motilidad bacteriana

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Annu. Rev. Microbiol. 2003. 57:249–73doi: 10.1146/annurev.micro.57.030502.091014

Copyright c© 2003 by Annual Reviews. All rights reserved

BACTERIAL MOTILITY ON A SURFACE:Many Ways to a Common Goal

Rasika M. HarsheySection of Molecular Genetics and Microbiology, Institute of Cellular andMolecular Biology, University of Texas at Austin, Austin, Texas, 78712;email: [email protected]

Key Words swarming, gliding, twitching, sliding, biofilms, flagella, pili,fruiting body

■ Abstract When free-living bacteria colonize biotic or abiotic surfaces, the re-sultant changes in physiology and morphology have important consequences on theirgrowth, development, and survival. Surface motility, biofilm formation, fruiting bodydevelopment, and host invasion are some of the manifestations of functional responsesto surface colonization. Bacteria may sense the growth surface either directly throughphysical contact or indirectly by sensing the proximity of fellow bacteria. Extracellu-lar signals that elicit new gene expression include autoinducers, amino acids, peptides,proteins, and carbohydrates. This review focuses mainly on surface motility and makescomparisons to features shared by other surface phenomenon.

CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250TYPES OF SURFACE MOTILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

Swimming and Swarming with Flagella. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250Twitching: Gliding with Pili or Social Gliding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253Gliding Without Pili . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254Sliding or Spreading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

INITIATION OF MOTILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255Conditions Favoring Motility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255Signals and Signal Transduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257Specialized Cells/Organelles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

MIGRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260Mechanism of Movement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260Regulation of Motility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

MOTILITY IN OTHER SURFACE PHENOMENON: SOMECOMMON THEMES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264Biofilm Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264Fruiting Body Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265Pattern Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266

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Host Invasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266CONCLUSIONS AND FUTURE PROSPECTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

INTRODUCTION

Bacteria colonize surfaces in many different environments. Depending on avail-ability of nutrients and surface conditions, bacteria can remain localized, moveout to colonize larger areas, invade host tissue, or elaborate fruiting bodies toproduce spores and await more favorable seasons. As a group, bacteria livingin surface colonies have several advantages over single cells. They can optimize(a) growth and survival by having distinct cell types perform specialized functions,(b) access to nutrients, and (c) defense mechanisms for protection from dessicationand antagonists. In the laboratory bacterial colonies are spatially organized, withdistinctive morphological zones and biochemical expression patterns (7, 80). Bac-teria interact dynamically with the surface to detect environmental signals as wellas the presence of other bacteria (28). They secrete polysaccharides that consti-tute important layers called biofilms, which promote bacterial adhesion, survival,and movement (69, 89). This review focuses primarily on bacterial movementover surfaces and how these studies relate to other surface phenomenon, drawingfrom separate reviews that appeared recently on swarming (24, 27, 59), twitching(57, 93), gliding (58), biofilm formation (69, 89), pattern formation (7, 54), andfruiting body formation (83).

TYPES OF SURFACE MOTILITY

Over a quarter century ago J. Henrichsen (36) surveyed surface motility in hundredsof strains from 40 bacterial species belonging to 18 different genera. He identifiedsix different categories: swimming, swarming, gliding, twitching, sliding, anddarting. Only swimming and swarming could be correlated with the presence offlagella. We have since learned a lot more about the first four forms of motility.Swimming and swarming are indeed dependent on flagella (33, 48); twitching hasbeen shown to require type IV pili, as do some forms of gliding (57); the mechanismof some other forms of gliding still remain a mystery (58); and sliding/spreadingare forms of passive translocation (49, 50). Nothing is known about darting. Table 1summarizes the main features of the various types of surface motility discussedbelow.

Swimming and Swarming with Flagella

Swimming in liquid medium inEscherichia coliand Salmonella typhimuriumhas been studied in great detail over the past three decades (48). Typically, fiveto eight peritrichous flagella randomly emerging from the cell surface are eachdriven by a motor at their base. The motor acts as a reversible rotary device, us-ing transmembrane proton potential (proton motive force) as the energy source.Each flagellum is composed of a long helical filament, a short curved structure


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TABLE 1 Main features of various types of surface motility

ColonyTypes of Motive Cell expansion Bacterialmotility organelles differentiation rates (µµm/s) Function generaa

Swarming Flagella Yes 2–10 Surface colonizationAeromonas, Azospirillum,Bacillus, Clostridium,Escherichia, Proteus,Pseudomonas, Rhodospirillum,Salmonella, Serratia, Vibrio,Yersinia

Twitching/ Type IV pili No 0.06–0.3 Surface colonization,Aeromonas, Acinetobacter,social biofilm formation, Azoarcus, Bacteroides,gliding/ fruiting body Branhamella, Comomonas,retractile development, Dichelobacter, Eikenella,motility bphage infection, Kingella, Legionella,

transformation, Moraxella, Myxococcus,conjugation Neisseria, Pasteurella,

Pseudomonas, Ralstonia,Shewanella, Streptococcus,Suttonella, Synechocystis,Vibrio, Wolinella

Gliding/ Not known No 0.025–10 Surface colonizationAnabaena, Cytophaga,adventurous Flavobacterium, Flexibacter,gliding Mycoplasma, Myxococcus,

Phormidium, Saprospira,Stigmatella

Sliding/ None No 0.03–6 Surface colonizationAcinetobacter, Alcaligenes,spreading Bacillus, Escherichia,

Flavobacterium, Mycobacterium,Serratia, Streptococcus, Vibrio

aNot all species in a genera display the indicated motility. See review articles cited in text.bType IV pili function, and not twitching motility per se, is required for phage infection, transformation and conjugation.

called the hook, and a basal body consisting of a central rod and several rings.The basal body is embedded in the cell surface, whereas the hook and filamentare external to the cell. The flagellar filament is normally a left-handed helix ofvariable length (typically 5 to 10µm), with a diameter of 20 nm. Motor rotation inthe counterclockwise (CCW) sense causes a helical wave to travel from proximalto distal and to exert a pushing motion on the cell. Mechanical and hydrodynamicforces cause the flagellar filaments to coalesce into a bundle, usually around thelong axis of the cell body. The concerted action of the flagellar bundle resultsin “running” or “smooth swimming,” pushing the bacterium along relatively lin-ear trajectories at speeds of up to 40µm/s. When motors rotate in the clockwise(CW) sense, the filaments are placed under a right-handed torsional load, whichinitiates a polymorphic transition to a right-handed waveform. This transition re-sults in “tumbling” because the flagellar filaments now have a poorly definedorientation, causing the cell to reorient its travel direction more or less randomly.The chemotaxis signal transduction network modulates the switch between theCCW and CW modes (88). In a constant environment a cell typically moves ina random walk of runs of approximately 1 s interspersed by tumbles of 0.1 s.


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When a cell finds itself swimming toward increasing attractant or decreasing re-pellent concentrations, it tumbles less frequently, thus biasing its random walk inthe preferred direction.

Whereas swimming is an individual endeavor, swarming is the movement of agroup of bacteria. Hauser first described swarming inProteusspecies in 1885 [see(97) for a review of earlier literature]. It has since been shown to be widespreadamong flagellated bacteria, suggesting that this mode of surface translocation mustplay an important role in the colonization of natural environments by microorgan-isms (33) (Table 1). In the laboratory the concentration of the agar used to solidifythe medium can be critical for swarming (0.5%–2%), likely owing to a certainlevel of wetness required for movement. More flagella are needed for swarmingon a surface than for swimming in liquid media, perhaps because of surface frictionand/or higher viscosity of the slime encasing a swarmer colony. While many bacte-ria have only one kind of flagella for both swimming and swarming, some specieshave separate and distinct flagella for the two modes of motility (59). Growth of abacterial colony on swarm medium leads to differentiation of vegetative cells intoswarmer cells, which are hyperflagellated and generally longer than their vegeta-tive counterparts. Continuous outward movement provides a constant supply offresh nutrients until the surface is fully colonized. Exceptions areProteusspecies,which swarm in temporal cycles (4).

Isolated swarmer cells generally do not move unless the agar concentration islow or the agar is supplemented with surfactant. Association of cells in a grouplikely facilitates movement by increasing fluid retention (54). If a drop of fluidis added to the surface of a swarming colony, the bacteria will disperse readilyand individual bacteria can be seen swimming efficiently, usually in the smoothswimming mode, i.e., without tumbling. Swarming may be viewed as a specializedcase of swimming on a surface. Being in a flat two-dimensional environment,swarmer cells can move only forward or backward. They are generally seen movingonly in one direction and reverse only on impact with another group of cellsor barriers such as the outer edge of the colony. Nothing is known about themechanics of motor rotation on the surface, although several flagellar bundleshave been observed along the length of elongated swarmer cells ofProteussp.(67).

The rate of surface colonization via swarming equals or often exceeds the rateat which swimmer cells of the same species colonize “swim” agar (0.2%–0.35%),where individual bacteria swim in the submerged water-filled spaces of the agar.The most active swarming occurs near the periphery of the colony, where thelongest and most flagellated cells are found. Cells in the interior of the colony areless motile and appear to dedifferentiate to a vegetative morphology. Bacteriathat can swarm on higher agar concentration (1%–2%) show a more strikingswarmer cell morphology, with longer cells and an increased number of flagella.Individual cells in the colony move at speeds ranging from 2 to 10µm/sec, andthe swarm front advances at similar rates, making this the fastest mode of surfacetranslocation. Often, multiple spearheaded rafts emerge at various points along theadvancing front (Figure 1a). The spearheads can meander and merge with other



Figure 1 Advancing edges of colonies exhibiting (a) swarming, (b) twitching,(c) gliding, and (d) sliding. The organisms shown areP. mirabilis, P. aeruginosa,Cytophaga hutch, andS. marcescens, respectively. Cell lengths range from 2µm (inpanela) to 10µm (in panelc). Mark McBride (University of Wisconsin at Milwaukee)kindly provided photo ofC. hutch.

groups, leading to the formation of a network of trails within which cells move inboth directions, sometimes pausing and reversing direction but always followingthe long axis of the cell. Growth eventually produces a confluent lawn of cells.Transfer of swarmer cells from the surface to the liquid medium results in a rapidreturn to vegetative morphology.

Swarming has been used loosely in the literature to describe outward migrationof either swimming bacteria in a chemotaxis gradient or gliding bacteria as theymove out in a group. In order to avoid confusion, I suggest this terminology berestricted to describe flagella-dependent surface motility.

Twitching: Gliding with Pili or Social Gliding

Twitching was originally defined as an intermittent and jerky movement predom-inantly displayed by single cells (36). This form of motility occurs in a widevariety of bacteria (36, 57, 93) (Table 1), requires a moist surface, and enablescells to move both forward and backward at speeds of a few tenths of a microm-eter per second. Henrichsen & Blom (37) suggested the possible involvement ofpolar pili in twitching. On the basis of observations regarding phage infection ofPseudomonas aeruginosa, Bradley (14) proposed that retraction of polar pili wasthe driving force of twitching motility. It is now clear that active extension and


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retraction of type IV pili is involved in twitching motility (63, 84, 90). These piliare about 6 nm in diameter and up to 4µm in length; they are typically foundat one or both cell poles (12, 95). Radial expansion rates of colonies via twitch-ing motility can approach 0.3µm/s (57). This form of movement underlies thephenomenon of social gliding and formation of fruiting bodies (81) and is alsoinvolved in biofilm formation (70). Functional type IV pili are required for bacte-riophage infection (13, 42), transformation (99), conjugation (102), and host cellresponses (62). Given that the historical distinction between social gliding motilityand twitching motility is now a matter of semantics, Mattick (57) has suggestedthe use of a more neutral and descriptively accurate new term such as “retractilemotility” to describe motility mediated by type IV pilus extension and retraction.

Like swarmer cells, rafts or spearhead-like clusters of aggregated cells inP. aeruginosa(Figure 1b) and inMyxococcus xanthushave been observed dur-ing twitching motility and social gliding motility, respectively. Within the rafts,cells are highly aligned in close cell-cell contact. The rafts move radially outward,following the long axis of the cells. Cells from one group join into another andform a lattice, much like swarming bacteria. Such cells at first move end forwardtoward the other cells until they touch with their poles and then rapidly snap intoan aligned position, which accounts for the characteristic jerky twitching motionobserved with this form of motility (57). Similar trails and cell reversals are alsoobserved in social gliding motility inM. xanthus(81). Although twitching motilityis primarily a social activity, individual cells can show limited movement whenin contact with inert substrates or on agar at low concentrations (57). Twitchingmotility can both promote outward movement of colonies under nutrient-rich con-ditions, as well as bring cells together to form fruiting bodies under conditions ofnutrient depletion.

Gliding Without Pili

Gliding is defined as a smooth movement of cells, generally along the long axis ofthe cell (36), and is particularly seen in three large bacterial groups, the myxobac-teria (motility rates range from 0.025 to 0.1µm/s), the cyanobacteria (speeds ap-proaching 10µm/s), and theCytophaga-Flavobacteriumgroup (2–4µm/s) (58)(Table 1). InM. xanthusthis form of motility is also known as adventurous motility.The advancing edges of these gliding bacteria look similar to those of swarming andtwitching bacteria, and movement is associated with reversals (Figure 1c). Manydifferent models have been proposed to explain gliding motility, including propul-sion by transport of macromolecules such as polysaccharides, movement of outermembrane components by protein complexes in the cytoplasmic membrane, as wellas contraction of fibrils in cell walls. However, there is no clear evidence supportingany specific model, and it is possible that several different mechanisms are involved.

Mycoplasmas are parasitic bacteria that lack the peptidoglycan layer (77a).Many of these have a distinct cell polarity and glide on glass and other solidsurfaces. Attachment organelles that protrude from the membrane around the necksof these bacteria (81a) are proposed to play a role in gliding motility [see (33a)].



Sliding or Spreading

Sliding is produced by the expansive forces of a growing colony in combinationwith reduced surface tension and has been observed in many bacteria (36) (Table 1).Sheets of cells packed in different directions can spread outward as a unit, suggest-ing that this is not an active form of movement (Figure 1d). Spreading of the colonyfront can range from relatively slow (0.03µm/s inMycobacterium smegmatis; 49)to moderately fast (2–6µm/s inSerratia marcescens; R.M. Harshey, unpublisheddata). There is a strong correlation between the production of surfactants such aslipopeptides, lipopolysaccharides (LPS), and glycolipids and the sliding/spreadingphenomenon (16, 50, 79). Although passive, this mode of translocation likely playsa significant role in bacterial surface colonization (78).

INITIATION OF MOTILITY

Conditions Favoring Motility

MOISTURE Surface motility is generally dependent on moist conditions. This isparticularly evident for swarming in species ofSerratia(1, 23),Escherichia(34),Salmonella(34),Aeromonas(33),Bacillus(33, 79a),Yersinia(33, 103), andPseu-domonas(45a, 76). These bacteria will swarm optimally on 0.5%–0.7% agar andgenerally not swarm at agar concentrations above 1%. The peculiar behavior ofE. coli K-12 strains in requiring a specific commercial source (Eiken) of agar forswarming (34) is likely due to superior wettability of this agar (91). This wasdeduced from the behavior of swarming mutants ofS. marcescensdefective inproduction of the surfactant serrawettin, as well as a large number of swarmingmutants ofS. typhimuriumdefective in LPS biosynthesis, which did not swarm onnormal agar but could be rescued for swarming on Eiken agar (50, 91). The latterobservation is in keeping with the absence of the LPS O-antigen layer in K-12strains, as well as the ability ofE. coli with an intact O-antigen to swarm on agarother than Eiken (34). Similar observations have been made withBacillus subtilismutants defective in surfactin production (65). External addition of surfactin canrescue the swarming defects of not only theseB. subtilismutants, but also LPSmutants ofS. typhimurium, suggesting that LPS normally acts as a surfactant (91).Surfactants such as serrawettin and surfactin are exolipids known to lower surfacetension and improve surface wettability, allowing liquids to spread on hydrophobicsurfaces (52, 55). These and other exolipids not only promote swarming motility(24, 45a, 65) but also allow cells to spread even in the absence of active motil-ity (50, 52) and are likely to play an important role in microbial colonization ofhydrophobic surfaces.

Some species of swarming bacteria such as those ofProteus(36, 77),Vibrio(59),Rhodospirillum(74), andAzospirillum(3, 36) can swarm on higher agar con-centrations (1.5%–2%). These bacteria likely excrete special forms of surfactants/polysaccharides to trap sufficient moisture for swarming on these relatively lessmoist surfaces. Indeed,Proteus mirabilis excretes an acidic polysaccharide


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Figure 2 Slime trail in the wake of a spearhead ofBacillussp. swarming on agar.Scenes were recorded at 1-minute intervals, left to right. Reprinted with permissionfrom Reference 51.

proposed to create an appropriate fluid microenvironment for swarming (27, 77).Polysaccharides and surfactants are components of what is generally referred toas slime surrounding motile surface colonies (Figure 2).

Twitching is usually also dependent on the availability of moisture (36). It isoften studied by stabbing a culture through the agar to the bottom of the plateso cells can move in the film of water between the agar and the petri dish (57).Twitching motility takes place on both organic and inorganic surfaces, includingagar gels, epithelial cells, plastics, glass, and metals. Social, or S, motility inmyxobacteria can work under drier conditions such as 1.5% agar. Surface-activelubricating compounds are also part of the extracellular slime released by thesebacteria (82).

Gliding without pili is likely to encompass several different phenomena, soit is hard to make generalizations. Most gliders (myxobacteria,Cytophaga, fil-amentous cyanobacteria, lysobacters) move well on media solidified with 1.5%agar. However, strain variation is extensive, with some strains showing motilityon media with less agar (1%) and some others on media with more agar (2%).Gliding bacteria also excrete surface-active molecules required for movementand have unusual sulfonolipids in their outer membranes that promote gliding(31, 32).

Like swarming and twitching, sliding is also dependent on agar concentra-tion, is optimally observed between 0.3%–0.7% agar, and is critically depen-dent on surface-active compounds such as peptidolipids, glycolipids, and LPS(16, 49, 50).



NUTRIENTS The abundance of nutrients is generally also an important factor inmotility. Swarming is not observed in many organisms on minimal media un-less casamino acids are included (23, 34, 77, 91). This likely reflects the highermetabolic cost of increased flagellar synthesis. Some bacteria such asVibrio para-haemolyticus, however, can swarm on minimal media (59). Biomass productionappears to be maximized during swarming (77), which also occurs under anaerobicconditions inE. coli andS. typhimurium(91). Twitching motility is also depen-dent on high nutrients (57), although fruiting body formation, which requires socialmotility, is a starvation response (83). For bacteria that glide without pili, as a gen-eral rule, nutrient-poor conditions favor motility and colony spreading, althoughthere is a lot of strain variation.Cytophagaspp., flavobacteria, and relatives glidewell on agar with no added nutrients or in water or buffer on a glass slide. Slidinghas been reported to occur on both rich and minimal media (49, 50).

SLIME Polysaccharides and surface-active compounds are major components ofslime. In addition, autoinducers such as homoserine lactones (HSLs), amino acids,and peptides (28, 64), and proteins such as proteases, lipases, and virulence deter-minants (23, 27, 48b, 79a, 82, 83), are included in the slime in various bacteria,where they serve multifarious roles. Polysaccharides and surfactants not only offerprotection from dessication by water retention, but also promote motility by pro-viding a hydrated milieu within which flagella and pili function (Figure 2). Theyallow the cells to spread in the absence of motility and have been implicated in sig-naling as well (91). Amino acids, peptides, and HSLs serve as cell density signalsthat regulate expression of many genes including those involved in production ofsurfactants and virulence determinants (24, 27, 28, 45a). Some proteins also serveas signals, such as the C-signal required for fruiting body formation inM. xanthus(83).

TEMPERATURE In S. marcescens, swimming and swarming motility as well assliding are inhibited at temperatures above 32◦C (1, 50, 56). Inhibition of sliding islikely due to the absence of serrawettin synthesis at the higher temperatures (55).Although absence of surfactant synthesis would be expected to affect swarming,even swimming motility is inhibited at 37◦C (1). Interestingly, pigment productionin this bacterium is also temperature sensitive (98). Surface translocation in mostother bacteria occurs in the normal temperature ranges.

Signals and Signal Transduction

Both physical and chemical signals have been implicated in initiating the de-velopment of swarmer cells, which have distinct cell morphology (longer andmore flagellated cells) compared with their broth-grown counterparts. InV. para-haemolyticusall conditions that slow down polar flagellar rotation lead to swarmercell induction (59). These include increasing viscosity, using antibodies to inhibitflagellar function, and decreasing motor speed by controlled use of a sodium


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channel–blocking drug (43). Thus, the polar flagellum appears to act as a mech-anosensor, although nothing is known about how such a signal is transduced to pro-gram gene expression. Iron starvation is also reported to be a signal for swarmer celldifferentiation in this organism. Amassing a critical cell density is closely linkedto initiation of swarming in most organisms. InSerratia liquefacienstwo quorum-sensing regulators, N-acyl HSLs, control swarming (24). They do so by binding to atranscription activator that regulates the production of the surfactant serrawettin re-quired for colony expansion; they do not control swarmer cell differentiation per se.Quorum sensing systems responsible for production of rhamnolipid appear to fa-cilitate swarming inP. aeruginosa(45a). Peptides and amino acids in the externalslime have been implicated as signals inProteussp. (27), while polysaccharideshave been proposed to perform this role in bacteria such asS. typhimuriumandE. coli (91). How any of these putative signals are transduced to program swarmercell development is not known. InE. coli andS. typhimuriummutations in thechemotaxis signaling pathway inhibit swarmer cell development, although chemo-taxis is not required for movement (18, 34). InP. mirabilis, two-component regu-lators such as RcsC-RcsB have been implicated in signal transduction (9).

In P. mirabilis, which undergoes multiple cycles of swarmer cell migration, ithas been suggested that both initiation and cessation of migration (consolidation)are a function of swarmer cell age. This model predicts that both events are gov-erned by population dynamics rather than by responses to nutrient depletion oraccumulation of chemotactic repellents (77). On the basis of properties of an LPSmutant that excretes copious amounts of slime and swarms at lower cell densities,it has been proposed that the cell density requirement may be related to slimeaccumulation (91). In this model slime buildup is initially constitutive. Multiplesources of polysaccharides (O-antigen sloughing, capsular polysaccharides, gly-colipid secretion) contribute to the initial slime, whose accumulation is dependenton cell metabolism and growth. The concentration of the polysaccharide signal inthe slime is thus directly related to growth. Unknown pathways detect the signalto elicit swarmer cell differentiation and secretion of more slime. This model isproposed to account for the periodic swarmer cell migration inP. mirabilis (91).As swarmer cells move out they carry the slime with them, thus depleting the sig-nal from the center and diluting it from the edge of the colony. The consolidationphase and the subsequent waves of swarming can be explained by reaccumulationof slime and signal in a time-dependent (and hence age-dependent) manner.

Unlike swarming cells, twitching and gliding bacteria do not undergo surface-induced morphological changes. Twitching motility is influenced by cell density,cell contact-dependent intercellular signals, as well as nutritional status (86,93, 94, 101). The environmental signals that control twitching motility are notwell understood (57). Phosphatidylethanolamine has been implicated as a signalin P. aeruginosaas well as inM. xanthus(44). InSynechocystissp. PCC6803, lightcontrols motility and production of extracellular type IV pili via a phototactic genecluster where signaling is linked to chromophore-binding photoreceptor domains(10). Chemosensory systems or gene clusters showing homologies to these systems



in M. xanthusandP. aeruginosaare involved in controlling reversal frequenciesas well as in fibril and type IV pili biogenesis, but sensory input and output detailsare still lacking (57).

Specialized Cells/Organelles

Only swarming bacteria are known to change morphology upon propagation on asurface. Some bacteria have distinct polar and peritrichous flagella for swimmingand swarming motility, respectively. Examples of these includeV. parahaemolyti-cus, V. alginolyticus, Rhodospirillum centenum, andAzospirillum brasilense(59).Others possess a single peritrichous system for both forms of motility. Examplesinclude Proteus, Serratia, Salmonella, Yersinia, and Escherichiaspecies (33).P. aeruginosahas been reported to swarm using multiple polar flagella (45a, 76).

The structure of the polar flagellum has been studied in some detail inV. para-haemolyticus(59). In this marine organism sodium motive force is used to powerthe polar flagellar motor, which achieves swimming speeds up to 60µm/s. Theflagellum is composed of subunits encoded by multiple genes and is sheathed byan apparent extension of the outer cell membrane. The mechanism that controlsflagellum rotation within the sheath is not understood. The lateral flagella used forswarming are not sheathed, and the filament is polymerized from a single flagellinsubunit. Lateral flagellar motors are powered by proton motive force, and althoughthe dual flagellar systems possess no shared structural components, movement iscoordinated by shared chemotaxis machinery (60).

The best-studied peritrichous flagellar system of a swarming bacterium is thatof S. typhimurium(48). Although a high-resolution crystal structure is not avail-able, much has been learned from three-dimensional reconstructions from electronmicrographs. A single subunit called flagellin is arranged in a helical conforma-tion with 11 subunits per turn in the outer filament. The filament is attached tothe hook-basal-body complex that spans the bacterial membrane. Assembly of thestructure is a sequential process that begins with insertion of the MS ring into theinner membrane. The individual extracytoplasmic flagellar subunits are secretedthrough the MS ring after assembly of an associated type III secretion system.Rotation of the hook-basal-body structure is achieved via the motor force gener-ators which act as stators against the C ring, a ring composed of three structuralproteins associated with the cytoplasmic face of the MS ring (48a). One of thesestructural proteins, FliM, interacts with the phosphorylated form of the chemotaxisresponse regulator CheY to control the direction of switching. Thus, the directionof movement is regulated by the chemotactic output. Regulation of flagellar assem-bly involves a combination of transcriptional, translational, and posttranslationalregulatory mechanisms (2).

Type IV pili are composed primarily of a single small protein subunit termedpilin, which is arranged in helical conformation with five subunits per turn andwhich may be glycosylated and/or phosphorylated in different species (57). Thethree-dimensional crystal structure ofNeisseria gonorrhoeaeMS11 pilin has been


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solved (71). A large number of proteins showing similarities to those in the type IIsecretion pathway are involved in assembly of the pilus (57). Different pili mayhave different binding specificities for different types of cells mediated both bystructural features of the pilin itself and/or by the presence of tip-exposed bindingsites or tip-associated adhesins, which may be quite specific to the species and itsecology (57).

In bacteria that glide without pili, a specific motor has yet to be identified. Insome filamentous cyanobacteria junction pores with a diameter of 14 to 16 nm havebeen found near septa that separate cells of a filament (58). It has been proposedthat polysaccharide extrusion may provide the propulsive force for gliding in thesebacteria. The mechanisms responsible for myxobacterial adventurous gliding orfor Cytophaga-Flavobacteriumgliding are still a matter of speculation.

Freeze-fracture electron microscopy has identified several 50-nm spikes usedas attachment organelles in many gliding mycoplasmas (81a). InMycoplasmamobile, two large proteins with distinct functions in gliding have been localizedto these organelles, which are proposed to comprise the motor [see (33a)].

MIGRATION

Mechanism of Movement

SWARMING MOTILITY Swarming motility involves movement of cells in groupsaligned along their long axis in multicellular rafts (Figure 1a). The observationthat isolated cells rarely move could be related to the amount of slime encasing agroup of cells versus individual cells because wetting agents in the slime providea hydrated environment for flagellar function (77, 91) (Figure 2). This notion isconsistent with inactivity of cells at the edge of the advancing swarm front, whilevigorous motility is evident just behind the front (6). The mechanism of flagellarrotation on a surface is almost entirely unknown, although darkfield images show-ing multiple flagellar bundles have been reported on swarmer cells ofProteussp.(67). Swarmer cells ofS. marcescens, S. typhimurium, andE. colishow prolongedsmooth swimming when suspended in a drop of liquid, suggesting that their mo-tors may only turn CCW on the surface (R.M. Harshey, unpublished data). It isnot known if CW rotation causes cells to pause, because they cannot tumble ona flat surface. It is possible that CCW-rotating flagella, because of their flexiblehooks, form bundles that can push the cell in either direction. Components ofthe chemotaxis system are critical for swarming migration inV. parahaemolyticusandR. centenumbut not for swarmer cell development (59). InS. marcescens,S. typhimurium, and E. coli, however, they are apparently required for devel-opment (34). In the latter two organisms chemotaxis per se is not required forswarming (18). It is not clear whether the chemotaxis system controls swarmercell development directly or whether it does so indirectly by controlling movement.

TWITCHING MOTILITY Twitching motility, like swarming, is also colonial in na-ture and involves cell-cell contact (Figure 1b). Isolated cells rarely move, except



when within a certain distance of others, which corresponds to the length of type IVpili (57). The motive force for twitching motility is pili retraction. This was first de-duced by Bradley (14) and recently confirmed by studies inN. gonorrhoeaeusingoptical tweezers (63), inM. xanthususing a tethering assay (90), and inP. aerug-inosausing fluorescently labeled pili (84). InN. gonorrhoeaecells were pulledat speeds of around 1µm/s with forces ranging up to 90 pN. Retraction eventswere sporadic, separated by 1–20 s.P. aeruginosapili were shown to extend andretract at approximately 0.5µm/s with a force of approximately 10 pN. Individualpili on the same cell appeared to extend and retract independently. Cell movementwas not associated with extension per se, but rather by retraction of pili after theyhad attached to the substratum at their distal tip. InM. xanthus, rates of twitchingmotility were around 0.4µm/s. Tethered wild-type cells appeared to be retractedtoward the surface via one pole and be pulled away from the initial attachment siteby extrusion of pili from the other pole by a similar tether-and-retract step. Thesestudies indicated that reversals of movement involve alternating the activity of pilifrom one cell pole to the other, the frequency of which was correlated with attach-ment time and is controlled by thefrz chemosensory system. Retraction of pilito bring cells into close alignment may involve specific recognition and possiblysensing by the pili of receptors present on the surface of neighboring cells. InM. xanthus, the extracellular polysaccharide fibrils serve as receptors [see (33a,101)]. Sensing tensional or torsional stress on the filament after attachment couldtrigger retraction, although retraction of individual pili is reported to occur in-dependently regardless of whether pili are attached inP. aeruginosa(84). Themechanism of pilus retraction is thought to be filament disassembly mediated byPilT (57), a process that has been estimated to occur at around 1000 pilin subunitsper sec (63). PilT is a nucleotide-binding protein and a member of the AAA familyof motor proteins. PilT is suggested to form a hexameric ATPase that surrounds thebase of the pilus, providing an axial rotary power stroke analogous to that providedby F1 ATPase (40).

GLIDING WITHOUT PILI Although no clear mechanism is at hand, mutants thatare impaired in gliding motility provide food for thought (58). For example, ad-venturous motility inM. xanthusis affected by mutations in outer membranelipoproteins. Similarities of these proteins to those that use proton motive forceto transport molecules across the outer membrane suggest that they may effectmotility by propulsion of macromolecules such as polysaccharides. Although un-usual chain-like strands that wrap helically around the cell have been observedby transmission electron microscopy in cell walls of some myxobacteria and glid-ing bacteria of theCytophaga-Flavobacteriumgroup, their role in motility is notknown.

Some cyanobacteria rotate as they glide. An extracellular protein is thought toform helical fibers on the cell surface that may serve as a passive screw thread. Somefilamentous cyanobacteria are proposed to glide by contraction of fibrils presentin their cell walls (58). Members of theCytophaga-Flavobacteriumgroup glide


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over glass surfaces at rates of approximately 2 to 4µm/s and occasionally reverse(Figure 1c). They also frequently attach to a glass slide by one pole and rotate. Evenwhen suspended in liquid, cells can attach to added latex spheres, which are seento propel along the cell surface. A sphere may travel the length of a cell, aroundthe pole, and down the opposite side. Two spheres may be propelled in differentdirections and may pass one another while going in opposite directions. A spheremay reverse direction, cross the width of the cell, and even follow a helical path.Some spheres become temporarily trapped near the cell pole and rotate in place.Sphere movement appears to be mediated by the same machinery that causes cellmovement, as judged by the effect of mutations. The energy for movement is likelyproton motive force. Polysaccharide extrusion is unlikely to account for this formof motility. A large number of nonmotile mutants ofFlavobacterium johnsoniaeare deficient in sulfonolipid synthesis, whose role is not understood. Genes withsequence similarities to ABC transporters are also required for motility in thisorganism. Whether these transport polysaccharide slime or proteins that make upthe motility machinery is unclear.

An unusual gliding mechanism has recently come to light inM. mobile[see(33a)]. Mycoplasmas do not have flagella, pili, or a chemosensory system (77a).M. mobileglides continuously on glass, exerting a force up to 27 pN. Two largeproteins with distinct functions have been localized to the attachment organellesthat protrude from the neck of this organism and grab the glass surface (33a). Amechanical cycle of gliding is proposed, where one of these proteins is involvedin attachment while the other is responsible for the stroke or force generation thatachieves gliding speeds of 2–4.5µm/s.

Regulation of Motility

SWARMING Swarmer cells need to upregulate the number of flagella in order tomove. InE. coli andS. typhimuriumthe flagellar genes are expressed in a hier-archical manner (48). The masterflhDCoperon encodes transcription factors thatcontrol a second level of genes that specify the hook-basal-body complex as well asthe transcription factor FliA, which in turn regulates late genes encoding the flag-ellar filament, the motor proteins, and the chemotaxis components. Expression oftheflhDCoperon is influenced by environmental signals such as glucose availabil-ity, osmolarity, heat shock, acetyl phosphate, as well as by cell cycle regulation.While the details might differ, it is likely that other swarming bacteria funda-mentally share this hierarchy of flagellar gene expression as well (24, 27, 59). InP. mirabilis the swarmer cells express 30-fold moreflhDC mRNA than the vege-tative cells do, and swarmer cell differentiation appears to be primarily controlledat this level (27). Because FlhDC proteins also appear to repress cell division invarious organisms (23, 29, 73), it is reasonable to think thatflhDC is the primarysite for swarm signal action. In this organism the leucine-responsive regulatoryprotein Lrp positively regulatesflhDC expression (27). Several additional regula-tory proteins have also been identified, some of which apparently network with the



flhDCoperon to coordinately control the expression of virulence genes along withswarmer cell differentiation (26). FlhD and FlhC proteins are also rapidly proteol-ysed following differentiation (20). InV. parahaemolyticusmutation of the LonSprotease results in a constitutive swarmer cell phenotype (87). InS. liquefacienstheflhDC operon controls the expression of at least 62 proteins (30). In addition,theswrI gene controls the synthesis of the HSLs, which regulate 28 other genes.The FlhDC and SwrI regulons are independent and control different aspects ofswarming, swarmer cell differentiation and migration, respectively. Even thoughoverexpression of theflhDCoperon can produce cells that resemble swarmer cellsin bothSerratia liquefaciensandSalmonella typhimurium, no significant increasein the transcription of this operon could be observed in cells swarming on thesurface in either organism [(92); Q. Wang & R.M. Harshey, unpublished resultsfor S. typhimurium].

The chemotaxis system is important for regulating swarming movement in allorganisms. InE. coliandS. typhimuriumthis system operates via methyl-acceptingchemotaxis proteins (MCPs), which induce autophosphorylation of a central histi-dine kinase. The kinase phosphorylates a response regulator (CheY) that controlsmotor reversals by binding to a switch protein complex at the base of the motor(88). Other chemotaxis proteins in the pathway act as adaptors between the methyl-accepting chemotaxis proteins and the kinase, or they modulate the methylationstate of the methyl-accepting chemotaxis proteins in order to reset the system toits prestimulus state. In the swimming mode the steady-state level of phospho-CheY controls the steady-state pattern of runs and tumbles. InS. marcescens,S. typhimurium, andE. coli chemotaxis components are apparently required fordevelopment, although in the latter two organisms, chemotaxis per se is not re-quired for swarming (34). It is not clear whether the chemotaxis signaling systemcontrols swarmer cell development by controlling gene expression in these organ-isms; such a function has not yet been directly ascribed to the response regulatorCheY. An alternative possibility is that the chemotaxis system controls movementon the surface, and that movement somehow indirectly controls development.

TWITCHING Biogenesis of type IV pili in bothP. aeruginosaandM. xanthusisinfluenced by typical and atypical two-component signaling pairs of regulatoryproteins (57). The signals to which these proteins respond are not known, butthey could include nutritional cues such as amino acids as well as cell density.The frz andchpchemosensory systems control twitching motility inM. xanthusandP. aeruginosa, respectively. These systems appear to be more complex thanthose ofE. coliandS. typhimurium. For example,P. aeruginosahas four completechemosensory systems encoded in its genome (22), one each for swimming andtwitching motility and two of unknown function [see also (33a)]. InM. xanthus,thefrizzyor frz system controls twitching, and a seconddif chemosensory systemcontrols the expression of extracellular fibrils that are required for social motility(86, 93, 101). Thefrz system controls the frequency of cell reversal and thereforethe direction and pattern of twitching motility both in normal vegetative growth and


264 HARSHEY

during fruiting body formation (94). FrzCD undergoes methylation and demethy-lation during social motility and fruiting body formation, but unlike well-knownmembers of the methylated chemotaxis protein family, it is a soluble cytoplasmicprotein rather than a membrane protein (83). Slime from developing cells wasshown to cause methylation of FrzCD, suggesting that an extracellular, quorum-dependent signal could be involved (81). Likefrz, thechpsystem inP. aeruginosaalso appears to control the speed and frequency of cell reversal in twitching motil-ity. Some Chp mutants show defects in the expression of many virulence factors,suggesting that this system is involved in the regulation and integration of a rangeof cellular responses, including fimbriae and alginate production during surfaceand host colonization (57). In contrast,N. gonorrhoeaeandN. meningitidisdo nothave any chemosensory systems encoded in their genomes. Not much is knownabout regulation of twitching motility in these obligate pathogens other than thatpilin expression is influenced by two genes that appear to encode an unusual two-component regulatory system responsive to GTP.

MOTILITY IN OTHER SURFACE PHENOMENON:SOME COMMON THEMES

Biofilm Formation

Biofilms are communities of microorganisms attached to a surface (89). They cancomprise a single microbial species or multiple species and can form on a rangeof biotic and abiotic surfaces. They are responsible for a variety of human infec-tions and are more resistant to antibiotics and host immune responses (21). Rapidprogress is being made in understanding their formation owing to the develop-ment of simple screens for isolation of biofilm-defective mutants (69).P. aerugi-nosabiofilms have been studied extensively, althoughP. fluorescens, E. coli, andV. choleraehave also been studied. Among gram-positive bacteria,Staphylococcusepidermidis, S. aureus, and the enterococci have been studied.

Bacteria seem to initiate biofilm development in response to environmental cuessuch as nutrient availability. They continue to develop as long as fresh nutrientsare provided, but they detach from the surface and return to a planktonic modeof growth when deprived of nutrients. Once attached, a common feature is theexpression of large quantities of exopolysaccharides (EPS), which not only pro-mote adherence but also protect the bacteria from dessication and, because of theiranionic nature, help hold minerals and nutrients near the cell (68). Confocal scan-ning laser microscopy has allowed visualization of fully hydrated biofilms. Thestructure of a matureP. aeruginosabiofilm comprises mushroom-shaped micro-colonies of bacteria that are surrounded by an extracellular polysaccharide matrixand separated by fluid-filled channels, reminiscent of fruiting bodies inM. xanthus.

In P. aeruginosaandV. cholerae, flagella and type IV pili were shown to playan important role in events leading to attachment (69). LPS is important depend-ing on the hydrophobicity of the surface. Mutants with various LPS defects show



differential attachment to hydrophilic versus hydrophobic surfaces, which couldeither be due to exposure of lipid moieties or proteins on the cell surface. Proteaseshave also been shown to have a role in development, although their mechanism isnot known. Flagella are important in swimming along the surface until an appropri-ate location for initial contact is found. After attachment, bacteria move by type IVpili–mediated motility, which appears to be dependent on contact with other cells.Type IV pili mutants fail to form the characteristic mushroom-like mounds, muchas these mutants fail to form fruiting bodies inM. xanthus(83). Surface contactappears to induce synthesis of the EPS and to downregulate flagellar biosynthesis.Like the type IV pili mutants, those that interfere with production of one of the twoHSLs made byP. aeruginosaalso fail to develop the mushroom structures. Interest-ingly, while the wild-type strain is resistant to biocides, a strain unable to produceacyl-HSL was reported to become extremely sensitive to the biocide sodium dode-cyl sulfate despite wild-type levels of EPS in the colony. HSLs inP. cepaciahavebeen recently implicated in late stages of biofilm development (38). Microarrayanalysis showed that gene expression in biofilm cells ofP. aeruginosais similarto that in free-living cells, but there are a small number of significant differences,which include expression of genes for some types of antibiotic resistance (96).

In E. coli, motility but not chemotaxis was seen to be important for initialattachment (69). Type I pili were also important. InS. typhimuriuman inverserelationship was observed between swarming and biofilm formation (65). Thus,surfactants that promote swarming motility were found to inhibit biofilm formationand vice versa.

In summary, biofilm formation shares several features with bacterial surfacetranslocation. These include motility mediated by flagella and type IV pili, celldensity-mediated events such as quorum sensing, secretion of extracellular polysac-charides, and formation of structures that resemble fruiting bodies.

Fruiting Body Formation

A comparison between fruiting body formation and biofilm formation has recentlybeen made (69). When deprived of nutrients on an appropriate surface,M. xan-thuscells use social gliding motility, or S motility, to aggregate into multicellularstructures called fruiting bodies within which cells sporulate. Biofilm formationalso occurs in response to nutritional signals and on a surface. Three cell surfacestructures—type IV pili, extracellular matrix fibrils, and LPS O-antigen—are allcritical for S motility. Similarly, type IV pili and EPS production are importantfor biofilm formation. Both phenomena require cell-cell contact. Like biofilms ofP. aeruginosathat require acyl-HSLs, the formation of fruiting bodies also requiresextracellular signaling molecules known as A-signal (a mixture of amino acids)and C-signal (a protein) (83). S motility inM. xanthusis controlled by thefrzanddif chemosensory systems (90, 100). Whereas thedif system is essential forS motility, thefrzsystem is not. Thedif system has been implicated in the biogenesisof the fibril material (101). Chemotaxis is apparently not required for biofilm for-mation (72), although it is required for the extent of migration in swarming and


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twitching, and for swarmer cell development in some organisms (33). The LPSO-antigen is necessary both for swarming (91) and for normal development andsocial motility inM. xanthus(11). The O-antigen has been postulated to serve a sur-factant function in swarming (65, 91). Surfactant synthesis is regulated by quorumsensing HSLs inS. liquefaciensand inP. aeruginosa(24, 45a). Recently, fruitingbody formation inB. subtiliswas found to resemble biofilm formation (15) inthat motile cells differentiated into aligned chains of attached cells that eventuallyproduced fruiting bodies. Fruiting body formation depended on regulatory genesrequired early in sporulation and on genes evidently needed for EPS and surfactinproduction. Like serrawettin and rhamnolipid production inS. liquefaciensandP.aeruginosa, surfactin production inB. subtilisis regulated by a quorum sensingsystem where the autoinducer is a peptide (64, 66).

In summary, motility appendages, EPS, surfactants, LPS O-antigen, and quorumsensing signals are common features of surface translocation, biofilm formation,and fruiting body formation.

Pattern Formation

Motile microbial colonies frequently develop complex patterns (7, 54). The moststriking of these are the periodic concentric rings generated by swarming movementof P. mirabilis (4, 54). These rings are the result of repeated cycles of swarmingfollowed by consolidation, where swarmer cells revert to a vegetative form. Littleis known about the mechanisms which cause the migrating swarmer cell popu-lations to revert, although several models have been proposed (6, 77, 91). Otherswarming bacteria such asClostridium sporogenesandV. parahaemolyticusgen-erate dendritic fractal patterns (59). NonchemotacticE. coli expressing either theserine chemoreceptor Tsr or the aspartate chemoreceptor Tar carrying point muta-tions that abolish binding to their respective ligands also produce unusual swarmpatterns (18). It is likely that CCW/CW switching patterns of the flagella motorsinfluence the direction of cell movement and hence the patterns. Che−mutants ofS. marcescensshow altered patterns, which are also influenced by the surfactantproduced by this organism (51, 53, 54, 56). These patterns are therefore differentfrom those formed by bacteria in semisolid media that are dependent on chemo-taxis (17). A variety of patterns can be induced inBacillus and Paenibacillusstrains in response to nutrient conditions, wetness of the agar, and surfactant pro-duction (7, 61). Microbial patterns have been modeled after concepts borrowedfrom studies of patterning from nonliving systems to suggest that cell-cell signal-ing, motility, and chemotaxis interact in complex ways under given nutrient andwetness conditions to generate these patterns.

Host Invasion

Host cells present microbes with surfaces for attachment and subsequent invasion(25, 41, 46). Development of convenient in vitro assays is helping us understandthe initial stages of colonization. Adhesion by type I pili is a common mode of



attachment not discussed in this review (39). Bacterial cell surface proteins andcomponents of the extracellular slime also contribute to adherence. Motility of-ten helps bacteria get to the surface (35). The role of type IV pili and flagella inbiofilm formation by pathogenic bacteria was discussed above (21, 69). Motilityregulons often also control the expression of virulence determinants. For example,the chemosensory gene cluster inP. aeruginosacontrols the expression of viru-lence factors, and pilin mutants ofP. aeruginosaandN. gonorrhoeaehave reducedvirulence (57). For the uro-pathogenProteus, swarming behavior is closely asso-ciated with modulation of virulence characteristics and the ability to invade humanuroepithelial cells (26, 27). InS. liquifaciens, Clostridium septicum, andBacilluscereus, expression of virulence proteins was seen to be associated with the swarmercell state (23, 48b, 79a). ForV. parahaemolyticus, differentiation into swarmer cellsplays an important role in adsorption and colonization of chitinaceous shells ofcrustaceans (8). The synthesis of virulence proteins is also influenced by localizedgrowth on a surface, which leads to a buildup of cell density. Quorum sensingmechanisms, wherein chemical signal molecules are detected to regulate a varietyof responses, also induce the synthesis of virulence factors (5). In addition, thetype III protein secretion complex, which is evolutionarily related to flagella andfound exclusively in gram-negative bacteria (45, 48a), provides a mechanism fordelivery of bacterial effector proteins across both bacterial membranes as well asthe eukaryotic plasma membrane into the host cell cytosol (19, 47, 75). Althoughwe understand many aspects of bacterial colonization of surfaces, much remainsto be learned.

CONCLUSIONS AND FUTURE PROSPECTS

In the past decade, examination of different modes of surface colonization by bacte-ria has led to a new appreciation and understanding of microbial physiology on thesurface. A common theme to emerge from these studies is that surface appendagesand secreted polysaccharides play central roles in surface colonization. Both havebeen proposed to signal bacteria to turn on new gene expression, although the sig-naling mechanisms are not yet at hand. Polysaccharides also play essential roles inbacterial colonization of plant surfaces, a topic not discussed in this review (85).More needs to be learned about the roles specific polysaccharides play in surfaceresponses. High cell densities are more readily achieved during localized growthon a surface and lead to accumulation of autoinducers and peptides that signal newgene expression. Among these, secretion of surfactants and polysaccharides aidsin both active and passive motility, and secretion of virulence determinants aids inhost infection. The chemotaxis machinery controls the direction of movement andthe extent of colonization. Cell-cell signaling, motility, and chemotaxis interact incomplex ways under given nutrient and wetness conditions to generate interestingpatterns of growth. Our knowledge of how flagellar bundles propel swarmer cellson a surface is lagging behind that of how pili function in twitching motility. Inthe era of genomics new signaling components readily come to light, yet we are


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still in the dark about what the signals are and how they are transduced to elicitfunctional responses. Detailed knowledge of all these pathways should provide agreater understanding of bacterial colonization of surfaces and multiple points ofintervention in bacterial infection.

ACKNOWLEDGMENTS

I thank Mark McBride and Tohey Matsuyama for contributing photographs, andJim Walker and members of my laboratory for helpful comments on the manuscript.Research in my laboratory is supported by grants from the NIH (GM 57400 andGM 33247) and the Welch Foundation (F1351).

The Annual Review of Microbiologyis online at http://micro.annualreviews.org

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