition. Through bioinformatic and experimental analyses, we showed that traA governs many distinct recognition groups within Myxococcales. This report provides an example in which a single locus influences social recognition across a wide phylogenetic range of natural populations.

Self-recognition underlies sociality in many group-living organisms. In bacteria, cells use various strategies to recognize kin to form social groups and, in some cases, to transition into multicellular life. One strategy relies on a single genetic locus that encodes a variable phenotypic tag (“greenbeard”) for recognizing other tag bearers. Previously, we discovered a polymorphic cell surface receptor called TraA that directs self-identification through homotypic interactions in the social bacterium Myxococcus xanthus. Recognition by TraA leads to cellular resource sharing in a process called outer membrane exchange (OME). A second gene in the traA operon, traB, is also required for OME but is not involved in recognition. Our prior studies of TraA identified only six recognition groups among closely related M. xanthus isolates. Here we hypothesize that the number of traA polymorphisms and, consequently, the diversity of recognition in wild isolates are much greater. To test this hypothesis, we expand the scope of TraA characterization to the order Myxococcales. From genomic sequences within the three suborders of Myxococcales, we identified 90 traA orthologs. Sequence analyses and functional characterization of traAB loci suggest that OME is well maintained among diverse myxobacterial taxonomic groups. Importantly, TraA orthologs are highly polymorphic within their variable domain, the region that confers selectivity in self-recognition. We experimentally defined 10 distinct recognition groups and, based on phylogenetic and experimental analyses, predicted >60 recognition groups among the 90 traA alleles. Taken together, our findings revealed a widespread greenbeard locus that mediates the diversity of self-recognition across the order Myxococcales.

Light-controlled motility in prokaryotes and the problem of directional light perception

The natural light environment is important to many prokaryotes. Most obviously, phototrophic prokaryotes need to acclimate their photosynthetic apparatus to the prevailing light conditions, and such acclimation is frequently complemented by motility to enable cells to relocate in search of more favorable illumination conditions. Non-phototrophic prokaryotes may also seek to avoid light at damaging intensities and wavelengths, and many prokaryotes with diverse lifestyles could potentially exploit light signals as a rich source of information about their surroundings and a cue for acclimation and behavior. Here we discuss our current understanding of the ways in which bacteria can perceive the intensity, wavelength and direction of illumination, and the signal transduction networks that link light perception to the control of motile behavior. We discuss the problems of light perception at the prokaryotic scale, and the challenge of directional light perception in small bacterial cells. We explain the peculiarities and the common features of light-controlled motility systems in prokaryotes as diverse as cyanobacteria, purple photosynthetic bacteria, chemoheterotrophic bacteria and haloarchaea.

Force generation by groups of migrating bacteria

From colony formation in bacteria to wound healing and embryonic development in multicellular organisms, groups of living cells must often move collectively. Although considerable study has probed the biophysical mechanisms of how eukaryotic cells generate forces during migration, little such study has been devoted to bacteria, in particular with regard to the question of how bacteria generate and coordinate forces during collective motion. This question is addressed here using traction force microscopy. We study two distinct motility mechanisms of Myxococcus xanthus, namely, twitching and gliding. For twitching, powered by type-IV pilus retraction, we find that individual cells exert local traction in small hotspots with forces on the order of 50 pN. Twitching bacterial groups also produce traction hotspots, but with forces around 100 pN that fluctuate rapidly on timescales of <1.5 min. Gliding, the second motility mechanism, is driven by lateral transport of substrate adhesions. When cells are isolated, gliding produces low average traction on the order of 1 Pa. However, traction is amplified approximately fivefold in groups. Advancing protrusions of gliding cells push, on average, in the direction of motion. Together, these results show that the forces generated during twitching and gliding have complementary characters, and both forces have higher values when cells are in groups.

Regulation of Cell Polarity in Motility and Cell Division in Myxococcus xanthus

Rod-shaped Myxococcus xanthus cells are polarized with proteins asymmetrically localizing to specific positions. This spatial organization is important for regulation of motility and cell division and changes over time. Dedicated protein modules regulate motility independent of the cell cycle, and cell division dependent on the cell cycle. For motility, a leading-lagging cell polarity is established that is inverted during cellular reversals. Establishment and inversion of this polarity are regulated hierarchically by interfacing protein modules that sort polarized motility proteins to the correct cell poles or cause their relocation between cell poles during reversals akin to a spatial toggle switch. For division, a novel self-organizing protein module that incorporates a ParA ATPase positions the FtsZ-ring at midcell. This review covers recent findings concerning the spatiotemporal regulation of motility and cell division in M. xanthus and illustrates how the study of diverse bacteria may uncover novel mechanisms involved in regulating bacterial cell polarity.

Mutational analysis and overproduction effects of MotX, an essential component for motor function of Na+-driven polar flagella of Vibrio

The bacterial flagellar motor is a rotary motor complex composed of various proteins. The motor contains a central rod, multiple ring-like structures and stators. The Na+-driven polar flagellar motor of the marine bacterium Vibrio alginolyticus has a specific ring, called the ‘T-ring’, which consists of two periplasmic proteins, MotX and MotY. The T-ring is essential for assembly of the torque-generating unit, the PomA/PomB stator complex, into the motor. To investigate the role of the T-ring for motor function, we performed random mutagenesis of the motX gene on a plasmid. The isolated MotX mutants showed nonmotile, slow-motile, and up-motile phenotypes by the expression from the plasmid. Deletion analysis indicated that the C-terminal region and the signal peptide in MotX are not always essential for flagellar motor function. We also found that overproduction of MotX caused the delay of growth and aberrant cell shape. MotX might have unexpected roles not only in flagellar motor function but also in cell morphology control.

Self-identity reprogrammed by a single residue switch in a cell surface receptor of a social bacterium

The ability to recognize close kin confers survival benefits on single-celled microbes that live in complex and changing environments. Microbial kinship detection relies on perceptible cues that reflect relatedness between individuals, although the mechanisms underlying recognition in natural populations remain poorly understood. In myxobacteria, cells identify related individuals through a polymorphic cell surface receptor, TraA. Recognition of compatible receptors leads to outer membrane exchange among clonemates and fitness consequences. Here, we investigated how a single receptor creates a diversity in recognition across myxobacterial populations. We first show that TraA requires its partner protein TraB to function in cell-cell adhesion. Recognition is shown to be traA allele-specific, where polymorphisms within TraA dictate binding selectivity. We reveal the malleability of TraA recognition, and seemingly minor changes to its variable region reprogram recognition outcomes. Strikingly, we identify a single residue (A/P205) as a molecular switch for TraA recognition. Substitutions at this position change the specificity of a diverse panel of environmental TraA receptors. In addition, we engineered a receptor with unique specificity by simply creating an A205P substitution, suggesting that modest changes in TraA can lead to diversification of new recognition groups in nature. We hypothesize that the malleable property of TraA has allowed it to evolve and create social barriers between myxobacterial populations and in turn avoid adverse interactions with relatives.

Reversals and collisions optimize protein exchange in bacterial swarms

Swarming groups of bacteria coordinate their behavior by self-organizing as a population to move over surfaces in search of nutrients and optimal niches for colonization. Many open questions remain about the cues used by swarming bacteria to achieve this self-organization. While chemical cue signaling known as quorum sensing is well-described, swarming bacteria often act and coordinate on time scales that could not be achieved via these extracellular quorum sensing cues. Here, cell-cell contact-dependent protein exchange is explored as a mechanism of intercellular signaling for the bacterium Myxococcus xanthus. A detailed biologically calibrated computational model is used to study how M. xanthus optimizes the connection rate between cells and maximizes the spread of an extracellular protein within the population. The maximum rate of protein spreading is observed for cells that reverse direction optimally for swarming. Cells that reverse too slowly or too fast fail to spread extracellular protein efficiently. In particular, a specific range of cell reversal frequencies was observed to maximize the cell-cell connection rate and minimize the time of protein spreading. Furthermore, our findings suggest that predesigned motion reversal can be employed to enhance the collective behavior of biological synthetic active systems.

The mechanism of force transmission at bacterial focal adhesion complexes

Various rod-shaped bacteria mysteriously glide on surfaces in the absence of appendages such as flagella or pili. In the deltaproteobacterium Myxococcus xanthus, a putative gliding motility machinery (the Agl-Glt complex) localizes to so-called focal adhesion sites (FASs) that form stationary contact points with the underlying surface. Here we show that the Agl-Glt machinery contains an inner-membrane motor complex that moves intracellularly along a right-handed helical path; when the machinery becomes stationary at FASs, the motor complex powers a left-handed rotation of the cell around its long axis. At FASs, force transmission requires cyclic interactions between the molecular motor and the adhesion proteins of the outer membrane via a periplasmic interaction platform, which presumably involves contractile activity of motor components and possible interactions with peptidoglycan. Our results provide a molecular model of bacterial gliding motility.

The Chemical Ecology of Predatory Soil Bacteria

The study of natural products is entering a renaissance, driven by the discovery that the majority of bacterial secondary metabolites are not produced under standard laboratory conditions. Understanding the ecological role of natural products is key to efficiently directing our screening efforts, and to ensuring that each screen efficiently captures the full biosynthetic repertoire of the producing organisms. Myxobacteria represent one of the most common and diverse groups of bacteria, with roughly 2500 strains publically available. Fed largely through predation, the myxobacteria have developed a large repertoire of natural products that target other microorganisms, including bacteria and fungi. Many of these interactions can be observed in predation assays, providing direct evidence for environmental interactions. With a focus on Myxococcus xanthus, this review will highlight how recent advances in myxobacteria are revealing the chemical ecology of bacterial natural products.

Novel mechanisms power bacterial gliding motility

For many bacteria, motility is essential for survival, growth, virulence, biofilm formation and intra/interspecies interactions. Since natural environments differ, bacteria have evolved remarkable motility systems to adapt, including swimming in aqueous media, and swarming, twitching and gliding on solid and semi-solid surfaces. Although tremendous advances have been achieved in understanding swimming and swarming motilities powered by flagella, and twitching motility powered by Type IV pili, little is known about gliding motility. Bacterial gliders are a heterogeneous group containing diverse bacteria that utilize surface motilities that do not depend on traditional flagella or pili, but are powered by mechanisms that are less well understood. Recently, advances in our understanding of the molecular machineries for several gliding bacteria revealed the roles of modified ion channels, secretion systems and unique machinery for surface movements. These novel mechanisms provide rich source materials for studying the function and evolution of complex microbial nanomachines. In this review, we summarize recent findings made on the gliding mechanisms of the myxobacteria, flavobacteria and mycoplasmas.

MglC, a Paralog of Myxococcus xanthus GTPase-Activating Protein MglB, Plays a Divergent Role in Motility Regulation

In order to optimize interactions with their environment and one another, bacteria regulate their motility. In the case of the rod-shaped cells of Myxococcus xanthus, regulated motility is essential for social behaviors. M. xanthus moves over surfaces using type IV pilus-dependent motility and gliding motility. These two motility systems are coordinated by a protein module that controls cell polarity and consists of three polarly localized proteins, the small G protein MglA, the cognate MglA GTPase-activating protein MglB, and the response regulator RomR. Cellular reversals are induced by the Frz chemosensory system, and the output response regulator of this system, FrzZ, interfaces with the MglA/MglB/RomR module to invert cell polarity. Using a computational approach, we identify a paralog of MglB, MXAN_5770 (MglC). Genetic epistasis experiments demonstrate that MglC functions in the same pathway as MglA, MglB, RomR, and FrzZ and is important for regulating cellular reversals. Like MglB, MglC localizes to the cell poles asymmetrically and with a large cluster at the lagging pole. Correct polar localization of MglC depends on RomR and MglB. Consistently, MglC interacts directly with MglB and the C-terminal output domain of RomR, and we identified a surface of MglC that is necessary for the interaction with MglB and for MglC function. Together, our findings identify an additional member of the M. xanthus polarity module involved in regulating motility and demonstrate how gene duplication followed by functional divergence can add a layer of control to the complex cellular processes of motility and motility regulation.

IMPORTANCE Gene duplication and the subsequent divergence of the duplicated genes are important evolutionary mechanisms for increasing both biological complexity and regulation of biological processes. The bacterium Myxococcus xanthus is a soil bacterium with an unusually large genome that carries out several social processes, including predation of other bacterial species and formation of multicellular, spore-filled fruiting bodies. One feature of the large M. xanthus genome is that it contains many gene duplications. Here, we compare the products of one example of gene duplication and divergence, in which a paralog of the cognate MglA GTPase-activating protein MglB has acquired a different and opposing role in the regulation of cellular polarity and motility, processes critical to the bacterium's social behaviors.

Motility in cyanobacteria: polysaccharide tracks and Type IV pilus motors

Motility in cyanobacteria is useful for purposes that range from seeking out favourable light environments to establishing symbioses with plants and fungi. No known cyanobacterium is equipped with flagella, but a diverse range of species is able to 'glide' or 'twitch' across surfaces. Cyanobacteria with this capacity range from unicellular species to complex filamentous forms, including species such as Nostoc punctiforme, which can generate specialised motile filaments called hormogonia. Recent work on the model unicellular cyanobacterium Synechocystis sp. PCC 6803 has shown that its means of propulsion has much in common with the twitching motility of heterotrophs such as Pseudomonas and Myxococcus. Movement depends on Type IV pili, which are extended, adhere to the substrate and then retract to pull the cell across the surface. Previous work on filamentous cyanobacteria suggested a very different mechanism, with movement powered by the directional extrusion of polysaccharide from pores close to the cell junctions. Now a new report by Khayatan and colleagues in this issue of Molecular Microbiology suggests that the motility of Nostoc hormogonia has much more in common with Synechocystis than was previously thought. In both cases, polysaccharide secretion is important for preparing the surface, but the directional motive force comes from Type IV pili.

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.

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.