Role of the sigmaD-dependent autolysins in Bacillus subtilis population heterogeneity

New mechanistic insights into the motile-to-sessile switch in various bacteria with particular emphasis on Bacillus subtilis and Pseudomonas aeruginosa: a review

A biofilm is a complex assemblage of microbial communities adhered to a biotic or an abiotic surface which is embedded within a self-produced matrix of extracellular polymeric substances. Many transcriptional regulators play a role in triggering a motile-sessile switch and in consequently producing the biofilm matrix. This review is aimed at highlighting the role of two nucleotide signaling molecules (c-di-GMP and c-di-AMP), toxin antitoxin modules and a novel transcriptional regulator BolA in biofilm formation in various bacteria. In addition, it highlights the common themes that have appeared in recent research regarding the key regulatory components and signal transduction pathways that help Bacillus subtilis and Pseudomonas aeruginosa to acquire the biofilm mode of life.

Genomic sequencing-based mutational enrichment analysis identifies motility genes in a genetically intractable gut microbe

A major roadblock to understanding how microbes in the gastrointestinal tract colonize and influence the physiology of their hosts is our inability to genetically manipulate new bacterial species and experimentally assess the function of their genes. We describe the application of population-based genomic sequencing after chemical mutagenesis to map bacterial genes responsible for motility in Exiguobacterium acetylicum, a representative intestinal Firmicutes bacterium that is intractable to molecular genetic manipulation. We derived strong associations between mutations in 57 E. acetylicum genes and impaired motility. Surprisingly, less than half of these genes were annotated as motility-related based on sequence homologies. We confirmed the genetic link between individual mutations and loss of motility for several of these genes by performing a large-scale analysis of spontaneous suppressor mutations. In the process, we reannotated genes belonging to a broad family of diguanylate cyclases and phosphodiesterases to highlight their specific role in motility and assigned functions to uncharacterized genes. Furthermore, we generated isogenic strains that allowed us to establish that Exiguobacterium motility is important for the colonization of its vertebrate host. These results indicate that genetic dissection of a complex trait, functional annotation of new genes, and the generation of mutant strains to define the role of genes in complex environments can be accomplished in bacteria without the development of species-specific molecular genetic tools.

When Phase Contrast Fails: ChainTracer and NucTracer, Two ImageJ Methods for Semi-Automated Single Cell Analysis Using Membrane or DNA Staining

Within bacterial populations, genetically identical cells often behave differently. Single-cell measurement methods are required to observe this heterogeneity. Flow cytometry and fluorescence light microscopy are the primary methods to do this. However, flow cytometry requires reasonably strong fluorescence signals and is impractical when bacteria grow in cell chains. Therefore fluorescence light microscopy is often used to measure population heterogeneity in bacteria. Automatic microscopy image analysis programs typically use phase contrast images to identify cells. However, many bacteria divide by forming a cross-wall that is not detectable by phase contrast. We have developed ‘ChainTracer’, a method based on the ImageJ plugin ObjectJ. It can automatically identify individual cells stained by fluorescent membrane dyes, and measure fluorescence intensity, chain length, cell length, and cell diameter. As a complementary analysis method we developed 'NucTracer', which uses DAPI stained nucleoids as a proxy for single cells. The latter method is especially useful when dealing with crowded images. The methods were tested with Bacillus subtilis and Lactococcus lactis cells expressing a GFP-reporter. In conclusion, ChainTracer and NucTracer are useful single cell measurement methods when bacterial cells are difficult to distinguish with phase contrast.

Expression of multiple Bacillus subtilis genes is controlled by decay of slrA mRNA from Rho-dependent 3' ends

Timely turnover of RNA is an important element in the control of bacterial gene expression, but relatively few specific targets of RNA turnover regulation are known. Deletion of the Bacillus subtilis pnpA gene, encoding the major 3' exonuclease turnover enzyme, polynucleotide phosphorylase (PNPase), was shown previously to cause a motility defect correlated with a reduced level of the 32-gene fla/che flagellar biosynthesis operon transcript.fla/che operon transcript abundance has been shown to be inhibited by an excess of the small regulatory protein, SlrA, and here we find that slrA mRNA accumulated in the pnpA-deletion mutant. Mutation of slrA was epistatic to mutation of pnpA for the motility-related phenotype. Further, Rho-dependent termination was required for PNPase turnover of slrA mRNA. When the slrA gene was provided with a Rho-independent transcription terminator, gene regulation was no longer PNPase-dependent. Thus we show that the slrA transcript is a direct target of PNPase and that regulation of RNA turnover is a major determinant of motility gene expression. The interplay of specific transcription termination and mRNA decay mechanisms suggests selection for fine-tuning of gene expression.

Controlling Autolysis During Flagella Insertion in Gram-Negative Bacteria

The flagellum is an important macromolecular machine for many pathogenic bacteria. It is a hetero-oligomeric structure comprised of three major sub-structures: basal body, hook and thin helical filament. An important step during flagellum assembly is the localized and controlled degradation of the peptidoglycan sacculus to allow for the insertion of the rod as well as to facilitate anchoring for proper motor function. The peptidoglycan lysis events require specialized lytic enzymes, β-N-acetylglucosaminidases and lytic transglycosylases, which differ in flagellated proteobacteria. Due to their autolytic activity, these enzymes need to be controlled in order to prevent cellular lysis. This review summarizes are current understanding of the peptidoglycan lysis events required for flagellum assembly and motility with a main focus on Gram-negative bacteria.

Biofilm formation by Bacillus subtilis requires an endoribonuclease-containing multisubunit complex that controls mRNA levels for the matrix gene repressor SinR

Biofilm formation by Bacillus subtilis is largely governed by a circuit in which the response regulator Spo0A turns on the gene for the anti-repressor SinI. SinI, in turn, binds to and inactivates SinR, a dedicated repressor of genes for matrix production. Mutants of the genes ylbF, ymcA and yaaT are blocked in biofilm formation, but the mechanism by which they act has been mysterious. A recent report attributed their role in biofilm formation to stimulating Spo0A activity. However, we detect no measurable effect on the transcription of sinI. Instead, we find that the block in biofilm formation is caused by an increase in the levels of SinR and of its mRNA. Evidence is presented that YlbF, YmcA and YaaT interact with, and control the activity of, RNase Y, which is known to destabilize sinR mRNA. We also show that the processing of another target of RNase Y, cggR-gapA mRNA, similarly depends on YlbF and YmcA. Our work suggests that sinR mRNA stability is an additional posttranscriptional control mechanism governing the switch to multicellularity and raises the possibility that YlbF, YmcA and YaaT broadly regulate mRNA stability as part of an RNase Y-containing, multi-subunit complex.

Water surface tension modulates the swarming mechanics of Bacillus subtilis

Many Bacillus subtilis strains swarm, often forming colonies with tendrils on agar medium. It is known that B. subtilis swarming requires flagella and a biosurfactant, surfactin. In this study, we find that water surface tension plays a role in swarming dynamics. B. subtilis colonies were found to contain water, and when a low amount of surfactin is produced, the water surface tension of the colony restricts expansion, causing bacterial density to rise. The increased density induces a quorum sensing response that leads to heightened production of surfactin, which then weakens water surface tension to allow colony expansion. When the barrier formed by water surface tension is breached at a specific location, a stream of bacteria swarms out of the colony to form a tendril. If a B. subtilis strain produces surfactin at levels that can substantially weaken the overall water surface tension of the colony, water floods the agar surface in a thin layer, within which bacteria swarm and migrate rapidly. This study sheds light on the role of water surface tension in regulating B. subtilis swarming, and provides insight into the mechanisms underlying swarming initiation and tendril formation.

Bacterial SPOR domains are recruited to septal peptidoglycan by binding to glycan strands that lack stem peptides

Bacterial SPOR domains bind peptidoglycan (PG) and are thought to target proteins to the cell division site by binding to "denuded" glycan strands that lack stem peptides, but uncertainties remain, in part because septal-specific binding has yet to be studied in a purified system. Here we show that fusions of GFP to SPOR domains from the Escherichia coli cell-division proteins DamX, DedD, FtsN, and RlpA all localize to septal regions of purified PG sacculi obtained from E. coli and Bacillus subtilis. Treatment of sacculi with an amidase that removes stem peptides enhanced SPOR domain binding, whereas treatment with a lytic transglycosylase that removes denuded glycans reduced SPOR domain binding. These findings demonstrate unequivocally that SPOR domains localize by binding to septal PG, that the physiologically relevant binding site is indeed a denuded glycan, and that denuded glycans are enriched in septal PG rather than distributed uniformly around the sacculus. Accumulation of denuded glycans in the septal PG of both E. coli and B. subtilis, organisms separated by 1 billion years of evolution, suggests that sequential removal of stem peptides followed by degradation of the glycan backbone is an ancient feature of PG turnover during bacterial cell division. Linking SPOR domain localization to the abundance of a structure (denuded glycans) present only transiently during biogenesis of septal PG provides a mechanism for coordinating the function of SPOR domain proteins with the progress of cell division.

CodY Regulates SigD Levels and Activity by Binding to Three Sites in the fla/che Operon

Exponentially growing cultures of Bacillus subtilis (PY79) are composed primarily of nonmotile, chained cells. The alternative sigma factor, SigD, promotes the phenotypic switch from nonmotile, chained cells to unchained, motile cells. In the present work, we investigated the role of the GTP-sensing protein CodY in the regulation of SigD. Deletion of codY resulted in a significant increase in SigD accumulation and activity and shifted the proportion of unchained cells up from ∼15% to ∼75%, suggesting that CodY is an important regulator of SigD. CodY was previously shown to bind to the PD3 and Pfla/che promoters located upstream of the first gene in the sigD-containing fla/che operon. Using electrophoretic mobility shift assays, we found that CodY also binds to two other previously uncharacterized sites within the fla/che operon. Mutations in any one of the three binding sites resulted in SigD levels similar to those seen with the ΔcodY mutant, suggesting that each site is sufficient to tip cells toward a maximal level of CodY-dependent SigD accumulation. However, mutations in all three sites were required to phenocopy the ΔcodY mutant's reduced level of cell chaining, consistent with the idea that CodY binding in the fla/che operon is also important for posttranslational control of SigD activity.

IMPORTANCE

One way that bacteria adapt quickly and efficiently to changes in environmental quality is to employ global transcriptional regulators capable of responding allosterically to key cellular metabolites. In this study, we found that the conserved GTP-sensing protein CodY directly regulates cell motility and chaining in B. subtilis by controlling expression and activity of SigD. Our results suggest that B. subtilis becomes poised for cell dispersal as intracellular GTP levels are depleted.

From cell differentiation to cell collectives: Bacillus subtilis uses division of labor to migrate

The organization of cells, emerging from cell-cell interactions, can give rise to collective properties. These properties are adaptive when together cells can face environmental challenges that they separately cannot. One particular challenge that is important for microorganisms is migration. In this study, we show how flagellum-independent migration is driven by the division of labor of two cell types that appear during Bacillus subtilis sliding motility. Cell collectives organize themselves into bundles (called "van Gogh bundles") of tightly aligned cell chains that form filamentous loops at the colony edge. We show, by time-course microscopy, that these loops migrate by pushing themselves away from the colony. The formation of van Gogh bundles depends critically on the synergistic interaction of surfactin-producing and matrix-producing cells. We propose that surfactin-producing cells reduce the friction between cells and their substrate, thereby facilitating matrix-producing cells to form bundles. The folding properties of these bundles determine the rate of colony expansion. Our study illustrates how the simple organization of cells within a community can yield a strong ecological advantage. This is a key factor underlying the diverse origins of multicellularity.

Adaptor-mediated Lon proteolysis restricts Bacillus subtilis hyperflagellation

The Lon AAA+ protease is a highly conserved intracellular protease that is considered an anticancer target in eukaryotic cells and a crucial virulence regulator in bacteria. Lon degrades both damaged, misfolded proteins and specific native regulators, but how Lon discriminates among a large pool of candidate targets remains unclear. Here we report that Bacillus subtilis LonA specifically degrades the master regulator of flagellar biosynthesis SwrA governed by the adaptor protein swarming motility inhibitor A (SmiA). SmiA-dependent LonA proteolysis is abrogated upon microbe-substrate contact causing SwrA protein levels to increase and elevate flagellar density above a critical threshold for swarming motility atop solid surfaces. Surface contact-dependent cellular differentiation in bacteria is rapid, and regulated proteolysis may be a general mechanism of transducing surface stimuli.

Physiological and cell morphology adaptation of Bacillus subtilis at near-zero specific growth rates: a transcriptome analysis

Nutrient scarcity is a common condition in nature, but the resulting extremely low growth rates (below 0.025 h(-1) ) are an unexplored research area in Bacillus subtilis. To understand microbial life in natural environments, studying the adaptation of B. subtilis to near-zero growth conditions is relevant. To this end, a chemostat modified for culturing an asporogenous B. subtilis sigF mutant strain at extremely low growth rates (also named a retentostat) was set up, and biomass accumulation, culture viability, metabolite production and cell morphology were analysed. During retentostat culturing, the specific growth rate decreased to a minimum of 0.00006 h(-1) , corresponding to a doubling time of 470 days. The energy distribution between growth and maintenance-related processes showed that a state of near-zero growth was reached. Remarkably, a filamentous cell morphology emerged, suggesting that cell separation is impaired under near-zero growth conditions. To evaluate the corresponding molecular adaptations to extremely low specific growth, transcriptome changes were analysed. These revealed that cellular responses to near-zero growth conditions share several similarities with those of cells during the stationary phase of batch growth. However, fundamental differences between these two non-growing states are apparent by their high viability and absence of stationary phase mutagenesis under near-zero growth conditions.

RelA inhibits Bacillus subtilis motility and chaining

The nucleotide second messengers pppGpp and ppGpp [(p)ppGpp] are responsible for the global downregulation of transcription, translation, DNA replication, and growth rate that occurs during the stringent response. More recent studies suggest that (p)ppGpp is also an important effector in many nonstringent processes, including virulence, persister cell formation, and biofilm production. In Bacillus subtilis, (p)ppGpp production is primarily determined by the net activity of RelA, a bifunctional (p)ppGpp synthetase/hydrolase, and two monofunctional (p)ppGpp synthetases, YwaC and YjbM. We observe that in B. subtilis, a relA mutant grows exclusively as unchained, motile cells, phenotypes regulated by the alternative sigma factor SigD. Our data indicate that the relA mutant is trapped in a SigD "on" state during exponential growth, implicating RelA and (p)ppGpp levels in the regulation of cell chaining and motility in B. subtilis. Our results also suggest that minor variations in basal (p)ppGpp levels can significantly skew developmental decision-making outcomes.

FlgM is secreted by the flagellar export apparatus in Bacillus subtilis

The bacterial flagellum is assembled from over 20 structural components, and flagellar gene regulation is morphogenetically coupled to the assembly state by control of the anti-sigma factor FlgM. In the Gram-negative bacterium Salmonella enterica, FlgM inhibits late-class flagellar gene expression until the hook-basal body structural intermediate is completed and FlgM is inhibited by secretion from the cytoplasm. Here we demonstrate that FlgM is also secreted in the Gram-positive bacterium Bacillus subtilis and is degraded extracellularly by the proteases Epr and WprA. We further demonstrate that, like in S. enterica, the structural genes required for the flagellar hook-basal body are required for robust activation of σ(D)-dependent gene expression and efficient secretion of FlgM. Finally, we determine that FlgM secretion is strongly enhanced by, but does not strictly require, hook-basal body completion and instead demands a minimal subset of flagellar proteins that includes the FliF/FliG basal body proteins, the flagellar type III export apparatus components FliO, FliP, FliQ, FliR, FlhA, and FlhB, and the substrate specificity switch regulator FliK.

Induced sensitivity of Bacillus subtilis colony morphology to mechanical media compression

Bacteria from several taxa, including Kurthia zopfii, Myxococcus xanthus, and Bacillus mycoides, have been reported to align growth of their colonies to small features on the surface of solid media, including anisotropies created by compression. While the function of this phenomenon is unclear, it may help organisms navigate on solid phases, such as soil. The origin of this behavior is also unknown: it may be biological (that is, dependent on components that sense the environment and regulate growth accordingly) or merely physical. Here we show that B. subtilis, an organism that typically does not respond to media compression, can be induced to do so with two simple and synergistic perturbations: a mutation that maintains cells in the swarming (chained) state, and the addition of EDTA to the growth media, which further increases chain length. EDTA apparently increases chain length by inducing defects in cell separation, as the treatment has only marginal effects on the length of individual cells. These results lead us to three conclusions. First, the wealth of genetic tools available to B. subtilis will provide a new, tractable chassis for engineering compression sensitive organisms. Second, the sensitivity of colony morphology to media compression in Bacillus can be modulated by altering a simple physical property of rod-shaped cells. And third, colony morphology under compression holds promise as a rapid, simple, and low-cost way to screen for changes in the length of rod-shaped cells or chains thereof.

The structure and regulation of flagella in Bacillus subtilis

Bacterial flagellar motility is among the most extensively studied physiological systems in biology, but most research has been restricted to using the highly similar Gram-negative species Escherichia coli and Salmonella enterica. Here, we review the recent advances in the study of flagellar structure and regulation of the distantly related and genetically tractable Gram-positive bacterium Bacillus subtilis. B. subtilis has a thicker layer of peptidoglycan and lacks the outer membrane of the Gram-negative bacteria; thus, not only phylogenetic separation but also differences in fundamental cell architecture contribute to deviations in flagellar structure and regulation. We speculate that a large number of flagella and the absence of a periplasm make B. subtilis a premier organism for the study of the earliest events in flagellar morphogenesis and the type III secretion system. Furthermore, B. subtilis has been instrumental in the study of heterogeneous gene transcription in subpopulations and of flagellar regulation at the translational and functional level.

Global analysis of mRNA decay intermediates in Bacillus subtilis wild-type and polynucleotide phosphorylase-deletion strains

Messenger RNA decay in Bacillus subtilis is accomplished by a combination of exoribonucleases and endoribonucleases. Intermediates in the decay process have not been readily detectable, and previous studies on mRNA decay have used a handful of highly expressed transcripts as models. Here, we use RNA-Seq analysis to probe mRNA turnover globally. A significant fraction of messages showed differential accumulation of RNA fragments that mapped near the 5' or 3' end of the coding sequence, consistent with initiation of decay from either the 5' end or from an internal cleavage site. Patterns of mRNA decay in the wild type were compared with patterns in a mutant strain lacking polynucleotide phosphorylase (PNPase), which is considered the major 3' exonuclease activity in mRNA decay and which is one of four known 3' exonucleases in B. subtilis. The results showed a striking dependence on PNPase for mRNA turnover in many cases, suggesting specificity in the ability of 3' exonucleases to degrade from 3'-hydroxyl termini. RNA-Seq data demonstrated a sharp decrease in expression of Sigma D in the PNPase-deletion strain. Reduction in sigD regulon expression explained the chain growth phenotype of the PNPase mutant and also predicted a defect in swarming motility.

Bacillaene and sporulation protect Bacillus subtilis from predation by Myxococcus xanthus

Myxococcus xanthus and Bacillus subtilis are common soil-dwelling bacteria that produce a wide range of secondary metabolites and sporulate under nutrient-limiting conditions. Both organisms affect the composition and dynamics of microbial communities in the soil. However, M. xanthus is known to be a predator, while B. subtilis is not. A screen of various prey led to the finding that M. xanthus is capable of consuming laboratory strains of B. subtilis, while the ancestral strain, NCIB3610, was resistant to predation. Based in part on recent characterization of several strains of B. subtilis, we were able to determine that the pks gene cluster, which is required for production of bacillaene, is the major factor allowing B. subtilis NCIB3610 cells to resist predation by M. xanthus. Furthermore, purified bacillaene was added exogenously to domesticated strains, resulting in resistance to predation. Lastly, we found that M. xanthus is incapable of consuming B. subtilis spores even from laboratory strains, indicating the evolutionary fitness of sporulation as a survival strategy. Together, the results suggest that bacillaene inhibits M. xanthus predation, allowing sufficient time for development of B. subtilis spores.

Cell motility and biofilm formation in Bacillus subtilis are affected by the ribosomal proteins, S11 and S21

Bacillus subtilis differentiates into various cellular states in response to environmental changes. It exists in two states during the exponential growth phase: motile cells and connected chains of sessile cells. Here, we identified new regulators of cell motility and chaining, the ribosomal proteins S21 (rpsU) and S11 (rpsK). Their mutants showed impaired cell motility (observed in a laboratory strain) and robust biofilm formation (observed in an undomesticated strain). The two major operons for biofilm formation, tapA-sipW-tasA and epsA-O, were strongly expressed in the rpsU mutant, whereas the flagellin-encoding hag gene and other SigD-dependent motility regulons were not. Genetic analysis revealed that the mutation of remA, the transcriptional activator of the eps operon, is epistatic to that of rpsU, whereas the mutation of antagonistic regulators of SinR is not. Our studies demonstrate that S11 and S21 participate in the regulation of bistability via the RemA/RemB pathway.

ParB spreading requires DNA bridging

The bacterial parABS system is employed for plasmid partitioning and chromosome segregation. ParB binds to parS sites and associates with broad regions of adjacent DNA, a phenomenon known as spreading. However, the molecular basis for spreading is unknown. Using single-molecule approaches, Graham et al. demonstrate DNA bridging by B. subtilis ParB (Spo0J). Spo0J mutations that disrupt DNA bridging lead to defective spreading and SMC condensin complex recruitment. This study suggests a novel, conserved mechanism by which ParB proteins function in chromosome organization and segregation.

The parABS system is a widely employed mechanism for plasmid partitioning and chromosome segregation in bacteria. ParB binds to parS sites on plasmids and chromosomes and associates with broad regions of adjacent DNA, a phenomenon known as spreading. Although essential for ParB function, the mechanism of spreading remains poorly understood. Using single-molecule approaches, we discovered that Bacillus subtilis ParB (Spo0J) is able to trap DNA loops. Point mutants in Spo0J that disrupt DNA bridging are defective in spreading and recruitment of structural maintenance of chromosomes (SMC) condensin complexes in vivo. DNA bridging helps to explain how a limited number of Spo0J molecules per parS site (∼20) can spread over many kilobases and suggests a mechanism by which ParB proteins could facilitate the loading of SMC complexes. We show that DNA bridging is a property of diverse ParB homologs, suggesting broad evolutionary conservation.

Characterization of the SigD regulon of C. difficile and its positive control of toxin production through the regulation of tcdR

Clostridium difficile intestinal disease is mediated largely by the actions of toxins A (TcdA) and B (TcdB), whose production occurs after the initial steps of colonization involving different surface or flagellar proteins. In B. subtilis, the sigma factor SigD controls flagellar synthesis, motility, and vegetative autolysins. A homolog of SigD encoding gene is present in the C.difficile 630 genome. We constructed a sigD mutant in C. difficile 630 ∆erm to analyze the regulon of SigD using a global transcriptomic approach. A total of 103 genes were differentially expressed between the wild-type and the sigD mutant, including genes involved in motility, metabolism and regulation. In addition, the sigD mutant displayed decreased expression of genes involved in flagellar biosynthesis, and also of genes encoding TcdA and TcdB as well as TcdR, the positive regulator of the toxins. Genomic analysis and RACE-PCR experiments allowed us to characterize promoter sequences of direct target genes of SigD including tcdR and to identify the SigD consensus. We then established that SigD positively regulates toxin expression via direct control of tcdR transcription. Interestingly, the overexpression of FlgM, a putative anti-SigD factor, inhibited the positive regulation of motility and toxin synthesis by SigD. Thus, SigD appears to be the first positive regulator of the toxin synthesis in C. difficile.

Memory and modularity in cell-fate decision making

Genetically identical cells sharing an environment can display markedly different phenotypes. It is often unclear how much of this variation derives from chance, external signals, or attempts by individual cells to exert autonomous phenotypic programs. By observing thousands of cells for hundreds of consecutive generations under constant conditions, we dissect the stochastic decision between a solitary, motile state and a chained, sessile state in Bacillus subtilis. The motile state is memoryless, exhibiting no autonomous control over the time spent in the state, whereas chaining is tightly timed. Timing enforces coordination among related cells in the multicellular state. Further, we show that the three-protein regulatory circuit governing the decision is modular, as initiation and maintenance of chaining are genetically separable functions. As stimulation of the same initiating pathway triggers biofilm formation, we argue that autonomous timing allows a trial commitment to multicellularity that external signals could extend.

Functional characterization of core components of the Bacillus subtilis cyclic-di-GMP signaling pathway

Bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) is an intracellular second messenger that regulates adaptation processes, including biofilm formation, motility, and virulence in Gram-negative bacteria. In this study, we have characterized the core components of a c-di-GMP signaling pathway in the model Gram-positive bacterium Bacillus subtilis. Specifically, we have directly identified and characterized three active diguanylate cyclases, DgcP, DgcK, and DgcW (formerly YtrP, YhcK, and YkoW, respectively), one active c-di-GMP phosphodiesterase, PdeH (formerly YuxH), and a cyclic-diguanylate (c-di-GMP) receptor, DgrA (formerly YpfA). Furthermore, elevation of c-di-GMP levels in B. subtilis led to inhibition of swarming motility, whereas biofilm formation was unaffected. Our work establishes paradigms for Gram-positive c-di-GMP signaling, and we have shown that the concise signaling system identified in B. subtilis serves as a powerful heterologous host for the study of c-di-GMP enzymes from bacteria predicted to possess larger, more-complex signaling systems.

Action and timing of BacC and BacD in the late stages of biosynthesis of the dipeptide antibiotic bacilysin

Biosynthesis of the dipeptide antibiotic bacilysin, encoded by the seven Bacillus subtilis genes bacA-G, involves diversion of flux from prephenate to the noncognate amino acid anticapsin. The anticapsin warhead is then ligated to the C-terminus of l-alanine to produce mature bacilysin. We have previously noted the formation of two diastereomers of tetrahydrotyrosine (4S- and 4R-H(4)Tyr) by tandem action of the four purified enzymes BacABGF. BacC (oxidase) and BacD (ligase) have been hypothesized to be remaining late stage enzymes in bacilysin biosynthesis. Using a combination of BacCD in vitro studies, B. subtilis deletion mutants, and isotopic feeding studies, we were able to determine that the H(4)Tyr diastereomers are actually shunt products that are not on-pathway to bacilysin biosynthesis. Dihydroanticapsin and dihydrobacilysin accumulate in extracts of a ΔbacC strain and are processed to anticapsin and then bacilysin upon addition of BacC and BacD, respectively. These results suggest the epoxide group in bacilysin is installed in an earlier step of bacilysin biosynthesis, while BacC oxidation of the C(7)-hydroxyl and the subsequent BacD ligation of anticapsin to l-Ala are the penultimate and ultimate steps of bacilysin biosynthesis, respectively.

The cell biology of peritrichous flagella in Bacillus subtilis

Bacterial flagella are highly conserved molecular machines that have been extensively studied for assembly, function and gene regulation. Less studied is how and why bacteria differ based on the number and arrangement of the flagella they synthesize. Here we explore the cell biology of peritrichous flagella in the model bacterium Bacillus subtilis by fluorescently labelling flagellar basal bodies, hooks and filaments. We find that the average B. subtilis cell assembles approximately 26 flagellar basal bodies and we show that basal body number is controlled by SwrA. Basal bodies are assembled rapidly (< 5 min) but the assembly of flagella capable of supporting motility is rate limited by filament polymerization (> 40 min). We find that basal bodies are not positioned randomly on the cell surface. Rather, basal bodies occupy a grid-like pattern organized symmetrically around the midcell and that flagella are discouraged at the poles. Basal body position is genetically determined by FlhF and FlhG homologues to control spatial patterning differently from what is seen in bacteria with polar flagella. Finally, spatial control of flagella in B. subtilis seems more relevant to the inheritance of flagella and motility of individual cells than the motile behaviour of populations.

Peptidoglycan maturation enzymes affect flagellar functionality in bacteria

The flagellar machinery is a highly complex organelle composed of a free rotating flagellum and a fixed stator that converts energy into movement. The assembly of the flagella and the stator requires interactions with the peptidoglycan layer through which the organelle has to pass for externalization. Lytic transglycosylases are peptidoglycan degrading enzymes that cleave the sugar backbone of peptidoglycan layer. We show that an endogenous lytic transglycosylase is required for full motility of Helicobacter pylori and colonization of the gastric mucosa. Deficiency of motility resulted from a paralysed phenotype implying an altered ability to generate flagellar rotation. Similarly, another Gram-negative pathogen Salmonella typhimurium and the Gram-positive pathogen Listeria monocytogenes required the activity of lytic transglycosylases, Slt or MltC, and a glucosaminidase (Auto), respectively, for full motility. Furthermore, we show that in absence of the appropriate lytic transglycosylase, the flagellar motor protein MotB from H. pylori does not localize properly to the bacterial pole. We present a new model involving the maturation of the surrounding peptidoglycan for the proper anchoring and functionality of the flagellar motor.

Molecular characterization of the flagellar hook in Bacillus subtilis

The structure of the Gram-positive flagellum is poorly understood, and Bacillus subtilis encodes three proteins homologous to the flagellar hook protein from Salmonella enterica. Here we generated a modified B. subtilis hook protein that could be fluorescently stained using a cysteine-reactive dye. We used the fluorescently labeled hook to demonstrate that FlgE is the hook structural protein and that FliK regulated hook length. We further demonstrate that two proteins of unknown function, FlhO and FlhP, and the putative hook cap, FlgD, were required for hook assembly, such that when flhO, flhP, or flgD was mutated, hook protein was secreted into the supernatant. All mutants defective in hook completion resulted in homogeneously reduced σ(D)-dependent gene expression due to the action of the anti-sigma factor FlgM.

SlrA/SinR/SlrR inhibits motility gene expression upstream of a hypersensitive and hysteretic switch at the level of σ(D) in Bacillus subtilis

Exponentially growing Bacillus subtilis cultures are epigenetically differentiated into two subpopulations in which cells are either ON or OFF for σ(d) -dependent gene expression: a pattern suggestive of bistability. The gene encoding σ(D) , sigD, is part of the 31-gene fla/che operon where its location at the 3' end, 25 kb away from the strong P(fla/che) promoter, determines its expression level relative to a threshold. Here we show that addition of a single extra copy of the slrA gene in the chromosome inhibited σ(d) -dependent gene expression. SlrA together with SinR and SlrR reduced sigD transcript by potentiating a distance-dependent decrease in fla/che operon transcript abundance that was not mediated by changes in expression from the P(fla/che) promoter. Consistent with acting upstream of σ(D) , SlrA/SinR/SlrR was bypassed by artificial ectopic expression of sigD and hysteretically maintained for 20 generations by engaging the sigD gene at the native locus. SlrA/SinR/SlrR was also bypassed by increasing fla/che transcription and resulted in a hypersensitive output in flagellin expression. Thus, flagellin gene expression demonstrated hypersensitivity and hysteresis and we conclude that σ(d) -dependent gene expression is bistable.

Single-cell analysis of Bacillus subtilis biofilms using fluorescence microscopy and flow cytometry

Biofilm formation is a general attribute to almost all bacteria( 1-6). When bacteria form biofilms, cells are encased in extracellular matrix that is mostly constituted by proteins and exopolysaccharides, among other factors (7-10). The microbial community encased within the biofilm often shows the differentiation of distinct subpopulation of specialized cells (11-17). These subpopulations coexist and often show spatial and temporal organization within the biofilm ( 18-21). Biofilm formation in the model organism Bacillus subtilis requires the differentiation of distinct subpopulations of specialized cells. Among them, the subpopulation of matrix producers, responsible to produce and secrete the extracellular matrix of the biofilm is essential for biofilm formation (11,19). Hence, differentiation of matrix producers is a hallmark of biofilm formation in B. subtilis. We have used fluorescent reporters to visualize and quantify the subpopulation of matrix producers in biofilms of B. subtilis (15,19,22-24). Concretely, we have observed that the subpopulation of matrix producers differentiates in response to the presence of self-produced extracellular signal surfactin (25). Interestingly, surfactin is produced by a subpopulation of specialized cells different from the subpopulation of matrix producers (15). We have detailed in this report the technical approach necessary to visualize and quantify the subpopulation of matrix producers and surfactin producers within the biofilms of B. subtilis. To do this, fluorescent reporters of genes required for matrix production and surfactin production are inserted into the chromosome of B. subtilis. Reporters are expressed only in a subpopulation of specialized cells. Then, the subpopulations can be monitored using fluorescence microscopy and flow cytometry (See Fig 1). The fact that different subpopulations of specialized cells coexist within multicellular communities of bacteria gives us a different perspective about the regulation of gene expression in prokaryotes. This protocol addresses this phenomenon experimentally and it can be easily adapted to any other working model, to elucidate the molecular mechanisms underlying phenotypic heterogeneity within a microbial community.

Cell individuality: the bistable gene expression of the type III secretion system in Dickeya dadantii 3937

Dickeya dadantii 3937 is a gram-negative phytopathogenic bacterium that expresses genes encoding a type III secretion system (T3SS) in a bistable pattern when cultured in a homogeneous minimal media. In this work, we further characterized the bistable gene expression of T3SS at the single-cell level. We demonstrated that bistable expression of the HrpL-regulon genes, such as hrpA and hrpN, is controlled by the same regulatory mechanism. We also showed that the expression level of the T3SS master regulatory gene hrpL plays an important role in the development of the bistable expression of hrpA. A high expression level of hrpL is required but unable to guarantee the high-state expression of hrpA in a cell. In addition, bistable expression patterns of T3SS genes in other gram-negative pathogens of the Enterobacteriaceae and Pseudomonadaceae families were also described in this study. This suggests that the T3SS bistability might be a conserved population behavior in several gram-negative bacterial pathogens.

Swarming motility and the control of master regulators of flagellar biosynthesis

Swarming motility is the movement of bacteria over a solid surface powered by rotating flagella. The expression of flagellar biosynthesis genes is governed by species-specific master regulator transcription factors. Mutations that reduce or enhance master regulator activity have a commensurate effect on swarming motility. Here we review what is known about the proteins that modulate swarming motility and appear to act upstream of the master flagellar regulators in diverse swarming bacteria. We hypothesize that environmental control of the master regulators is important to the swarming phenotype perhaps at the level of controlling flagellar number.

Extracellular secretion of a recombinant therapeutic peptide by Bacillus halodurans utilizing a modified flagellin type III secretion system

Background

Through modification of the flagellin type III secretion pathway of Bacillus halodurans heterologous peptides could be secreted into the medium as flagellin fusion monomers. The stability of the secreted monomers was significantly enhanced through gene-targeted inactivation of host cell extracellular proteases. In evaluating the biotechnological potential of this extracellular secretion system an anti-viral therapeutic peptide, Enfuvirtide, was chosen. Currently, Enfuvirtide is synthesised utilizing 106 chemical steps. We used Enfuvirtide as a model system in an effort to develop a more cost-effective biological process for therapeutic peptide production.

Results

An attempt was made to increase the levels of the fusion peptide by two strategies, namely strain improvement through gene-targeted knock-outs, as well as vector and cassette optimization. Both approaches proved to be successful. Through chromosomal inactivation of the spo0A, lytC and lytE genes, giving rise to strain B. halodurans BhFDL05S, the secretion of recombinant peptide fusions was increased 10-fold. Cassette optimization, incorporating an expression vector pNW33N and the N- and C-terminal regions of the flagellin monomer as an in-frame peptide fusion, resulted in a further 3.5-fold increase in the secretion of recombinant peptide fusions.

Conclusions

The type III flagellar secretion system of B. halodurans has been shown to successfully secrete a therapeutic peptide as a heterologous flagellin fusion. Improvements to both the strain and expression cassette led to increased levels of recombinant peptide, showing promise for a biotechnological application.

DegU-phosphate activates expression of the anti-sigma factor FlgM in Bacillus subtilis

The bacterial flagellum is a complex molecular machine that is assembled by more than 30 proteins and is rotated to propel cells either through liquids or over solid surfaces. Flagellar gene expression is extensively regulated to co-ordinate flagellar assembly in both space and time. In Bacillus subtilis, the proteins of unknown function, SwrA and SwrB, and the alternative sigma factor σ(D) are required to activate expression of the flagellar filament protein, flagellin. Here we determine that in the absence of SwrA and SwrB, the phosphorylated form of the response regulator DegU inhibits σ(D) -dependent gene expression indirectly by binding to the P(flgM) promoter region and activating expression of the anti-sigma factor FlgM. We further demonstrate that DegU-P-dependent activation of FlgM is essential to inhibit flagellin expression when flagellar basal body assembly is disrupted. Regulation of FlgM is poorly understood outside of Salmonella, and differential control of FlgM expression may be a common means of coupling flagellin expression to flagellar assembly.

Single-cell analysis in situ in a Bacillus subtilis swarming community identifies distinct spatially separated subpopulations differentially expressing hag (flagellin), including specialized swarmers

The non-domesticated Bacillus subtilis strain 3610 displays, over a wide range of humidity, hyper-branched, dendritic, swarming-like migration on a minimal agar medium. At high (70 %) humidity, the laboratory strain 168 sfp+ (producing surfactin) behaves very similarly, although this strain carries a frameshift mutation in swrA, which another group has shown under their conditions (which include low humidity) is essential for swarming. We reconcile these different results by demonstrating that, while swrA is essential for dendritic migration at low humidity (30-40 %), it is dispensable at high humidity. Dendritic migration (flagella- and surfactin-dependent) of strains 168 sfp+ swrA and 3610 involves elongation of dendrites for several hours as a monolayer of cells in a thin fluid film. This enabled us to determine in situ the spatiotemporal pattern of expression of some key players in migration as dendrites develop, using gfp transcriptional fusions for hag (encoding flagellin), comA (regulation of surfactin synthesis) as well as eps (exopolysaccharide synthesis). Quantitative (single-cell) analysis of hag expression in situ revealed three spatially separated subpopulations or cell types: (i) networks of chains arising early in the mother colony (MC), expressing eps but not hag; (ii) largely immobile cells in dendrite stems expressing intermediate levels of hag; and (iii) a subpopulation of cells with several distinctive features, including very low comA expression but hyper-expression of hag (and flagella). These specialized cells emerge from the MC to spearhead the terminal 1 mm of dendrite tips as swirling and streaming packs, a major characteristic of swarming migration. We discuss a model for this swarming process, emphasizing the importance of population density and of the complementary roles of packs of swarmers driving dendrite extension, while non-mobile cells in the stems extend dendrites by multiplication.

Genetic evidence for involvement of the alternative sigma factor SigI in controlling expression of the cell wall hydrolase gene lytE and contribution of LytE to heat survival of Bacillus subtilis

The Bacillus subtilis cell wall hydrolase LytE is involved in cell wall turnover and cell separation during vegetative growth. lytE transcription is known to be driven by a YycF-activated SigA-dependent promoter. The cell wall regulator SigI is an alternative sigma factor that has been shown to be heat stress-inducible and to be essential for survival of B. subtilis at high temperature. However, none of the previously identified target genes of SigI contribute to heat resistance. We now demonstrate that lytE expression is heat-inducible and that heat induction of lytE expression is strongly dependent on SigI. We have also found that the lytE mutant shows the same growth defect at high temperature as the sigI mutant. Introducing an extra copy of lytE into the sigI mutant could rescue its growth defect. Our data strongly suggest that SigI-dependent lytE expression under heat stress is important for heat survival of B. subtilis.

Reversal of an epigenetic switch governing cell chaining in Bacillus subtilis by protein instability

Bacillus subtilis forms long chains of cells during growth and biofilm formation. Cell separation is mediated by autolysins, whose genes are under the negative control of a heteromeric complex composed of the proteins SinR and SlrR. Formation of the SinR-SlrR complex is governed by a self-reinforcing, double-negative feedback loop in which SinR represses the gene for SlrR and SlrR, by forming the SinR-SlrR complex, titrates SinR and prevents it from repressing slrR. The loop is a bistable switch and exists in a SlrR(LOW) state in which autolysin genes are on, and a SlrR(HIGH) state in which autolysin genes are repressed by SinR-SlrR. Cells in the SlrR(LOW) state are driven into the SlrR(HIGH) state by SinI, an antirepressor that binds to and inhibits SinR. However, the mechanism by which cells in the SlrR(HIGH) state revert back to the SlrR(LOW) state is unknown. We report that SlrR is proteolytically unstable and present evidence that self-cleavage via a LexA-like autopeptidase and ClpC contribute to its degradation. Cells producing a self-cleavage-resistant mutant of SlrR exhibited more persistent chaining during growth and yielded biofilms with enhanced structural complexity. We propose that degradation of SlrR allows cells to switch from the SlrR(HIGH) to the SlrR(LOW) state.

The sensitivity of Bacillus subtilis to diverse antimicrobial compounds is influenced by Abh

Abh is a transition state regulator of Bacillus subtilis that controls biofilm formation and the production of several diverse antimicrobial compounds. Using a high-throughput non-biased technique, we show for the first time that Abh influences the sensitivity of B. subtilis to diverse antimicrobial compounds. Following up on these findings with a combination of classical genetics and antibiotic susceptibility assays, we demonstrate that Abh influences cellular processes such as the remodelling of the cell wall. We present data demonstrating that the extracytoplasmic function sigma factor σ(X) controls resistance to β-lactam antibiotics by activating abh transcription. Downstream from Abh, activation of slrR expression by Abh is responsible for controlling the sensitivity of B. subtilis to such antibiotics due to the role that SlrR plays in regulating autolysin biosynthesis. The abh mutant additionally exhibits increased resistance to aminoglycoside antimicrobials. We confirm that aminoglycoside killing of B. subtilis is likely to be caused by oxidative damage but rule out the possibility that the increased resistance of the abh mutant to aminoglycosides is due to a general increase in resistance to oxidative stress.

A field guide to bacterial swarming motility

How bacteria regulate, assemble and rotate flagella to swim in liquid media is reasonably well understood. Much less is known about how some bacteria use flagella to move over the tops of solid surfaces in a form of movement called swarming. The focus of bacteriology is changing from planktonic to surface environments, and so interest in swarming motility is on the rise. Here, I review the requirements that define swarming motility in diverse bacterial model systems, including an increase in the number of flagella per cell, the secretion of a surfactant to reduce surface tension and allow spreading, and movement in multicellular groups rather than as individuals.

Muropeptide rescue in Bacillus subtilis involves sequential hydrolysis by beta-N-acetylglucosaminidase and N-acetylmuramyl-L-alanine amidase

We identified a pathway in Bacillus subtilis that is used for recovery of N-acetylglucosamine (GlcNAc)-N-acetylmuramic acid (MurNAc) peptides (muropeptides) derived from the peptidoglycan of the cell wall. This pathway is encoded by a cluster of six genes, the first three of which are orthologs of Escherichia coli genes involved in N-acetylmuramic acid dissimilation and encode a MurNAc-6-phosphate etherase (MurQ), a MurNAc-6-phosphate-specific transcriptional regulator (MurR), and a MurNAc-specific phosphotransferase system (MurP). Here we characterized two other genes of this cluster. The first gene was shown to encode a cell wall-associated beta-N-acetylglucosaminidase (NagZ, formerly YbbD) that cleaves the terminal nonreducing N-acetylglucosamine of muropeptides and also accepts chromogenic or fluorogenic beta-N-acetylglucosaminides. The second gene was shown to encode an amidase (AmiE, formerly YbbE) that hydrolyzes the N-acetylmuramyl-L-Ala bond of MurNAc peptides but not this bond of muropeptides. Hence, AmiE requires NagZ, and in conjunction these enzymes liberate MurNAc by sequential hydrolysis of muropeptides. NagZ expression was induced at late exponential phase, and it was 6-fold higher in stationary phase. NagZ is noncovalently associated with lysozyme-degradable particulate material and can be released from it with salt. A nagZ mutant accumulates muropeptides in the spent medium and displays a lytic phenotype in late stationary phase. The evidence for a muropeptide catabolic pathway presented here is the first evidence for cell wall recovery in a Gram-positive organism, and this pathway is distinct from the cell wall recycling pathway of E. coli and other Gram-negative bacteria.

An epigenetic switch governing daughter cell separation in Bacillus subtilis

Growing cells of Bacillus subtilis are a bistable mixture of individual motile cells in which genes for daughter cell separation and motility are ON, and chains of sessile cells in which these genes are OFF. How this ON/OFF switch is controlled has been mysterious. Here we report that a complex of the SinR and SlrR proteins binds to and represses genes involved in cell separation and motility. We also report that SinR and SlrR constitute a double-negative feedback loop in which SinR represses the gene for SlrR (slrR), and, by binding to (titrating) SinR, SlrR prevents SinR from repressing slrR. Thus, SlrR indirectly derepresses its own gene, creating a self-reinforcing loop. Finally, we show that, once activated, the loop remains locked in a high SlrR state in which cell separation and motility genes are OFF for extended periods of time. SinR and SlrR constitute an epigenetic switch for controlling genes involved in cell separation and motility.

Gene position in a long operon governs motility development in Bacillus subtilis

Growing cultures of Bacillus subtilis bifurcate into subpopulations of motile individuals and non-motile chains of cells that are differentiated at the level of gene expression. The motile cells are ON and the chaining cells are OFF for transcription that depends on RNA polymerase and the alternative sigma factor sigma(D). Here we show that chaining cells were OFF for sigma(D)-dependent gene expression because sigma(D) levels fell below a threshold and sigma(D) activity was inhibited by the anti-sigma factor FlgM. The probability that sigma(D) exceeded the threshold was governed by the position of the sigD gene. The proportion of ON cells increased when sigD was artificially moved forward in the 27 kb fla/che operon. In addition, we identified a new sigma(D)-dependent promoter that increases sigD expression and may provide positive feedback to stabilize the ON state. Finally, we demonstrate that ON/OFF motility states in B. subtilis are a form of development because mosaics of stable and differentiated epigenotypes were evident when the normally dispersed bacteria were forced to grow in one dimension.