Isolation of bacteriophage MX4, a generalized transducing phage for Myxococcus xanthus

Genome sequences of Mx1, the first Myxococcus phage isolated, and Mx4, a generalized transducing myxophage

Myxococcus xanthus is the best-studied member of the phylum Myxococcota, but the bacteriophages infecting it and their characterization remain limited. Here, we present complete genomes of Mx1, the first Myxococcus phage isolated, and of an Mx4 derivative widely used for generalized transduction, both unclassified Caudoviricetes with long, contractile tails.

Genetic components of Escherichia coli involved in its complex prey-predator interaction with Myxococcus xanthus

Myxococcus xanthus and Escherichia coli represent a well-studied microbial predator-prey pair frequently examined in laboratory settings. While significant progress has been made in comprehending the mechanisms governing M. xanthus predation, various aspects of the response and defensive mechanisms of E. coli as prey remain elusive. In this study, the E. coli MG1655 large-scale chromosome deletion library was screened, and a mutant designated as ME5012 was identified to possess significantly reduced susceptibility to predation by M. xanthus. Within the deleted region of ME5012 encompassing seven genes, the significance of dusB and fis genes in driving the observed phenotype became apparent. Specifically, the deletion of fis resulted in a notable reduction in flagellum production in E. coli, contributing to a certain level of resistance against predation by M. xanthus. Meanwhile, the removal of dusB in E. coli led to diminished inducibility of myxovirescin A production by M. xanthus, accompanied by a slight decrease in susceptibility to myxovirescin A. These findings shed light on the molecular mechanisms underlying the complex interaction between M. xanthus and E. coli in a predatory context.

Myxococcus xanthus PilB interacts with c-di-GMP and modulates motility and biofilm formation

The regulation of biofilm and motile states as alternate bacterial lifestyles has been studied extensively in flagellated bacteria, where the second messenger cyclic-di-GMP (cdG) plays a crucial role. However, much less is known about the mechanisms of such regulation in motile bacteria without flagella. The bacterial type IV pilus (T4P) serves as a motility apparatus that enables Myxococcus xanthus to move on solid surfaces. PilB, the T4P assembly ATPase, is, therefore, required for T4P-dependent motility in M. xanthus. Interestingly, T4P is also involved in the regulation of exopolysaccharide as the biofilm matrix material in this bacterium. A newly discovered cdG-binding domain, MshEN, is conserved in the N-terminus of PilB (PilBN) in M. xanthus and other bacteria. This suggests that cdG may bind to PilB to control the respective outputs that regulate biofilm development and T4P-powered motility. In this study, we aimed to validate M. xanthus PilB as a cdG effector protein. We performed a systematic mutational analysis of its cdG-binding domain to investigate its relationship with motility, piliation, and biofilm formation. Excluding those resulting in low levels of PilB protein, all other substitution mutations in PilBN resulted in pilB mutants with distinct and differential phenotypes in piliation and biofilm levels in M. xanthus. This suggests that the PilBN domain plays dual roles in modulating motility and biofilm levels, and these two functions of PilB can be dependent on and independent of each other in M. xanthus. IMPORTANCE The regulation of motility and biofilm by cyclic-di-GMP in flagellated bacteria has been extensively investigated. However, our knowledge regarding this regulation in motile bacteria without flagella remains limited. Here, we aimed to address this gap by investigating a non-flagellated bacterium with motility powered by bacterial type-IV pilus (T4P). Previous studies hinted at the possibility of Myxococcus xanthus PilB, the T4P assembly ATPase, serving as a cyclic-di-GMP effector involved in regulating both motility and biofilm. Our findings strongly support the hypothesis that PilB directly interacts with cyclic-di-GMP to act as a potential switch to promote biofilm formation or T4P-dependent motility. These results shed light on the bifurcation of PilB functions and its pivotal role in coordinating biofilm formation and T4P-mediated motility.

Discovery of Two Inhibitors of the Type IV Pilus Assembly ATPase PilB as Potential Antivirulence Compounds

With the pressing antibiotic resistance pandemic, antivirulence has been increasingly explored as an alternative strategy against bacterial infections. The bacterial type IV pilus (T4P) is a well-documented virulence factor and an attractive target for small molecules for antivirulence purposes. The PilB ATPase is essential for T4P biogenesis because it catalyzes the assembly of monomeric pilins into the polymeric pilus filament. Here, we describe the identification of two PilB inhibitors by a high-throughput screen (HTS) in vitro and their validation as effective inhibitors of T4P assembly in vivo. We used Chloracidobacterium thermophilum PilB as a model enzyme to optimize an ATPase assay for the HTS. From a library of 2,320 compounds, benserazide and levodopa, two approved drugs for Parkinson's disease, were identified and confirmed biochemically to be PilB inhibitors. We demonstrate that both compounds inhibited the T4P-dependent motility of the bacteria Myxoccocus xanthus and Acinetobacter nosocomialis. Additionally, benserazide and levodopa were shown to inhibit A. nosocomialis biofilm formation, a T4P-dependent process. Using M. xanthus as a model, we showed that both compounds inhibited T4P assembly in a dose-dependent manner. These results suggest that these two compounds are effective against the PilB protein in vivo. The potency of benserazide and levodopa as PilB inhibitors both in vitro and in vivo demonstrate potentials of the HTS and its two hits here for the development of anti-T4P chemotherapeutics. IMPORTANCE Many bacterial pathogens use their type IV pilus (T4P) to facilitate and maintain an infection in a human host. Small-molecule inhibitors of the production or assembly of the T4P are promising for the treatment and prevention of infections by these bacteria, especially in our fight against antibiotic-resistant pathogens. Here, we report the development and implementation of a method to identify anti-T4P chemicals from compound libraries by high-throughput screen. This led to the identification and validation of two T4P inhibitors both in the test tubes and in bacteria. The discovery and validation pipeline reported here as well as the confirmation of two anti-T4P inhibitors provide new venues and leads for the development of chemotherapeutics against antibiotic-resistant infections.

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.

Complete Genome Sequence of the Nonmotile Myxococcus xanthus Strain NM

Myxobacteria exhibit multicellular swarming behavior, which depends on the coordination of cell motility. Unlike other myxobacteria, Myxococcus xanthus NM is not capable of forming swarms due to a defective motility system. Here, we present the 9.35-Mbp genome sequence of this nonmotile myxobacterium.

Complete Genome Sequence of Myxococcus Phage Mx4

Myxococcus xanthus is a bacterial model in microbial developmental biology and social evolution. Here, I present the 57.0-kb circular genomic sequence of the wild-type Myxococcus phage Mx4, with a GC content of 70.1%. Annotation predicted 97 protein-coding genes. Head-neck-tail protein classification assigns Mx4 to the tailed, Mu-like members of the family Myoviridae of group type 1 (cluster 8).

Bacteriophages of Myxococcus xanthus, a Social Bacterium

Bacteriophages have been used as molecular tools in fundamental biology investigations for decades. Beyond this, however, they play a crucial role in the eco-evolutionary dynamics of bacterial communities through their demographic impact and the source of genetic information they represent. The increasing interest in describing ecological and evolutionary aspects of bacteria–phage interactions has led to major insights into their fundamental characteristics, including arms race dynamics and acquired bacterial immunity. Here, we review knowledge on the phages of the myxobacteria with a major focus on phages infecting Myxococcus xanthus, a bacterial model system widely used to study developmental biology and social evolution. In particular, we focus upon the isolation of myxophages from natural sources and describe the morphology and life cycle parameters, as well as the molecular genetics and genomics of the major groups of myxophages. Finally, we propose several interesting research directions which focus on the interplay between myxobacterial host sociality and bacteria–phage interactions.

PlpA, a PilZ-like protein, regulates directed motility of the bacterium Myxococcus xanthus

The rod-shaped bacterium Myxococcus xanthus moves on surfaces along its long cell axis and reverses its moving direction regularly. Current models propose that the asymmetric localization of a Ras-like GTPase, MglA, to leading cell poles determines the moving direction of cells. However, cells are still motile in the mutants where MglA localizes symmetrically, suggesting the existence of additional regulators that control moving direction. In this study, we identified PlpA, a PilZ-like protein that regulates the direction of motility. PlpA and MglA localize into opposite asymmetric patterns. Deletion of the plpA gene abolishes the asymmetry of MglA localization, increases the frequency of cellular reversals and leads to severe defects in cell motility. By tracking the movements of single motor particles, we demonstrated that PlpA and MglA co-regulated the direction of gliding motility through direct interactions with the gliding motor. PlpA inhibits the reversal of individual gliding motors while MglA promotes motor reversal. By counteracting MglA near lagging cell poles, PlpA reinforces the polarity axis of MglA and thus stabilizes the direction of motility.

High concentrations of intracellular Ap4A and/or Ap5A in developing Myxococcus xanthus cells inhibit sporulation

Diadenosine polyphosphates (ApnA) are thought to act as signalling molecules regulating stress responses and biofilm formation in prokaryotes. However, ApnA function in Myxococcus xanthus remains unknown. Here, we investigated the role of ApnA in M. xanthus, using the wild-type and ApnA hydrolase (apaH) mutant strains exposed to various stress conditions. In both wild-type and apaH mutant cells cultured on starvation medium (CF agar), the levels of intracellular diadenosine tetraphosphate (Ap4A) and pentaphosphate (Ap5A) increased several fold during the first 16 h of development and decreased gradually thereafter. The levels of Ap4A and Ap5A in the apaH mutant were about 5- and 11-fold higher than those in the wild-type strain at 16 h, respectively. ApnA hydrolase activity of the wild-type strain increased 1.5-fold during the first 8 h of development, and it then gradually decreased. The apaH mutant formed spores 1-2 days after the wild-type strain did, and the yield of viable spores was 5.5 % of that in the wild-type strain 5 days after inoculation onto CF agar. These results suggest the possibility that high intracellular levels of Ap4A and/or Ap5A may inhibit M. xanthus sporulation at the early stage of development and that the bacteria reduce intracellular Ap4A and Ap5A accumulation through ApnA hydrolase activity.

Exopolysaccharides promote Myxococcus xanthus social motility by inhibiting cellular reversals

The biofilm-forming bacterium Myxococcus xanthus moves on surfaces as structured swarms utilizing type IV pili-dependent social (S) motility. In contrast to isolated cells that reverse their moving direction frequently, individual cells within swarms rarely reverse. The regulatory mechanisms that inhibit cellular reversal and promote the formation of swarms are not well understood. Here we show that exopolysaccharides (EPS), the major extracellular components of M. xanthus swarms, inhibit cellular reversal in a concentration-dependent manner. Thus, individual wild-type cells reverse less frequently in swarms due to high local EPS concentrations. In contrast, cells defective in EPS production hyper-reverse their moving direction and show severe defects in S-motility. Surprisingly, S-motility and wild-type reversal frequency are restored in double mutants that are defective in both EPS production and the Frz chemosensory system, indicating that EPS regulates cellular reversal in parallel to the Frz pathway. Here we clarify that besides functioning as the structural scaffold in biofilms, EPS is a self-produced signal that coordinates the group motion of the social bacterium M. xanthus.

More Is Better: Selecting for Broad Host Range Bacteriophages

Bacteriophages are viruses that infect bacteria. In this perspective, we discuss several aspects of a characteristic feature of bacteriophages, their host range. Each phage has its own particular host range, the range of bacteria that it can infect. While some phages can only infect one or a few bacterial strains, other phages can infect many species or even bacteria from different genera. Different methods for determining host range may give different results, reflecting the multiple mechanisms bacteria have to resist phage infection and reflecting the different steps of infection each method depends on. This makes defining host range difficult. Another difficulty in describing host range arises from the inconsistent use of the words "narrow" and especially "broad" when describing the breadth of the host range. Nearly all bacteriophages have been isolated using a single host strain of bacteria. While this procedure is fairly standard, it may more likely produce narrow rather than broad host range phage. Our results and those of others suggest that using multiple host strains during isolation can more reliably produce broader host range phages. This challenges the common belief that most bacteriophages have a narrow host range. We highlight the implications of this for several areas that are affected by host range including horizontal gene transfer and phage therapy.

Interplay between type IV pili activity and exopolysaccharides secretion controls motility patterns in single cells of Myxococcus xanthus

Myxococcus xanthus performs coordinated social motility of cell groups through the extension and retraction of type IV pili (TFP) on solid surfaces, which requires both TFP and exopolysaccharides (EPS). By submerging cells in a liquid medium containing 1% methylcellulose, M. xanthus TFP-driven motility was induced in isolated cells and independently of EPS. We measured and analyzed the movements of cells using community tracking algorithms, which combine single-cell resolution with statistics from large sample populations. Cells without significant multi-cellular social interactions have surprisingly complex behaviors: EPS⁻ cells exhibited a pronounced increase in the tendency to stand vertically and moved with qualitatively different characteristics than other cells. A decrease in the EPS secretion of cells correlates with a higher instantaneous velocity, but with lower directional persistence in trajectories. Moreover, EPS⁻ cells do not adhere to the surface as strongly as wild-type and EPS overproducing cells, and display a greater tendency to have large deviations between the direction of movement and the cell axis, with cell velocity showing only minimal dependence on the direction of movement. The emerging picture is that EPS does not simply provide rheological resistance to a single mechanism but rather that the availability of EPS impacts motility pattern.

Dual biochemical oscillators may control cellular reversals in Myxococcus xanthus

Myxococcus xanthus is a Gram-negative, soil-dwelling bacterium that glides on surfaces, reversing direction approximately once every 6 min. Motility in M. xanthus is governed by the Che-like Frz pathway and the Ras-like Mgl pathway, which together cause the cell to oscillate back and forth. Previously, Igoshin et al. (2004) suggested that the cellular oscillations are caused by cyclic changes in concentration of active Frz proteins that govern motility. In this study, we present a computational model that integrates both the Frz and Mgl pathways, and whose downstream components can be read as motor activity governing cellular reversals. This model faithfully reproduces wildtype and mutant behaviors by simulating individual protein knockouts. In addition, the model can be used to examine the impact of contact stimuli on cellular reversals. The basic model construction relies on the presence of two nested feedback circuits, which prompted us to reexamine the behavior of M. xanthus cells. We performed experiments to test the model, and this cell analysis challenges previous assumptions of 30 to 60 min reversal periods in frzCD, frzF, frzE, and frzZ mutants. We demonstrate that this average reversal period is an artifact of the method employed to record reversal data, and that in the absence of signal from the Frz pathway, Mgl components can occasionally reverse the cell near wildtype periodicity, but frz- cells are otherwise in a long nonoscillating state.

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

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

Killing of Escherichia coli by Myxococcus xanthus in aqueous environments requires exopolysaccharide-dependent physical contact

Nutrient or niche-based competition among bacteria is a widespread phenomenon in the natural environment. Such interspecies interactions are often mediated by secreted soluble factors and/or direct cell-cell contact. As ubiquitous soil bacteria, Myxococcus species are able to produce a variety of bioactive secondary metabolites to inhibit the growth of other competing bacterial species. Meanwhile, Myxococcus spp. also exhibit sophisticated predatory behavior, an extreme form of competition that is often stimulated by close contact with prey cells and largely depends on the availability of solid surfaces. Myxococcus spp. can also be isolated from aquatic environments. However, studies focusing on the interaction between Myxococcus and other bacteria in such environments are still limited. In this study, using the well-studied Myxococcus xanthus DK1622 and Escherichia coli as model interspecies interaction pair, we demonstrated that in an aqueous environment, M. xanthus was able to kill E. coli in a cell contact-dependent manner and that the observed contact-dependent killing required the formation of co-aggregates between M. xanthus and E. coli cells. Further analysis revealed that exopolysaccharide (EPS), type IV pilus, and lipopolysaccharide mutants of M. xanthus displayed various degrees of attenuation in E. coli killing, and it correlated well with the mutants' reduction in EPS production. In addition, M. xanthus showed differential binding ability to different bacteria, and bacterial strains unable to co-aggregate with M. xanthus can escape the killing, suggesting the specific nature of co-aggregation and the targeted killing of interacting bacteria. In conclusion, our results demonstrated EPS-mediated, contact-dependent killing of E. coli by M. xanthus, a strategy that might facilitate the survival of this ubiquitous bacterium in aquatic environments.

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

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

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

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

Characterization of Myxococcus xanthus MazF and implications for a new point of regulation

During development, Myxococcus xanthus cells undergo programmed cell death (PCD) whereby 80% of vegetative cells die. Previously, the MazF RNA interferase has been implicated in this role. Recently, it was shown that deletion of the mazF gene does not eliminate PCD in wild-type strain DK1622 as originally seen in DZF1. To clarify the role of MazF, recombinant enzyme was characterized using a highly sensitive assay in the presence and absence of the proposed antitoxin MrpC. In contrast to previous reports that MrpC inhibits MazF activity, the hydrolysis rate was enhanced in a concentration-dependent manner with MrpC or MrpC2, an N-terminally truncated form of MrpC. Furthermore, MazF transcripts were not detected until 6-8 h post-induction, suggesting an antitoxin is unnecessary earlier. Potential MazF targets were identified and their transcript levels were shown to decline in DK1622 while remaining steady in a mazF deletion strain. Elimination of the mazF hydrolysis site in the nla6 transcript resulted in overproduction of the mRNA. Thus, MazF negatively regulates specific transcripts. Additionally, we show that discrepancies in the developmental phenotypes caused by removal of mazF in DK1622 and DZF1 are due to the presence of the pilQ1 allele in the latter strain.

Earthquake-like dynamics in Myxococcus xanthus social motility

Myxococcus xanthus is a bacterium capable of complex social organization. Its characteristic social ("S")-motility mechanism is mediated by type IV pili (TFP), linear actuator appendages that propel the bacterium along a surface. TFP are known to bind to secreted exopolysaccharides (EPS), but it is unclear how M. xanthus manages to use the TFP-EPS technology common to many bacteria to achieve its unique coordinated multicellular movements. We examine M. xanthus S-motility, using high-resolution particle-tracking algorithms, and observe aperiodic stick-slip movements. We show that they are not due to chemotaxis, but are instead consistent with a constant TFP-generated force interacting with EPS, which functions both as a glue and as a lubricant. These movements are quantitatively homologous to the dynamics of earthquakes and other crackling noise systems. These systems exhibit critical behavior, which is characterized by a statistical hierarchy of discrete "avalanche" motions described by a power law distribution. The measured critical exponents from M. xanthus are consistent with mean field theoretical models and with other crackling noise systems, and the measured Lyapunov exponent suggests the existence of highly branched EPS. Such molecular architectures, which are common for efficient lubricants but rare in bacterial EPS, may be necessary for S-motility: We show that the TFP of leading "locomotive" cells initiate the collective motion of follower cells, indicating that lubricating EPS may alleviate the force generation requirements on the lead cell and thus make S-motility possible.

DNA builds and strengthens the extracellular matrix in Myxococcus xanthus biofilms by interacting with exopolysaccharides

One intriguing discovery in modern microbiology is the extensive presence of extracellular DNA (eDNA) within biofilms of various bacterial species. Although several biological functions have been suggested for eDNA, including involvement in biofilm formation, the detailed mechanism of eDNA integration into biofilm architecture is still poorly understood. In the biofilms formed by Myxococcus xanthus, a Gram-negative soil bacterium with complex morphogenesis and social behaviors, DNA was found within both extracted and native extracellular matrices (ECM). Further examination revealed that these eDNA molecules formed well organized structures that were similar in appearance to the organization of exopolysaccharides (EPS) in ECM. Biochemical and image analyses confirmed that eDNA bound to and colocalized with EPS within the ECM of starvation biofilms and fruiting bodies. In addition, ECM containing eDNA exhibited greater physical strength and biological stress resistance compared to DNase I treated ECM. Taken together, these findings demonstrate that DNA interacts with EPS and strengthens biofilm structures in M. xanthus.

The clpB gene is involved in the stress response of Myxococcus xanthus during vegetative growth and development

The Clp/HSP100 family of molecular chaperones is ubiquitous in both prokaryotes and eukaryotes. These proteins play important roles in refolding, disaggregating and degrading proteins damaged by stress. As a subclass of the Clp/HSP100 family, ClpB has been shown to be involved in various stress responses as well as other functions in bacteria. In the present study, we investigated the role of a predicted ClpB-encoding gene, MXAN5092, in the stress response during vegetative growth and development of Myxococcus xanthus. Transcriptional analysis confirmed induction of this clpB homologue under different stress conditions, and further phenotypic analysis revealed that an in-frame deletion mutant of MXAN5092 was more sensitive to various stress treatments than the wild-type strain during vegetative growth. Moreover, the absence of the MXAN5092 gene resulted in decreased heat tolerance of myxospores, indicating the involvement of this clpB homologue in the stress response during the development of myxospores. The M. xanthus recombinant ClpB (MXAN5092) protein also showed a general chaperone activity in vitro. Overall, our genetic and phenotypic analysis of the predicted ATP-dependent chaperone protein ClpB (MXAN5092) demonstrated that it functions as a chaperone protein and plays an important role in cellular stress tolerance during both vegetative growth and development of M. xanthus.

Effects of exopolysaccharide production on liquid vegetative growth, stress survival, and stationary phase recovery in Myxococcus xanthus

Exopolysaccharide (EPS) of Myxococcus xanthus is a well-regulated cell surface component. In addition to its known functions for social motility and fruiting body formation on solid surfaces, EPS has also been proposed to play a role in multi-cellular clumping in liquid medium, though this phenomenon has not been well studied. In this report, we confirmed that M. xanthus clumps formed in liquid were correlated with EPS levels and demonstrated that the EPS encased cell clumps exhibited biofilm-like structures. The clumps protected the cells at physiologically relevant EPS concentrations, while cells lacking EPS exhibited significant reduction in long-term viability and resistance to stressful conditions. However, excess EPS production was counterproductive to vegetative growth and viable cell recovery declined in extended late stationary phase as cells became trapped in the matrix of clumps. Therefore, optimal EPS production by M. xanthus is important for normal physiological functions in liquid.

Direct visualization of the interaction between pilin and exopolysaccharides of Myxococcus xanthus with eGFP-fused PilA protein

Type IV pili (TFP) and exopolysaccharides (EPS) are important components for social behaviors in Myxococcus xanthus, including gliding motility and fruiting body formation. Although specific interactions between TFP and EPS have been proposed, there have as yet been no direct observations of these interactions under native conditions. In this study, we found that a truncated PilA protein (PilACt) containing only the C-terminal domain (amino acids 32-208) is sufficient for EPS binding in vitro. Furthermore, an enhanced green fluorescent protein (eGFP) and PilACt fusion protein were constructed and used to label the native EPS in M. xanthus. Under confocal laser scanning microscope, the eGFP-PilACt-bound fruiting bodies, trail structures and biofilms exhibited similar patterns as the wheat germ agglutinin lectin-labeled EPS structures. This study showed that eGFP-PilACt fusion protein was able efficiently to label the EPS of M. xanthus, providing evidence for the first time of the direct interaction between the PilA protein and EPS under native conditions.

A Myxococcus xanthus bacterial tyrosine kinase, BtkA, is required for the formation of mature spores

A Myxococcus xanthus cytoplasmic bacterial tyrosine kinase, BtkA, showed phosphorylation activity in the presence of Exo. Phosphorylated BtkA was expressed late after starvation induction and early after glycerol induction. The btkA mutant was unable to complete maturation to heat- and sonication-resistant spores under both starvation- and glycerol-induced developmental conditions.

Alanine 32 in PilA is important for PilA stability and type IV pili function in Myxococcus xanthus

Type IV pili (TFP) are membrane-anchored filaments with a number of important biological functions. In the model organism Myxococcus xanthus, TFP act as molecular engines that power social (S) motility through cycles of extension and retraction. TFP filaments consist of several thousand copies of a protein called PilA or pilin. PilA contains an N-terminal α-helix essential for TFP assembly and a C-terminal globular domain important for its activity. The role of the PilA sequence and its structure-function relationship in TFP-dependent S motility remain active areas of research. In this study, we identified an M. xanthus PilA mutant carrying an alanine to valine substitution at position 32 in the α-helix, which produced structurally intact but retraction-defective TFP. Characterization of this mutant and additional single-residue variants at this position in PilA demonstrated the critical role of alanine 32 in PilA stability, TFP assembly and retraction.

Natural transformation of Myxococcus xanthus

Myxococcus xanthus belongs to the delta class of the proteobacteria and is notable for its complex life-style with social behaviors and relatively large genome. Although previous observations have suggested the existence of horizontal gene transfer in M. xanthus, its ability to take up exogenous DNA via natural transformation has not been experimentally demonstrated. In this study, we achieved natural transformation in M. xanthus using the autonomously replicating myxobacterial plasmid pZJY41 as donor DNA. M. xanthus exopolysaccharide (EPS) was shown to be an extracellular barrier for transformation. Cells deficient in EPS production, e.g., mutant strains carrying ΔdifA or ΔepsA, became naturally transformable. Among the inner barriers to transformation were restriction-modification systems in M. xanthus, which could be partially overcome by methylating DNA in vitro using cell extracts of M. xanthus prior to transformation. In addition, the incubation time of DNA with cells and the presence of divalent magnesium ion affected transformation frequency of M. xanthus. Furthermore, we also observed a potential involvement of the type IV pilus system in the DNA uptake machinery of M. xanthus. The natural transformation was totally eliminated in the ΔpilQ/epsA and Δtgl/epsA mutants, and null mutation of pilB or pilC in an ΔepsA background diminished the transformation rate. Our study, to the best of our knowledge, provides the first example of a naturally transformable species among deltaproteobacteria.

Myxobacteria gliding motility requires cytoskeleton rotation powered by proton motive force

Myxococcus xanthus is a Gram-negative bacterium that glides over surfaces without the aid of flagella. Two motility systems are used for locomotion: social-motility, powered by the retraction of type IV pili, and adventurous (A)-motility, powered by unknown mechanism(s). We have shown that AgmU, an A-motility protein, is part of a multiprotein complex that spans the inner membrane and periplasm of M. xanthus. In this paper, we present evidence that periplasmic AgmU decorates a looped continuous helix that rotates clockwise as cells glide forward, reversing its rotation when cells reverse polarity. Inhibitor studies showed that the AgmU helix rotation is driven by proton motive force (PMF) and depends on actin-like MreB cytoskeletal filaments. The AgmU motility complex was found to interact with MotAB homologs. Our data are consistent with a mechanochemical model in which PMF-driven motors, similar to bacterial flagella stator complexes, run along an endless looped helical track, driving rotation of the track; deformation of the cell surface by the AgmU-associated proteins creates pressure waves in the slime, pushing cells forward.

Exopolysaccharide-independent social motility of Myxococcus xanthus

Social motility (S motility), the coordinated movement of large cell groups on agar surfaces, of Myxococcus xanthus requires type IV pili (TFP) and exopolysaccharides (EPS). Previous models proposed that this behavior, which only occurred within cell groups, requires cycles of TFP extension and retraction triggered by the close interaction of TFP with EPS. However, the curious observation that M. xanthus can perform TFP-dependent motility at a single-cell level when placed onto polystyrene surfaces in a highly viscous medium containing 1% methylcellulose indicated that "S motility" is not limited to group movements. In an apparent further challenge of the previous findings for S motility, mutants defective in EPS production were found to perform TFP-dependent motility on polystyrene surface in methylcellulose-containing medium. By exploring the interactions between pilin and surface materials, we found that the binding of TFP onto polystyrene surfaces eliminated the requirement for EPS in EPS(-) cells and thus enabled TFP-dependent motility on a single cell level. However, the presence of a general anchoring surface in a viscous environment could not substitute for the role of cell surface EPS in group movement. Furthermore, EPS was found to serve as a self-produced anchoring substrate that can be shed onto surfaces to enable cells to conduct TFP-dependent motility regardless of surface properties. These results suggested that in certain environments, such as in methylcellulose solution, the cells could bypass the need for EPS to anchor their TPF and conduct single-cell S motility to promote exploratory movement of colonies over new specific surfaces.

Introduction of transposon Tn5 into Myxococcus for analysis of developmental and other nonselectable mutants

The transposon Tn5, which carries a gene for kanamycin resistance, can be introduced into Myxococcus xanthus, an organism that undergoes a primitive cycle of development, from Escherichia coli by the specialized transducing phage P1::Tn5. Tn5 DNA sequences, but no P1 sequences, are found in the stable kanamycin-resistant transductants. Tn5 transposes from P1 to many different chromosomal sites in Myxococcus. In each independent transductant of Myxococcus examined, the Tn5 element is found in a different DNA fragment produced by cleaving cell DNA with a restriction endonuclease. Moreover, different Tn5 insertions have been found linked to the first 20 different genetic sites tested. Once inserted into the Myxococcus chromosome, Tn5 remains fixed in position during growth and when transferred to another strain by generalized transduction. To analyze developmental or other mutants that have no selectable phenotype themselves, a general method has been devised, and tested, for the systematic isolation of a Tn5 insertion near any arbitrary locus.

A multi-protein complex from Myxococcus xanthus required for bacterial gliding motility

Myxococcus xanthus moves by gliding motility powered by Type IV pili (S-motility) and a second motility system, A-motility, whose mechanism remains elusive despite the identification of approximately 40 A-motility genes. In this study, we used biochemistry and cell biology analyses to identify multi-protein complexes associated with A-motility. Previously, we showed that the N-terminal domain of FrzCD, the receptor for the frizzy chemosensory pathway, interacts with two A-motility proteins, AglZ and AgmU. Here we characterized AgmU, a protein that localized to both the periplasm and cytoplasm. On firm surfaces, AgmU-mCherry colocalized with AglZ as distributed clusters that remained fixed with respect to the substratum as cells moved forward. Cluster formation was favoured by hard surfaces where A-motility is favoured. In contrast, AgmU-mCherry clusters were not observed on soft agar surfaces or when cells were in large groups, conditions that favour S-motility. Using glutathione-S-transferase affinity chromatography, AgmU was found to interact either directly or indirectly with multiple A-motility proteins including AglZ, AglT, AgmK, AgmX, AglW and CglB. These proteins, important for the correct localization of AgmU and AglZ, appear to be organized as a motility complex, spanning the cytoplasm, inner membrane and the periplasm. Identification of this complex may be important for uncovering the mechanism of A-motility.

Improving cellular properties for genetic manipulation by dispersed growing mutagenesis in Myxococcus fulvus HW-1

One of the key limitations to genetic manipulation in myxobacteria is that the cells grow in clumps in liquid. A salt-tolerant strain HW-1 of Myxococcus fulvus was treated with UV irradiation and produced a completely dispersedly growing mutant UV684. There were no significant differences between the parent HW-1 and the mutant UV684 in terms of salt-tolerant growth. The mutant UV684 and the parent strain had the similar abilities of the fruiting body formation and S motility. Interestingly, the mutant exhibited high transformation/transposition efficiency with 10(5)-10(6) colony-forming units per microg DNA, which was about 10(3)-10(5) fold higher than HW-1. The results indicate that the mutation that led to dispersed growth in the UV684 mutant strain had a few impacts on social behavior, but greatly facilitated molecular genetic manipulation.

Bacterial motility complexes require the actin-like protein, MreB and the Ras homologue, MglA

Gliding motility in the bacterium Myxococcus xanthus uses two motility engines: S-motility powered by type-IV pili and A-motility powered by uncharacterized motor proteins and focal adhesion complexes. In this paper, we identified MreB, an actin-like protein, and MglA, a small GTPase of the Ras superfamily, as essential for both motility systems. A22, an inhibitor of MreB cytoskeleton assembly, reversibly inhibited S- and A-motility, causing rapid dispersal of S- and A-motility protein clusters, FrzS and AglZ. This suggests that the MreB cytoskeleton is involved in directing the positioning of these proteins. We also found that a DeltamglA motility mutant showed defective localization of AglZ and FrzS clusters. Interestingly, MglA-YFP localization mimicked both FrzS and AglZ patterns and was perturbed by A22 treatment, consistent with results indicating that both MglA and MreB bind to motility complexes. We propose that MglA and the MreB cytoskeleton act together in a pathway to localize motility proteins such as AglZ and FrzS to assemble the A-motility machineries. Interestingly, M. xanthus motility systems, like eukaryotic systems, use an actin-like protein and a small GTPase spatial regulator.

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.

AglZ regulates adventurous (A-) motility in Myxococcus xanthus through its interaction with the cytoplasmic receptor, FrzCD

Myxococcus xanthus moves by gliding motility powered by type IV pili (S-motility) and distributed motor complexes (A-motility). The Frz chemosensory pathway controls reversals for both motility systems. However, it is unclear how the Frz pathway can communicate with these different systems. In this article, we show that FrzCD, the Frz pathway receptor, interacts with AglZ, a protein associated with A-motility. Affinity chromatography and cross-linking experiments showed that the FrzCD-AglZ interaction occurs between the uncharacterized N-terminal region of FrzCD and the N-terminal pseudo-receiver domain of AglZ. Fluorescence microscopy showed AglZ-mCherry and FrzCD-GFP localized in clusters that occupy different positions in cells. To study the role of the Frz system in the regulation of A-motility, we constructed aglZ frzCD double mutants and aglZ frzCD pilA triple mutants. To our surprise, these mutants, predicted to show no A-motility (A-S+) or no motility at all (A-S-), respectively, showed restored A-motility. These results indicate that AglZ modulates a FrzCD activity that inhibits A-motility. We hypothesize that AglZ-FrzCD interactions are favoured when cells are isolated and moving by A-motility and inhibited when S-motility predominates and A-motility is reduced.

Generalized transduction

Transduction is the process in which bacterial DNA is transferred from one bacterial cell to another by means of a phage particle. There are two types of transduction, generalized transduction and specialized transduction. In this chapter two of the best-studied systems - Escherichia coli-phage P1, and Salmonella enterica-phage P22 - are discussed from theoretical and practical perspectives.

Site-specific receptor methylation of FrzCD in Myxococcus xanthus is controlled by a tetra-trico peptide repeat (TPR) containing regulatory domain of the FrzF methyltransferase

Myxococcus xanthus is a gliding bacterium with a complex life cycle that includes swarming, predation and fruiting body formation. Directed movements in M. xanthus are regulated by the Frz chemosensory system, which controls cell reversals. The Frz pathway requires the activity of FrzCD, a cytoplasmic methyl-accepting chemotaxis protein, and FrzF, a methyltransferase (CheR) containing an additional domain with three tetra trico-peptide repeats (TPRs). To investigate the role of the TPRs in FrzCD methylation, we used full-length FrzF and FrzF lacking its TPRs (FrzF^CheR) to methylate FrzCD in vitro. FrzF methylated FrzCD on a single residue, E182, while FrzF^CheR methylated FrzCD on three residues, E168, E175 and E182, indicating that the TPRs regulate site-specific methylation. E168 and E182 were predicted consensus methylation sites, but E175 is methylated on an HE pair. To determine the roles of these sites in vivo, we substituted each methylatable glutamate with either an aspartate or an alanine residue and determined the impact of the point mutants on single cell reversals, swarming and fruiting body formation. Single, double and triple methylation site mutants revealed that each site played a unique role in M. xanthus behaviour and that the pattern of receptor methylation determined receptor activity. This work also shows that methylation can both activate and inactivate the receptor.

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

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

Discovery of the autonomously replicating plasmid pMF1 from Myxococcus fulvus and development of a gene cloning system in Myxococcus xanthus

Myxobacteria are very important due to their unique characteristics, such as multicellular social behavior and the production of diverse and novel bioactive secondary metabolites. However, the lack of autonomously replicating plasmids has hindered genetic manipulation of myxobacteria for decades. To determine whether indigenous plasmids are present, we screened about 150 myxobacterial strains, and a circular plasmid designated pMF1 was isolated from Myxococcus fulvus 124B02. Sequence analysis showed that this plasmid was 18,634 bp long and had a G+C content of 68.7%. Twenty-three open reading frames were found in the plasmid, and 14 of them were not homologous to any known sequence. Plasmids containing the gene designated pMF1.14, which encodes a large unknown protein, were shown to transform Myxococcus xanthus DZ1 and DK1622 at high frequencies ( approximately 10(5) CFU/microg DNA), suggesting that the locus is responsible for the autonomous replication of pMF1. Shuttle vectors were constructed for both M. xanthus and Escherichia coli. The pilA gene, which is essential for pilus formation and social motility in M. xanthus, was cloned into the shuttle vectors and introduced into the pilA-deficient mutant DK10410. The transformants subsequently exhibited the ability to form pili and social motility. Autonomously replicating plasmid pMF1 provides a new tool for genetic manipulation in Myxococcus.

Genome evolution and the emergence of fruiting body development in Myxococcus xanthus

Background

Lateral gene transfer (LGT) is thought to promote speciation in bacteria, though well-defined examples have not been put forward.

Methodology/Principle Findings

We examined the evolutionary history of the genes essential for a trait that defines a phylogenetic order, namely fruiting body development of the Myxococcales. Seventy-eight genes that are essential for Myxococcus xanthus development were examined for LGT. About 73% of the genes exhibit a phylogeny similar to that of the 16S rDNA gene and a codon bias consistent with other M. xanthus genes suggesting vertical transmission. About 22% have an altered codon bias and/or phylogeny suggestive of LGT. The remaining 5% are unique. Genes encoding signal production and sensory transduction were more likely to be transmitted vertically with clear examples of duplication and divergence into multigene families. Genes encoding metabolic enzymes were frequently acquired by LGT. Myxobacteria exhibit aerobic respiration unlike most of the δ Proteobacteria. M. xanthus contains a unique electron transport pathway shaped by LGT of genes for succinate dehydrogenase and three cytochrome oxidase complexes.

Conclusions/Significance

Fruiting body development depends on genes acquired by LGT, particularly those involved in polysaccharide production. We suggest that aerobic growth fostered innovation necessary for development by allowing myxobacteria access to a different gene pool from anaerobic members of the δ Proteobacteria. Habitat destruction and loss of species diversity could restrict the evolution of new bacterial groups by limiting the size of the prospective gene pool.

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

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

Proteins associated with the Myxococcus xanthus extracellular matrix

Fruiting body formation of Myxococcus xanthus, like biofilm formation of many other organisms, involves the production of an extracellular matrix (ECM). While the polysaccharide component has been studied, the protein component has been largely unexplored. Proteins associated with the ECM were solubilized from purified ECM by boiling with sodium dodecyl sulfate and were identified by liquid chromatography-tandem mass spectrometry of tryptic fragments. The ECM is enriched in proteins of novel function; putative functions were assigned for only 5 of the 21 proteins. Thirteen putative ECM proteins had lipoprotein secretion signals. The genes for many ECM proteins were disrupted in the wild-type (WT), fibA, and pilA backgrounds. Disruption of the MXAN4860 gene had no effect in the WT or fibA background but in the pilA background resulted in a 24-h delay in aggregation and sporulation compared to its parent. The results of this study show that the M. xanthus ECM proteome is diverse and novel.

Adaptation of salt-tolerant Myxococcus strains and their motility systems to the ocean conditions

More and more studies have indicated that myxobacteria are able to live in seawater conditions, which, however, can decrease the fruiting body formation ability and also the adventurous (A) and social (S) motility systems of the myxobacteria. To learn the adaptation mechanism of the salt-tolerant myxobacteria to marine conditions, we analyzed 10 salt-tolerant Myxococcus strains of their fruiting body formation and motility. The isolates were from marine samples and possessed different levels of salt tolerance. They had the dual motility system and formed fruiting bodies in the presence of suitable seawater concentrations. Some high salt-tolerant strains even lost their fruiting abilities in the absence of seawater. In response to the presence of seawater, the S-motility was found to be increased in the high salt-tolerants but decreased in the low salt-tolerants. The A-motility, on the other hand, was observed in all the salt-tolerant Myxococcus strains, but increased or decreased in response to the presence of seawater. Perceived shifts of fruiting body formation abilities and motilities discovered in the salt-tolerant Myxococcus strains suggested an ecological adaptation of myxobacterial social behaviors to the marine environments.