Bacterial Swarming: A Re-examination of Cell-Movement Patterns

Genetic analysis of the regulation of type IV pilus function by the Chp chemosensory system of Pseudomonas aeruginosa

The virulence of the opportunistic pathogen Pseudomonas aeruginosa involves the coordinate expression of many virulence factors, including type IV pili, which are required for colonization of host tissues and for twitching motility. Type IV pilus function is controlled in part by the Chp chemosensory system, which includes a histidine kinase, ChpA, and two CheY-like response regulators, PilG and PilH. How the Chp components interface with the type IV pilus motor proteins PilB, PilT, and PilU is unknown. We present genetic evidence confirming the role of ChpA, PilG, and PilB in the regulation of pilus extension and the role of PilH and PilT in regulating pilus retraction. Using informative double and triple mutants, we show that (i) ChpA, PilG, and PilB function upstream of PilH, PilT, and PilU; (ii) that PilH enhances PilT function; and (iii) that PilT and PilB retain some activity in the absence of signaling input from components of the Chp system. By site-directed mutagenesis, we demonstrate that the histidine kinase domain of ChpA and the phosphoacceptor sites of both PilG and PilH are required for type IV pilus function, suggesting that they form a phosphorelay system important in the regulation of pilus extension and retraction. Finally, we present evidence suggesting that pilA transcription is regulated by intracellular PilA levels. We show that PilA is a negative regulator of pilA transcription in P. aeruginosa and that the Chp system functionally regulates pilA transcription by controlling PilA import and export.

Novel ultramicrobacterial isolates from a deep Greenland ice core represent a proposed new species, Chryseobacterium greenlandense sp. nov

Three novel orange, ultramicrobacterial isolates, UMB10, UMB14, and UMB34(T) were isolated from enrichment cultures inoculated with a melted 3,043 m deep Greenland ice core sample. Phylogenetic analysis of the 16S rRNA gene sequences indicated that the isolates belonged to a single species within the genus Chryseobacterium. They were most closely related to Chryseobacterium aquaticum (99.3%), Chryseobacterium soli (97.1%), and Chryseobacterium soldanellicola (96.9%). Genomic hybridization showed low levels of relatedness between UMB34(T) and C. aquaticum and C. soldanellicola (19-30%) and C. soli and Chryseobacterium jejuense (45-56%). Comparative genomic fingerprinting analysis using the enterobacterial repetitive intergenic consensus (ERIC) sequence showed nearly identical banding patterns for the three isolates and these patterns were distinct from those of C. aquaticum, C. soldanellicola, C. soli, and C. jejuense. The cells were short rods, lacked flagella, had cell volumes of <0.1 mum(3), formed buds and smaller protrusions (blebs), produced copious extracellular material and a flexirubin type pigment. UMB34(T) produced acids from carbohydrates and utilized glucose and maltose although it did not assimilate mannose. The DNA G + C was 39.6-41.6 mol%. Based on the differences from validly named Chryseobacterium species, it was concluded that these isolates represent a new species for which the name, Chryseobacterium greenlandense is proposed. The type strain is UMB34(T) (=CIP 110007T = NRRL B-59357).

Coordinated surface activities in Variovorax paradoxus EPS

Background

Variovorax paradoxus is an aerobic soil bacterium frequently associated with important biodegradative processes in nature. Our group has cultivated a mucoid strain of Variovorax paradoxus for study as a model of bacterial development and response to environmental conditions. Colonies of this organism vary widely in appearance depending on agar plate type.

Results

Surface motility was observed on minimal defined agar plates with 0.5% agarose, similar in nature to swarming motility identified in Pseudomonas aeruginosa PAO1. We examined this motility under several culture conditions, including inhibition of flagellar motility using Congo Red. We demonstrated that the presence of a wetting agent, mineral, and nutrient content of the media altered the swarming phenotype. We also demonstrated that the wetting agent reduces the surface tension of the agar. We were able to directly observe the presence of the wetting agent in the presence and absence of Congo Red, and found that incubation in a humidified chamber inhibited the production of wetting agent, and also slowed the progression of the swarming colony. We observed that swarming was related to both carbon and nitrogen sources, as well as mineral salts base. The phosphate concentration of the mineral base was critical for growth and swarming on glucose, but not succinate. Swarming on other carbon sources was generally only observed using M9 salts mineral base. Rapid swarming was observed on malic acid, d-sorbitol, casamino acids, and succinate. Swarming at a lower but still detectable rate was observed on glucose and sucrose, with weak swarming on maltose. Nitrogen source tests using succinate as carbon source demonstrated two distinct forms of swarming, with very different macroscopic swarm characteristics. Rapid swarming was observed when ammonium ion was provided as nitrogen source, as well as when histidine, tryptophan, or glycine was provided. Slower swarming was observed with methionine, arginine, or tyrosine. Large effects of mineral content on swarming were seen with tyrosine and methionine as nitrogen sources. Biofilms form readily under various culture circumstances, and show wide variance in structure under different conditions. The amount of biofilm as measured by crystal violet retention was dependent on carbon source, but not nitrogen source. Filamentous growth in the biofilm depends on shear stress, and is enhanced by continuous input of nutrients in chemostat culture.

Conclusion

Our studies have established that the beta-proteobacterium Variovorax paradoxus displays a number of distinct physiologies when grown on surfaces, indicative of a complex response to several growth parameters. We have identified a number of factors that drive sessile and motile surface phenotypes. This work forms a basis for future studies using this genetically tractable soil bacterium to study the regulation of microbial development on surfaces.

Adhesion and fitness in the bean phyllosphere and transmission to seed of Xanthomonas fuscans subsp. fuscans

Deciphering the mechanisms enabling plant-pathogenic bacteria to disperse, colonize, and survive on their hosts provides the necessary basis to set up new control methods. We evaluated the role of bacterial attachment and biofilm formation in host colonization processes for Xanthomonas fuscans subsp. fuscans on its host. This bacterium is responsible for the common bacterial blight of bean (Phaseolus vulgaris), a seedborne disease. The five adhesin genes (pilA, fhab, xadA1, xadA2, and yapH) identified in X. fuscans subsp. fuscans CFBP4834-R strain were mutated. All mutants were altered in their abilities to adhere to polypropylene or seed. PilA was involved in adhesion and transmission to seed, and mutation of pilA led to lower pathogenicity on bean. YapH was required for adhesion to seed, leaves, and abiotic surfaces but not for in planta transmission to seed or aggressiveness on leaves. Transmission to seed through floral structures did not require any of the known adhesins. Conversely, all mutants tested, except in yapH, were altered in their vascular transmission to seed. In conclusion, we showed that adhesins are implicated in the various processes leading to host phyllosphere colonization and transmission to seed by plant-pathogenic bacteria.

Systems level analysis of two-component signal transduction systems in Erwinia amylovora: role in virulence, regulation of amylovoran biosynthesis and swarming motility

BACKGROUND

Two-component signal transduction systems (TCSTs), consisting of a histidine kinase (HK) and a response regulator (RR), represent a major paradigm for signal transduction in prokaryotes. TCSTs play critical roles in sensing and responding to environmental conditions, and in bacterial pathogenesis. Most TCSTs in Erwinia amylovora have either not been identified or have not yet been studied.

RESULTS

We used a systems approach to identify TCST and related signal transduction genes in the genome of E. amylovora. Comparative genomic analysis of TCSTs indicated that E. amylovora TCSTs were closely related to those of Erwinia tasmaniensis, a saprophytic enterobacterium isolated from apple flowers, and to other enterobacteria. Forty-six TCST genes in E. amylovora including 17 sensor kinases, three hybrid kinases, 20 DNA- or ligand-binding RRs, four RRs with enzymatic output domain (EAL-GGDEF proteins), and two kinases were characterized in this study. A systematic TCST gene-knockout experiment was conducted, generating a total of 59 single-, double-, and triple-mutants. Virulence assays revealed that five of these mutants were non-pathogenic on immature pear fruits. Results from phenotypic characterization and gene expression experiments indicated that several groups of TCST systems in E. amylovora control amylovoran biosynthesis, one of two major virulence factors in E. amylovora. Both negative and positive regulators of amylovoran biosynthesis were identified, indicating a complex network may control this important feature of pathogenesis. Positive (non-motile, EnvZ/OmpR), negative (hypermotile, GrrS/GrrA), and intermediate regulators for swarming motility in E. amylovora were also identified.

CONCLUSION

Our results demonstrated that TCSTs in E. amylovora played major roles in virulence on immature pear fruit and in regulating amylovoran biosynthesis and swarming motility. This suggested presence of regulatory networks governing expression of critical virulence genes in E. amylovora.

Identification of non-flagellar genes involved in swarm cell differentiation using a Bacillus thuringiensis mini-Tn10 mutant library

Swarming is a social phenomenon that enables motile bacteria to move co-ordinately over solid surfaces. The molecular basis regulating this process is not completely known and may vary among species. Insertional mutagenesis of a swarming-proficient Bacillus thuringiensis strain was performed, by use of the transposon mini-Tn10, to identify novel genetic determinants of swarming that are dispensable for flagellation, swimming motility, chemotaxis and active growth. Among the 67 non-swarming mutants obtained, six were selected that showed no defect in flagellar assembly and function, chemotaxis or growth rate. Sequence analysis of DNA flanking the transposon insertion led to the identification of previously uncharacterized genes that are involved in the development of swarming colonies by B. thuringiensis and that are highly conserved in all members of the Bacillus cereus sensu lato group. These genes encode non-flagellar proteins with putative activity as sarcosine oxidase, catalase-2, amino acid permease, ATP-binding cassette transporter, dGTP triphosphohydrolase and acetyltransferase. Functional analysis of two of the isolated mutants demonstrated that swarming differentiation depends on the intracellular levels of the osmoprotectant glycine betaine and on the quantity of synthesized phenazine secondary metabolites. The finding that proteins involved in diverse physiological processes have a role in swarming motility underlines the complexity of the molecular mechanisms governing this behaviour in B. thuringiensis.

Periodic reversal of direction allows Myxobacteria to swarm

Many bacteria can rapidly traverse surfaces from which they are extracting nutrient for growth. They generate flat, spreading colonies, called swarms because they resemble swarms of insects. We seek to understand how members of any dense swarm spread efficiently while being able to perceive and interfere minimally with the motion of others. To this end, we investigate swarms of the myxobacterium, Myxococcus xanthus. Individual M. xanthus cells are elongated; they always move in the direction of their long axis; and they are in constant motion, repeatedly touching each other. Remarkably, they regularly reverse their gliding directions. We have constructed a detailed cell- and behavior-based computational model of M. xanthus swarming that allows the organization of cells to be computed. By using the model, we are able to show that reversals of gliding direction are essential for swarming and that reversals increase the outflow of cells across the edge of the swarm. Cells at the swarm edge gain maximum exposure to nutrient and oxygen. We also find that the reversal period predicted to maximize the outflow of cells is the same (within the errors of measurement) as the period observed in experiments with normal M. xanthus cells. This coincidence suggests that the circuit regulating reversals evolved to its current sensitivity under selection for growth achieved by swarming. Finally, we observe that, with time, reversals increase the cell alignment, and generate clusters of parallel cells.

Pantoea stewartii subsp. stewartii exhibits surface motility, which is a critical aspect of Stewart's wilt disease development on maize

Pantoea stewartii subsp. stewartii is a plant-pathogenic bacterium that causes Stewart's vascular wilt in maize. The organism is taxonomically described as aflagellated and nonmotile. We recently showed that P. stewartii colonizes the xylem of maize as sessile, cell-wall-adherent biofilms. Biofilm formation is a developmental process that generally involves some form of surface motility. For that reason, we reexamined the motility properties of P. stewartii DC283 based on the assumption that the organism requires some form of surface motility for biofilm development. Here, we show that the organism is highly motile on agar surfaces. This motility is flagella dependent, shown by the fact that a fliC mutant, impaired in flagellin subunit synthesis, is nonmotile. Motility also requires the production of stewartan exopolysaccharide. Moreover, surface motility plays a significant role in the colonization of the plant host.

Living on a surface: swarming and biofilm formation

Swarming is the fastest known bacterial mode of surface translocation and enables the rapid colonization of a nutrient-rich environment and host tissues. This complex multicellular behavior requires the integration of chemical and physical signals, which leads to the physiological and morphological differentiation of the bacteria into swarmer cells. Here, we provide a review of recent advances in the study of the regulatory pathways that lead to swarming behavior of different model bacteria. It has now become clear that many of these pathways also affect the formation of biofilms, surface-attached bacterial colonies. Decision-making between rapidly colonizing a surface and biofilm formation is central to bacterial survival among competitors. In the second part of this article, we review recent developments in the understanding of the transition between motile and sessile lifestyles of bacteria.

Genetic circuitry controlling motility behaviors of Myxococcus xanthus

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

The surprisingly diverse ways that prokaryotes move

Prokaryotic cells move through liquids or over moist surfaces by swimming, swarming, gliding, twitching or floating. An impressive diversity of motility mechanisms has evolved in prokaryotes. Movement can involve surface appendages, such as flagella that spin, pili that pull and Mycoplasma 'legs' that walk. Internal structures, such as the cytoskeleton and gas vesicles, are involved in some types of motility, whereas the mechanisms of some other types of movement remain mysterious. Regardless of the type of motility machinery that is employed, most motile microorganisms use complex sensory systems to control their movements in response to stimuli, which allows them to migrate to optimal environments.

Regulation of swarming motility and flhDC(Sm) expression by RssAB signaling in Serratia marcescens

Serratia marcescens cells swarm at 30 degrees C but not at 37 degrees C, and the underlying mechanism is not characterized. Our previous studies had shown that a temperature upshift from 30 to 37 degrees C reduced the expression levels of flhDC(Sm) and hag(Sm) in S. marcescens CH-1. Mutation in rssA or rssB, cognate genes that comprise a two-component system, also resulted in precocious swarming phenotypes at 37 degrees C. To further characterize the underlying mechanism, in the present study, we report that expression of flhDC(Sm) and synthesis of flagella are significantly increased in the rssA mutant strain at 37 degrees C. Primer extension analysis for determination of the transcriptional start site(s) of flhDC(Sm) revealed two transcriptional start sites, P1 and P2, in S. marcescens CH-1. Characterization of the phosphorylated RssB (RssB approximately P) binding site by an electrophoretic mobility shift assay showed direct interaction of RssB approximately P, but not unphosphorylated RssB [RssB(D51E)], with the P2 promoter region. A DNase I footprinting assay using a capillary electrophoresis approach further determined that the RssB approximately P binding site is located between base pair positions -341 and -364 from the translation start codon ATG in the flhDC(Sm) promoter region. The binding site overlaps with the P2 "-35" promoter region. A modified chromatin immunoprecipitation assay was subsequently performed to confirm that RssB-P binds to the flhDC(Sm) promoter region in vivo. In conclusion, our results indicated that activated RssA-RssB signaling directly inhibits flhDC(Sm) promoter activity at 37 degrees C. This inhibitory effect was comparatively alleviated at 30 degrees C. This finding might explain, at least in part, the phenomenon of inhibition of S. marcescens swarming at 37 degrees C.

Social interactions in myxobacterial swarming

Swarming, a collective motion of many thousands of cells, produces colonies that rapidly spread over surfaces. In this paper, we introduce a cell-based model to study how interactions between neighboring cells facilitate swarming. We chose to study Myxococcus xanthus, a species of myxobacteria, because it swarms rapidly and has well-defined cell–cell interactions mediated by type IV pili and by slime trails. The aim of this paper is to test whether the cell contact interactions, which are inherent in pili-based S motility and slime-based A motility, are sufficient to explain the observed expansion of wild-type swarms. The simulations yield a constant rate of swarm expansion, which has been observed experimentally. Also, the model is able to quantify the contributions of S motility and A motility to swarming. Some pathogenic bacteria spread over infected tissue by swarming. The model described here may shed some light on their colonization process.

Many bacteria are able to spread rapidly over the surface using a strategy called swarming. When the cells cover a surface at high density and compete with each other for nutrients, swarming permits them to maintain rapid growth at the swarm edge. Swarming with flagella has been investigated for many years, and much has been learned about its regulation. Nevertheless, its choreography, which is somewhat related to the counterflow of pedestrians on a city sidewalk, has remained elusive. It is the bacterial equivalent of dancing toward the exit in a crowd of moving bodies that usually are in close contact. Myxococcus xanthus expands its swarms at 1.6 μm/min, about a third the speed of individual cells gliding over the same surface. Each cell has pilus engines at its front end and slime secretion engines at its rear. Using the known mechanics of these engines and the ways they are coordinated, we have developed a cell-based model to study the role of social interactions in bacterial swarming. The model is able to quantify the contributions of individual motility engines to swarming. It also shows that microscopic social interactions help to form the ordered collective motion observed in swarms.

The wetting agent required for swarming in Salmonella enterica serovar typhimurium is not a surfactant

We compared the abilities of media from agar plates surrounding swarming and nonswarming cells of Salmonella enterica serovar Typhimurium to wet a nonpolar surface by measuring the contact angles of small drops. The swarming cells were wild type for chemotaxis, and the nonswarming cells were nonchemotactic mutants with motor biases that were counterclockwise (cheY) or clockwise (cheZ). The latter strains have been shown to be defective for swarming because the agar remains dry (Q. Wang, A. Suzuki, S. Mariconda, S. Porwollik, and R. M. Harshey, EMBO J. 24:2034-2042, 2005). We found no differences in the abilities of the media surrounding these cells, either wild type or mutant, to wet a low-energy surface (freshly prepared polydimethylsiloxane); although, their contact angles were smaller than that of the medium harvested from the underlying agar. So the agent that promotes wetness produced by wild-type cells is not a surfactant; it is an osmotic agent.