Calcium requirement for gliding motility in myxobacteria

Calcium promotes persistent soil organic matter by altering microbial transformation of plant litter

Calcium (Ca) can contribute to soil organic carbon (SOC) persistence by mediating physico-chemical interactions between organic compounds and minerals. Yet, Ca is also crucial for microbial adhesion, potentially affecting colonization of plant and mineral surfaces. The importance of Ca as a mediator of microbe-mineral-organic matter interactions and resulting SOC transformation has been largely overlooked. We incubated 44Ca labeled soils with 13C15N labeled leaf litter to study how Ca affects microbial transformation of litter and formation of mineral associated organic matter. Here we show that Ca additions promote hyphae-forming bacteria, which often specialize in colonizing surfaces, and increase incorporation of litter into microbial biomass and carbon use efficiency by approximately 45% each. Ca additions reduce cumulative CO2 production by 4%, while promoting associations between minerals and microbial byproducts of plant litter. These findings expand the role of Ca in SOC persistence from solely a driver of physico-chemical reactions to a mediator of coupled abiotic-biotic cycling of SOC.

Watershed-scale liming reveals the short- and long-term effects of pH on the forest soil microbiome and carbon cycling

Soil microbial community composition routinely correlates with pH, reflecting both direct pH effects on microbial physiology and long-term biogeochemical feedbacks. We used two watershed-scale liming experiments to identify short- (2 years) and long-term (25 years) changes in the structure and function of bacterial and fungal communities in organic horizons (O_e and O_a ) of acid forest soils. Liming increased soil pH, extractable calcium, and soil carbon stocks, reduced biomass-specific respiration, and caused major changes in the soil microbiome in the short and long term. More taxa responded to liming in the short term (70%) than in the long term (30%), with most showing consistent directional responses at both sites. The ratio of change in relative abundance between limed and reference sites was twofold higher at the long than the short-term site, indicating that the effects of liming grew over time. Liming impacts were most pronounced in fungi, as steep declines of dominant ectomycorrhizal fungi (Cenococcum and Russula) occurred at both sites. Liming favoured neutrophilic bacteria over acidophilic populations according to estimated environmental pH optima. Collectively, these results demonstrate that a liming-induced change of one pH unit has an immediate and persistent effect on the structure and function of microbial communities in acid forest soils. The corresponding suppression of respiration indicates that anthropogenic alterations of soil pH, as driven by acid deposition or liming, can affect forest floor C stocks due to pH-driven shifts in community structure.

Magnesium and calcium ions: roles in bacterial cell attachment and biofilm structure maturation

The ubiquitous divalent cations magnesium and calcium are important nutrients required by bacteria for growth and cell maintenance. Multi-faceted roles are shown both in bacterial initial attachment and biofilm maturation. The effects of calcium and magnesium can be highlighted in physio-chemical interactions, gene regulation and bio-macromolecular structural modification, which lead to either promotion or inhibition of biofilms. This review outlines recent research addressing phenotypic changes and mechanisms undertaken by calcium and magnesium in affecting bacterial biofilm formation.

Assessing Travel Conditions: Environmental and Host Influences On Bacterial Surface Motility

The degree to which surface motile bacteria explore their surroundings is influenced by aspects of their local environment. Accordingly, regulation of surface motility is controlled by numerous chemical, physical, and biological stimuli. Discernment of such regulation due to these multiple cues is a formidable challenge. Additionally inherent ambiguity and variability from the assays used to assess surface motility can be an obstacle to clear delineation of regulated surface motility behavior. Numerous studies have reported single environmental determinants of microbial motility and lifestyle behavior but the translation of these data to understand surface motility and bacterial colonization of human host or environmental surfaces is unclear. Here, we describe the current state of the field and our understanding of exogenous factors that influence bacterial surface motility.

Structure of thrombospondin type 3 repeats in bacterial outer membrane protein A reveals its intra-repeat disulfide bond-dependent calcium-binding capability

Eukaryotic thrombospondin type 3 repeat (TT3R) is an efficient calcium ion (Ca²⁺) binding motif only found in mammalian thrombospondin family. TT3R has also been found in prokaryotic cellulase Cel5G, which was thought to forfeit the Ca²⁺-binding capability due to the formation of intra-repeat disulfide bonds, instead of the inter-repeat ones possessed by eukaryotic TT3Rs. In this study, we have identified an enormous number of prokaryotic TT3R-containing proteins belonging to several different protein families, including outer membrane protein A (OmpA), an important structural protein connecting the outer membrane and the periplasmic peptidoglycan layer in gram-negative bacteria. Here, we report the crystal structure of the periplasmic region of OmpA from Capnocytophaga gingivalis, which contains a linker region comprising five consecutive TT3Rs. The structure of OmpA-TT3R exhibits a well-ordered architecture organized around two tightly-coordinated Ca²⁺ and confirms the presence of abnormal intra-repeat disulfide bonds. Further mutagenesis studies showed that the Ca²⁺-binding capability of OmpA-TT3R is indeed dependent on the proper formation of intra-repeat disulfide bonds, which help to fix a conserved glycine residue at its proper position for Ca²⁺ coordination. Additionally, despite lacking inter-repeat disulfide bonds, the interfaces between adjacent OmpA-TT3Rs are enhanced by both hydrophobic and conserved aromatic-proline interactions.

Characterization of four type IV pilin homologues in Stigmatella aurantiaca DSM17044 by heterologous expression in Myxococcus xanthus

As prokaryotic models for multicellular development, Stigmatella aurantiaca and Myxococcus xanthus share many similarities in terms of social behaviors, such as gliding motility. Our current understanding of myxobacterial grouped-cell motilities comes mainly from the research on M. xanthus, which shows that filamentous type IV pili (TFP), composed of type IV pilin (also called PilA protein) subunits, are the key apparatus for social motility (S-motility). However, little is known about the pilin protein in S. aurantiaca. We cloned and sequenced four genes (pilA(Sa1~4)) from S. aurantiaca DSM17044 that are homologous to pilA(Mx) (pilA gene in M. xanthus DK1622). The homology and similarities among pilA(Sa) proteins and other myxobacterial homologues were systematically analyzed. To determine their potential biological functions, the four pilA(Sa) genes were expressed in M. xanthus DK10410 (ΔpilA(Mx)), which did not restore S-motility on soft agar or EPS production to host cells. After further analysis of the motile behaviors in a methylcellulose solution, the M. xanthus strains were categorized into three types. YL6101, carrying pilA(Sa1), and YL6104, carrying pilA(Sa4), produced stable but unretractable surface pili; YL6102, carrying pilA(Sa2), produced stable surface pili and exhibited reduced TFP-dependent motility in methylcellulose; YL6103, carrying pilA(Sa3), produced unstable surface pili. Based on these findings, we propose that pilA(Sa2) might be responsible for the type IV pilin production involved in group motility in S. aurantiaca DSM17044. After examining the developmental processes, it was suggested that the expression of PilA(Sa4) protein might have positive effects on the fruiting body formation of M. xanthus DK10410 cells. Moreover, the formation of fruiting body in M. xanthus cells with stable exogenous TFPSa were compensated by mixing them with S. aurantiaca DSM17044 cells. Our results shed some light on the features and functions of type IV pilin homologues in S. aurantiaca.

Effects of chelating reagents on colonial appearance of Paenibacillus alvei isolated from canine oral cavity

A bacterial strain isolated from the oral cavity of a healthy dog revealed an unusual colony formation in nebular appearance on agar plates. The isolated bacterial strain was Gram-positive, spore-forming rod with peritrichous flagella, and grown under aerobic conditions, but unable to grow at 45 degrees C. The strain was tentatively classified as Paenibacillus alvei according to the biochemical properties and the 16S rRNA gene sequence. The isolate exhibits collective locomotion on solid agar plates. The bacterial motility was inhibited with EDTA and was restored by adding magnesium. We concluded that magnesium ion is essential for collective locomotion of P. alvei. This suggests that EDTA is useful for inhibition of biofilm formation.

How myxobacteria glide

BACKGROUND

Many microorganisms, including myxobacteria, cyanobacteria, and flexibacteria, move by gliding. Although gliding always describes a slow surface-associated translocation in the direction of the cell's long axis, it can result from two very different propulsion mechanisms: social (S) motility and adventurous (A) motility. The force for S motility is generated by retraction of type 4 pili. A motility may be associated with the extrusion of slime, but evidence has been lacking, and how force might be generated has remained an enigma. Recently, nozzle-like structures were discovered in cyanobacteria from which slime emanated at the same rate at which the bacteria moved. This strongly implicates slime extrusion as a propulsion mechanism for gliding.

RESULTS

Here we show that similar but smaller nozzle-like structures are found in Myxococcus xanthus and that they are clustered at both cell poles, where one might expect propulsive organelles. Furthermore, light and electron microscopical observations show that slime is secreted in ribbons from the ends of cells. To test whether the slime propulsion hypothesis is physically reasonable, we construct a mathematical model of the slime nozzle to see if it can generate a force sufficient to propel M. xanthus at the observed velocities. The model assumes that the hydration of slime, a cationic polyelectrolyte, is the force-generating mechanism.

CONCLUSIONS

The discovery of nozzle-like organelles in various gliding bacteria suggests their role in prokaryotic gliding. Our calculations and our observations of slime trails demonstrate that slime extrusion from such nozzles can account for most of the observed properties of A motile gliding.

Effects of organic antagonists of Ca(2+), Na(+), and K(+) on chemotaxis and motility of escherichia coli

Various Ca(2+) antagonists used in animal research, many of them known to be Ca(2+) channel blockers, inhibited Escherichia coli chemotaxis (measured as entry of cells into a capillary containing attractant). The most effective of these, acting in the nanomolar range, was omega-conotoxin GVIA. The next most effective were gallopamil and verapamil. At concentrations around 100-fold higher than that needed for inhibition of chemotaxis, each of these antagonists inhibited motility (measured as entry of cells into a capillary lacking attractant). Various other Ca(2+) antagonists were less effective, though chemotaxis was almost always more sensitive to inhibition than was motility. Cells treated with each of these Ca(2+) antagonists swam with a running bias, i.e., tumbling was inhibited. Similarly, some Na(+) antagonists used in animal research inhibited bacterial chemotaxis. E. coli chemotaxis was inhibited by saxitoxin at concentrations above 10(-7) M, while more than 10(-4) M was needed to inhibit motility. Cells treated with saxitoxin swam with a tumbling bias. In the case of other Na(+) antagonists in animals, aconitine inhibited bacterial chemotaxis 10 times more effectively than it inhibited motility, and two others inhibited chemotaxis and motility at about the same concentration. In the case of K(+) antagonists used in animal research, 4-aminopyridine blocked E. coli chemotaxis between 10(-3) M and, totally, 10(-2) M, while motility was not affected at 10(-2) M; on the other hand, tetraethylammonium chloride failed to inhibit either chemotaxis or motility at 10(-2) M.

Gliding motility in bacteria: insights from studies of Myxococcus xanthus

Gliding motility is observed in a large variety of phylogenetically unrelated bacteria. Gliding provides a means for microbes to travel in environments with a low water content, such as might be found in biofilms, microbial mats, and soil. Gliding is defined as the movement of a cell on a surface in the direction of the long axis of the cell. Because this definition is operational and not mechanistic, the underlying molecular motor(s) may be quite different in diverse microbes. In fact, studies on the gliding bacterium Myxococcus xanthus suggest that two independent gliding machineries, encoded by two multigene systems, operate in this microorganism. One machinery, which allows individual cells to glide on a surface, independent of whether the cells are moving alone or in groups, requires the function of the genes of the A-motility system. More than 37 A-motility genes are known to be required for this form of movement. Depending on an additional phenotype, these genes are divided into two subclasses, the agl and cgl genes. Videomicroscopic studies on gliding movement, as well as ultrastructural observations of two myxobacteria, suggest that the A-system motor may consist of multiple single motor elements that are arrayed along the entire cell body. Each motor element is proposed to be localized to the periplasmic space and to be anchored to the peptidoglycan layer. The force to glide which may be generated here is coupled to adhesion sites that move freely in the outer membrane. These adhesion sites provide a specific contact with the substratum. Based on single-cell observations, similar models have been proposed to operate in the unrelated gliding bacteria Flavobacterium johnsoniae (formerly Cytophaga johnsonae), Cytophaga strain U67, and Flexibacter polymorphus (a filamentous glider). Although this model has not been verified experimentally, M. xanthus seems to be the ideal organism with which to test it, given the genetic tools available. The second gliding motor in M. xanthus controls cell movement in groups (S-motility system). It is dependent on functional type IV pili and is operative only when cells are in close proximity to each other. Type IV pili are known to be involved in another mode of bacterial surface translocation, called twitching motility. S-motility may well represent a variation of twitching motility in M. xanthus. However, twitching differs from gliding since it involves cell movements that are jerky and abrupt and that lack the organization and smoothness observed in gliding. Components of this motor are encoded by genes of the S-system, which appear to be homologs of genes involved in the biosynthesis, assembly, and function of type IV pili in Pseudomonas aeruginosa and Neisseria gonorrhoeae. How type IV pili generate force in S-motility is currently unknown, but it is to be expected that ongoing physiological, genetic, and biochemical studies in M. xanthus, in conjunction with studies on twitching in P. aeruginosa and N. gonorrhoeae, will provide important insights into this microbial motor. The two motility systems of M. xanthus are affected to different degrees by the MglA protein, which shows similarity to a small GTPase. Bacterial chemotaxis-like sensory transduction systems control gliding motility in M. xanthus. The frz genes appear to regulate gliding movement of individual cells and movement by the S-motility system, suggesting that the two motors found in this bacterium can be regulated to result in coordinated multicellular movements. In contrast, the dif genes affect only S-system-dependent swarming.

The Myxococcus xanthus pilQ (sglA) gene encodes a secretin homolog required for type IV pilus biogenesis, social motility, and development

The Myxococcus xanthus sglA1 spontaneous mutation was originally isolated because it allowed dispersed cell growth in liquid yet retained the ability to form fruiting bodies. Consequently, most of today's laboratory strains either contain the sglA1 mutation or were derived from strains that carry it. Subsequent work showed that sglA was a gene for social gliding motility, a process which is mediated by type IV pili. Here sglA is shown to map to the major pil cluster and to encode a 901-amino-acid open reading frame (ORF) that is homologous to the secretin superfamily of proteins. Secretins form a channel in the outer membrane for the transport of macromolecules. The closest homologs found were PilQ proteins from Pseudomonas aeruginosa and Neisseria gonorrhoeae, which are required for type IV pili biogenesis and twitching motility. To signify these molecular and functional similarities, we have changed the name of sglA to pilQ. The hypomorphic pilQ1 (sglA1) allele was sequenced and found to contain two missense mutations at residues 741 (G-->S) and 762 (N-->G). In addition, 19 independent social (S)-motility mutations are shown to map to the pilQ locus. In-frame deletions of pilQ and its downstream gene, orfL, were constructed. pilQ is shown to be essential for pilus biogenesis, S-motility, rippling, and fruiting body formation, while orfL is dispensable for these processes. The pilQ1 allele, but not the DeltapilQ allele, was found to render cells hypersensitive to vancomycin, suggesting that PilQ1 alters the permeability properties of the outer membrane. Many differences between pilQ1 and pilQ+ strains have been noted in the literature. We discuss some of these observations and how they may be rationalized in the context of our molecular and functional findings.

Inhibition of development of Myxococcus xanthus by eukaryotic protein kinase inhibitors

Myxococcus xanthus is a social bacterium that lives in the soil and undergoes spectacular development to form multicellular fruiting bodies. It contains a large family of eukaryote-like serine/threonine protein kinases. We found that a number of inhibitors for eukaryotic protein serine, threonine, and tyrosine kinases could inhibit the development and sporulation of M. xanthus to various degrees. These results suggest that serine/threonine and tyrosine phosphorylation may be involved in development of M. xanthus. None of the inhibitors tested had any effect on vegetative growth of M. xanthus. Most of them seemed to act during the early stages of development. However, the expression of a very early development-specific gene, Omega4521, was not significantly affected by the inhibitors. The patterns of protein phosphorylation during development were also not significantly altered by the inhibitors, suggesting that the targets of the inhibitors are minor or unstable phosphoproteins but play key roles in fruiting-body formation in M. xanthus.

Contact stimulation of Tgl and type IV pili in Myxococcus xanthus

Myxococcus xanthus tgl mutants lack social motility and type IV pili but can be transiently stimulated to swarm and to make pili by contacting tgl+ cells. The absence of pili in tgl mutants is shown not to be due to the absence of pilin. The rate of pilus elongation after Tgl stimulation is shown to be similar to the rate of pilus elongation in wild-type cells, using a new more rapid assay for stimulation.

Calcium is required for swimming by the nonflagellated cyanobacterium Synechococcus strain WH8113

The marine cyanobacterium Synechococcus strain WH8113 swims in the absence of any recognizable organelles of locomotion. We have found that calcium is required for this motility. Cells deprived of calcium stopped swimming, while addition of calcium completely restored motility. No other divalent ions tested could replace calcium. Terbium, a lanthanide ion, blocked motility even when calcium was present at 10(5)-fold-higher concentrations, presumably by occupying calcium binding sites. Calcium chelators, EGTA or EDTA, blocked motility, even when calcium was present at 25-fold-higher concentrations, presumably by acting as calcium ionophores. Finally, motility was blocked by verapamil and nitrendipine, molecules known to block voltage-gated calcium channels of eukaryotic cells by an allosteric mechanism. These results suggest that a calcium potential is involved in the mechanism of motility.

Chemotactic properties of Escherichia coli mutants having abnormal Ca2+ content

The calA, calC, and calD mutants of Escherichia coli are known to be sensitive to Ca2+ (R. N. Brey and B. P. Rosen, J. Bacteriol. 139:824-834, 1979). In the absence of any added stimuli for chemotaxis, both the calC and the calD mutants swam with a tumbly bias. Both the calC and the calD mutants were defective in chemotaxis as measured by computer analysis, use of swarm plates, and capillary assays. The calA mutant was only slightly defective in motility and only slightly impaired in chemotaxis. Chemotactically wild-type cells had an intra-cellular free-Ca2+ level of about 105 nM. The intracellular free-Ca2+ levels of the mutants, as determined by use of the fluorescent Ca2+ indicator dye fura-2 or fluo-3, were about 90, about 1,130, and about 410 nM for calA, calC, and calD, respectively. Lowering the intracellular free-Ca2+ levels in wild-type cells and in the tumbly cal mutants by use of Ca2+ chelators promoted running (smooth swimming). Overexpression of CheZ (which causes dephosphorylation of CheY-phosphate) in the wild type and in the tumbly cal mutants decreased the level of tumbliness (which is caused by CheY-phosphate). The calA mutant was 4- to 10-fold more resistant than the wild type to the inhibitory effect of omega-conotoxin on chemotaxis. omega-Conotoxin had no effect on Ca2+ extrusion by wild-type E. coli; that result suggests that omega-conotoxin affects Ca2+ transport at the point of entry instead of exit.

Cytoplasmic free-Ca2+ level rises with repellents and falls with attractants in Escherichia coli chemotaxis

Cytoplasmic free-Ca2+ levels in Escherichia coli were measured by use of the fluorescent Ca(2+)-indicator dye fura-2. Chemotactically wild-type E. coli regulated cytoplasmic free Ca2+ at approximately 100 nM when no stimuli were encountered, but changes in bacterial behavior correlated with changes in cytoplasmic free-Ca2+ concentration. For chemotactically wild-type E. coli, addition of a repellent resulted in cells tumbling and a transient increase in cytoplasmic free-Ca2+ levels. Conversely, addition of an attractant to wild-type cells caused running and produced a transient decrease in cytoplasmic free-Ca2+ levels. Studies with mutant strains showed that the chemoreceptors were required for the observed changes in cytoplasmic free-Ca2+ levels in response to chemical stimuli.

Calcium ions are involved in Escherichia coli chemotaxis

Escherichia coli regulates intracellular free Ca2+ at about 90 nM [Gangola, P. & Rosen, B. P. (1987) J. Biol. Chem. 262, 12570-12574]. To increase intracellular free Ca2+, nitr-5/Ca2+, a "caged" Ca2+ compound, was electroporated into cells and then its affinity for Ca2+ was reduced by exposure to 370-nm light. Upon release of the Ca2+ ions, the cells tumbled. Studies on mutant strains showed that the receptor proteins (methyl-accepting chemotaxis proteins, MCPs) were not required for the Ca(2+)-induced tumbling but that CheA, CheW, and CheY proteins were required. Similar results were obtained with DM-nitrophen/Ca2+, another caged calcium compound that releases Ca2+ upon illumination at 340 nm. Diazo-2, a caged Ca2+ chelator that takes up Ca2+ upon illumination at 340 nm, was used to decrease intracellular free Ca2+, and this caused smooth swimming.

Effect of mechanical removal of pili on gliding motility of Myxococcus xanthus

Gliding motility of Myxococcus xanthus is governed by both the adventurous (A) and the social (S) motility gene systems. The presence of pili has previously been shown to be correlated with a genetically intact S-motility system (D. Kaiser, Proc. Natl. Acad. Sci. USA 76:5952-5956, 1979). The purpose of the present work was to study the direct effect of mechanical removal of pill on the social motility of M. xanthus. Depiliation resulted in (i) a loss of streaming motility of A- S+ mutants, i.e., strains which are able to move by virtue of the S-motility system only, (ii) no effect on motility in A+ S- mutants, i.e., strains capable of movement by the A-motility system only, and (iii) a retardation of streaming speed in the wild-type strain (A+ S+). Cell-cell cohesion, another characteristic of social behavior, was not affected by mechanical removal of pill. The observation that mechanical depiliation perturbed the motility of strains which rely on the S-motility system strongly supports a role for pili in social motility of M. xanthus.

Cell surface modifications induced by calcium ion in the myxobacterium Stigmatella aurantiaca

Calcium ion induces in the myxobacterium Stigmatella aurantiaca the ability to glide on solid surfaces and to become cohesive (D. F. Gilmore and D. White, J. Bacteriol. 161:113-117, 1985; B. J. Womack, D. F. Gilmore, and D. White, J. Bacteriol. 171:6093-6096, 1989). The addition of calcium ion to the growth medium resulted in the formation of extracellular fibrils, the appearance in the membrane fractions of a 30-kDa protein, and the accumulation in a low-speed centrifugal pellet of 10 polypeptides that cross-reacted with affinity-purified antibody to one of the polypeptides. One of the polypeptides, a 55-kDa protein, was present in the membrane fraction of control cells not incubated with calcium ion and was apparently translocated to the extracellular matrix during incubation in medium containing calcium ion. The 55-kDa protein was immunologically related to a 65-kDa protein located on the fibrils of another myxobacterium, Myxococcus xanthus.

Upstream gene of the mgl operon controls the level of MglA protein in Myxococcus xanthus

The mgl operon contains two open reading frames (ORFs) which are transcribed together. A collection of nonmotile mutants helped to define the downstream ORF as the mglA gene. Single mutations at the mglA locus completely abolish motility. A series of deletion mutations was constructed to determine the role of the upstream ORF (now called mglB). A strain carrying a deletion in mglB and with an intact mglA produces small colonies. The cells are motile, but their rate of swarm spreading is reduced. Measurements of cell movement showed that mglB mutant cells advanced, on average, less than 0.1 cell length in 5 min. The mglB+ cells advanced an average of 1.3 cell lengths in the same time. Extracts of delta mglB cells contain 15 to 20% as much of the 22-kDa MglA protein as do mglB+ cells, as measured in Western immunoblots and enzyme-linked immunosorbent assays. However, the amount of mgl transcript is the same in the delta mglB mutants as in the mglB+ strain. Heterozygous partial diploids mglB/mglA with the wild-type alleles in trans have normal motility, demonstrating that the largest of the mglB deletions is not polar on mglA. Like other motility defects, a delta mglB mutation alters fruiting body development and sporulation. The mglB mutants delayed aggregation, produced small immature fruiting bodies, and sporulated at 45 to 50% wild-type levels. All aspects of the mglB mutant phenotype are explained by the reduced levels of mglA protein and the assumption that it limits the amount of gliding.

Social and developmental biology of the myxobacteria

Myxobacteria are soil bacteria whose unusually social behavior distinguishes them from other groups of procaryotes. Perhaps the most remarkable aspect of their social behavior occurs during development, when tens of thousands of cells aggregate and form a colorful fruiting body. Inside the fruiting body the vegetative cells convert into dormant, resistant myxospores. However, myxobacterial social behavior is not restricted to the developmental cycle, and three other social behaviors have been described. Vegetative cells have a multigene social motility system in which cell-cell contact is essential for gliding in multicellular swarms. Cell growth on protein is cooperative in that the growth rate increases with the cell density. Rippling is a periodic behavior in which the cells align themselves in ridges and move in waves. These social behaviors indicate that myxobacterial colonies are not merely collections of individual cells but are societies in which cell behavior is synchronized by cell-cell interactions. The molecular basis of these social behaviors is becoming clear through the use of a combination of behavioral, biochemical, and genetic experimental approaches.

Social and developmental biology of the myxobacteria

Myxobacteria are soil bacteria whose unusually social behavior distinguishes them from other groups of procaryotes. Perhaps the most remarkable aspect of their social behavior occurs during development, when tens of thousands of cells aggregate and form a colorful fruiting body. Inside the fruiting body the vegetative cells convert into dormant, resistant myxospores. However, myxobacterial social behavior is not restricted to the developmental cycle, and three other social behaviors have been described. Vegetative cells have a multigene social motility system in which cell-cell contact is essential for gliding in multicellular swarms. Cell growth on protein is cooperative in that the growth rate increases with the cell density. Rippling is a periodic behavior in which the cells align themselves in ridges and move in waves. These social behaviors indicate that myxobacterial colonies are not merely collections of individual cells but are societies in which cell behavior is synchronized by cell-cell interactions. The molecular basis of these social behaviors is becoming clear through the use of a combination of behavioral, biochemical, and genetic experimental approaches.