Proc. Natl. Acad. Sci. USA Vol. 90, pp. 3378-3382, April 1993 Microbiology The two motility systems of Myxococcus xanthus show different selective advantages on various surfaces (gliding bacterium/swarming/fruiting/video microscopy) WENYUAN SHI AND DAVID R. ZUSMAN Department of Molecular and Cell Biology, 401 Barker Hall, University of California, Berkeley, CA 94720 Communicated by Horace A. Barker, January 8, 1993 (received for review August 26, 1992)

ABSTRACT Myxococcus xanthus, a bacterium that forms In this paper, we report that A-motility and S-motility show fruiting bodies, moves by gliding motility utilizing dual motility different selective advantages on different surfaces: A-mo- systems that differ both genetically and morphologically [sys- tility allows cells to move better than S-motility on relatively tem A, having at least 21 genetic loci and moving mainly single firm and dry surfaces, whereas S-motility allows cells to cells, and system S, having at least 10 genetic loci and moving move much better on relatively soft and wet surfaces. These groups (rafts) of cells] [Hodgkin, J. & Kaiser, D. (1979) Mol. results show that, like flagellated bacteria, the dual motility Gen. Genet. 172, 177-191]. In this study, we found that A- and systems in gliding bacteria allow cells to adapt to a variety of S-gliding-motility systems have different selective advantages physiological and ecological environments. on surfaces containing different concentrations of agar. We observed that colonies of A+S- cells (A-motile cells) swarmed better than A-S+ cells (S-motile cells) on relatively firm and MATERIALS AND METHODS dry surfaces (e.g., 1.5% agar). In contrast, colonies of A-S+ Strains and Culture Conditions. M. xanthus strains used in cells swarmed much better than A+S- cells on soft and wet this study are listed in Table 1. M. xanthus cells were grown surfaces (e.g., 0.3% agar). Individual A-motile cells moved at in a medium consisting of casitone (10 g/liter), yeast extract a rate of 2-4 jam/min on 1.5% agar but they barely moved on (5 g/liter), and 8 mM MgSO4 in 10 mM Mops buffer (pH 7.6; 0.3% agar (<0.5 ,um/min); in contrast S-motile cells moved CYE) (24) at 32°C on a rotary shaker at 225 rpm. Sometimes 3-5 times faster on 0.3% agar than on 1.5% agar. Wild-type a less-rich growth medium (CMM) was used, which consists cells with both A- and S-motility systems were able to move well of casitone (5 g/liter), 8 mM MgSO4, and 10 mM Mops buffer over a wide range of surfaces. These results suggest that dual (pH 7.6). CF medium, used for testing fruiting body forma- motility systems enable the myxobacteria to adapt to a variety tion, was prepared as described (25). Different concentra- of physiological and ecological environments and show simi- tions of agar (Difco), agarose (Sigma), and Gel-Gro (ICN) larities in function to the dual motility systems of flagellated were added to the various media (CYE, CMM, or CF) to bacteria such as Vibrio spp. make swarming or fruiting plates. Swarming and Fruiting. For swarming assays, 2 /,u of cells Dual motility systems are common in the microbial world. at a concentration of 1000 Klett units with a red filter (1 x 107 Many flagellated bacteria (e.g., Vibrio and Proteus) are able cells) was added to the center of the swarming plates; to produce two types of flagella under different conditions alternatively, a wooden stick was used to inoculate bacteria (1-6). Most of these bacteria inhabit very complex environ- from the CYE plates on the center ofthe swarming plates. All ments and their multiple motility systems have different swarming plates were incubated at 32°C for 3-4 days. For selective advantages that enable them to adapt to a variety of fruiting, 20 ,ul of cells at 1000 Klett units (2 x 108 cells) was physiological and ecological environments. One of the best added to the fruiting plates and incubated at 32°C for 2-3 understood examples is Vibrio parahaemolyticus. When the days. The morphology ofthe colonies on swarming or fruiting bacterium is grown in liquid, it produces a single sheathed plates was recorded by a video camera (COHU, model polar flagellum that is used for swimming; when grown on a 4815-2000) with a video macrolens. The morphology of the solidified medium, it produces numerous unsheathed lateral edges of colonies was recorded by the same video camera flagella that are responsible for swarming over the solid through a Zeiss microscope (model 47 60 05-9901). The video surface (5, 7-9). images were printed by a video printer (Hitachi VY-50). Myxococcus xanthus is a gliding bacterium that can move Microscopic Observation of Gliding Motility. Gliding mo- on a solid surface without flagella (for recent reviews, see tility on agar surfaces was observed with a Zeiss microscope. refs. 10 and 11). The mechanism of gliding motility is still A designated medium (5 ml) with a designated concentration largely unknown; however, genetic and morphological anal- ofagar was added to a Falcon tissue culture dish (60 x 15 mm; yses suggest that M. xanthus also contains dual motility Becton Dickinson). After the agar solidified, 10 ,ul ofbacterial systems: (i) system A is required for the movement of single cells was added to the center of the plate. After a 30-min cells or small groups of cells (Fig. lj) and has at least 21 incubation, the bacterial behavior was recorded by video genetic loci; (ii) system S is mainly involved in the movement microscopy for further analysis. Due to the slow movement of cells in groups (Fig. lk) and has at least 10 genetic loci of the M. xanthus cells, a time-lapse video cassette recorder (12-14). Although the genes for A- and S-motility (12-17) and (JVC, model BR-9000U) was used. Bacterial movements the cell surface structure related to A- and S-motility (18-22) were recorded at a 120 times slower rate and played back at have been partially characterized, little is known about the normal speed. The bacteria were maintained at ==22°C, unless physiological differences of the two motility systems in M. otherwise indicated. xanthus. 13-Galactosidase Assay of Tn5-lac Transcriptional Fusions to A- or S-Motility Genes. Cells were harvested directly from The publication costs of this article were defrayed in part by page charge the swarming colonies on different surfaces and broken by payment. This article must therefore be hereby marked "advertisement" sonication. The amounts of protein in these extracts were in accordance with 18 U.S.C. §1734 solely to indicate this fact. determined by the BCA protein assay (Pierce). Protein (80 3378 Downloaded by guest on September 26, 2021 Microbiology: Shi and Zusman Proc. Natl. Acad. Sci. USA 90 (1993) 3379

A+ S A+ S A S+ A S

1 .5% agar

a b c d

0.3% agar I

e f g h

1.5% agar

i k

0.3% agar I I m n 0 p

FIG. 1. Swarming colonies and morphologies of the edges of colonies on different concentrations of agar. A+S+, DK1622; A+S-, DK1300; A-S+, DK1217; and A-S-, DZF4150. The medium used was CYE medium with 1.5% or 0.3% agar. The cells (2 Al) were inoculated onto the plates (the initial size of the colonies was around 0.2 cm) and incubated at 32°C for 3 days. The diameters of the swarming colonies (a-h) were 1.6, 0.8, 0.6, 0.35, 2.1, 0.5, 1.7, and 0.35 cm, respectively. The approximate dimensions for the edges of colonies presented in the photos are as follows: i and m, 450 um x 340 ,um; j- and n-p, 110 pm x 90 Am. ug) was assayed for /-galactosidase activity, as described by formed much larger swarming colonies on 0.3% agar (Fig. Miller (26). le). However, the swarming ofA+S- cells was lower on 0.3% agar plate (Fig. lf) than on 1.5% agar (Fig. lb). In contrast, RESULTS A-S+ cells showed limited swarming on 1.5% agar (Fig. lc) but greatly expanded swarming on 0.3% agar (Fig. lg). As a M. xanthus Displays Different Swarming Behaviors on Plates control, A-S- cells did not swarm on 0.3% agar either (Fig. Containing Different Concentrations of Agar. As reported lh). Table 2 compares the colony spreading on 0.3% agar to (12-15, 27, 28), when M. xanthus cells were placed on a solid that on 1.5% agar and shows that A+S+ cells spread more on agar plate (1.5% agar) containing abundant nutrients (such as 0.3% agar, A+S- cells spread less on 0.3% agar, and A-S+ CYE medium), the cells grew vegetatively and showed cells spread much more on 0.3% agar. These results illustrate cooperative swarming movements. Wild-type (A+S+) cells that changing the agar concentration from 1.5% to 0.3% moved away from the colony center to form a large swarming causes A-motility to be reduced whereas S-motility is in- colony (Fig. la). Mutants defective in S-motility (A+S- cells) creased. or mutants defective in A-motility (A-S+ cells) also swarmed Fig. 2 shows a detailed study on the effects of agar on 1.5% agar to expand their colonies (Fig. 1 b and c); concentration on A- and S-motility. At low concentrations of however, both ofthem had less swarming than wild-type cells agar (<0.7% agar), cells having only S-motility (A-S+ cells) (ref. 14 and Table 2) and A+S- cells swarmed better than moved much better than cells having only A-motility (A+S- A-S+ cells (ref. 28 and Table 2). Nonmotile mutants that lack cells); at high concentrations of agar (>1.0%), cells having both motility systems (A-S- cells) did not swarm and, only A-motility moved better than cells having only S-mo- therefore, formed small colonies (Fig. ld). tility. When agar concentrations were too low (<0.3%) or too When the same M. xanthus cells were inoculated onto high (>1.5%), neither motility system appeared to function CYE medium with 0.3% agar, the swarming behavior of very well (Fig. 2). Again, these results suggest that the two colonies was dramatically changed. Wild-type (A+S+) cells motility systems of M. xanthus function with different effec- Downloaded by guest on September 26, 2021 3380 Microbiology: Shi and Zusman Proc. Natl. Acad. Sci. USA 90 (1993) Table 1. M. xanthus strains used in this study Table 2. Effect of agar concentration on swarming Motility Ref. or Swarming Strain Relevant genotype phenotype source colony size, DZ2 Wild type A+S+ 23 cm2 0.3%/1.5% agar DK1622 Wild type A+S+ 20 Strain Motility type 1.5% 0.3% size ratio DZF1 sglA (leaky) A+S- 24 1.9 A+S- 14 DZ2 A+S+ 2.0 3.8 DK1300 sglGl DK1622 A+S+ 1.8 2.8 1.6 DK1253 tgl-l A+S- 14 A+S- 1.0 0.7 0.7 in an DZF1 (leaky) MXH1651 Tn5-lac insertion DK1300 A+S- 0.4 0.2 0.5 S-motility gene A+S- P. Hartzell DK1253 A+S- 0.3 0.2 0.7 MXH1226 TnS-4ac insertion in an DK1218 A-S+ 0.2 1.4 7.0 S-motility gene A+S- P. Hartzell DK1217 A-S+ 0.3 2.0 7.0 DK1217 aglBI A-S+ 14 DZF4150 A-S- 0.07 0.07 1.0 DK1218 cglB2 A-S+ 14 MXH1216 Tn5-ac insertion in an All plates were made with CYE broth and 1.5% or 0.3% agar, A-motility gene A-S+ P. Hartzell inoculated with 5 x 106 cells initially (duplicates for each strain), and MXH1273 insertion in an then incubated at 32°C for 3 days. The swarming colony sizes were TriS-4ac the average the newly colonized areas. The error is within 10%o. A-motility gene A-S+ P. Hartzell of DZF4150 Mutations in A- and shown). These results suggest that the firmness and wetness S-motility genes A-S- This paper of the substratum are responsible for the pattern of cell P. Hartzell is at the University of California, Los Angeles. swarming. Gliding Motility on Plates with Different Concentrations of tiveness on surfaces made by different concentrations of Agar. To study the changed swarming behavior on different agar. It is particularly interesting to note that wild-type cells concentrations of agar, we examined the effect of agar were able to move well over a wide range of surfaces concentration on gliding motility. Gliding motility on 1.5% (0.3-1.5%) with a combination of A- and S-motilities (Fig. 2) agar has been studied by examination of the morphology of and the rate ofmovement ofwild-type cells was more than the the edges of colonies (12-15, 27, 28). A nonmotile strain sum of A- and S-motility alone (Fig. 1). (A-S- cells) gave an unorganized smooth colony edge (Fig. What causes different swarming behavior on different 1 1 and p). A+S- cells had a rough colony edge with many concentrations of agar? It could simply be due to the reduced single cells moving out of the colony (Fig. lj) and A-S+ cells firmness of the surface, since the surface of 0.3% agar is had a rough colony edge with many rafts of cells moving as softer and wetter than that of 1.5% agar; alternatively, flares out of the colony (Fig. lk). Wild-type (A+S+) cells on changing the agar concentration from 1.5% to 0.3% could 1.5% agar had a rough edge with both single cells and rafts of reduce some "key" chemical contaminant in the agar that cells (Fig. ii). We also examined the morphology ofthe edges controls A- or S-motility. We prepared plates with ultrapure of swarming colonies on 0.3% agar and found that A+S- cells electrophoresis grade agarose or Gel-Gro (an agar substitute gave a smooth edge (indicating little A-motility, Fig. ln) made by ICN) and used these substrates as agar substitutes. whereas A-S+ cells had a rough edge with highly extended At 1.5% agarose and 0.6% Gel-Gro, M. xanthus formed flares (indicating activated S-motility, Fig. lo). These data colonies with morphologies similar to that found on 1.5% are consistent with the colony-spreading behavior. It is agar; whereas at 0.5% agarose and 0.1% Gel-Gro, M. xanthus interesting to note that wild-type cells formed a highly colony morphology was similar to cells on 0.3% agar (data not organized edge (Fig. lm): under the microscope, we ob- 4.0

3.0- E 0 N2.0- c 0 0

Agar concentration (%) FIG. 2. Effect of agar concentration on A-motility and S-motility (see Table 2 for detailed explanations ofthe experiment). A, Wild-type cells (DK1622); O, A+S- cells (DK1300); *, A-S+ cells (DK1217). Downloaded by guest on September 26, 2021 Microbiology: Shi and Zusman Proc. Natl. Acad. Sci. USA 90 (1993) 3381 served many A+S+ cells aligned with each other in large strains that have acquired the ability to swarm on 0.3% agar groups moving outward away from the colony center; single surface (6 from DZF1, 12 from DK1300, and 10 from cells later filled the gaps in between. MXH1226) and found that the vast majority (96%) of these We also used time-lapse video microscopy to more directly mutants (or revertants) agglutinated (S. Tavazoie, W.S., and study gliding motility of M. xanthus. Consistent with the D.R.Z., unpublished data). swarming data and morphology of the edges of colonies Expression of A- and S-Motility Genes on Different Agar presented above, we found that A+S- cells moved at a rate Surfaces. One possible hypothesis to explain what makes of 2-4 Am/min on 1.5% agar but barely moved on 0.3% agar A+S- and A-S+ motile cells behave differently on 1.5% and (the moving speed was <0.5 ,um/min and many cells ap- 0.3% agar surfaces is that different agar surfaces affect the peared to vibrate in place) (Table 3). On the other hand, A-S+ gene expression of A- and S-motility genes. However, we cells did not exhibit much movement when initially plated were unable to obtain data to support this hypothesis. For onto 1.5% agar; however, motility became better after 1.0 or example, we put A+S- cells grown in liquid medium onto 1.5 h. When S-motile cells were plated on 0.3% agar, they 1.5% and 0.3% agar surfaces at the same time and found that were motile immediately. In addition, the speed of cell the cells on 1.5% agar had good motility without a lag whereas movement was 3-5 times faster on 0.3% agar than on 1.5% the same cells on 0.3% agar had much reduced motility. Since agar, based on analysis of the video images (Table 3). Such these cells were preadapted to a moist environment, they movement could often be observed in real time and the should have been more motile on a moist surface than the dry maximum rate of movement we observed was >20 ,um/min surface ifsurface moisture were responsible for the induction at 22°C. We also observed discontinuous gliding movements of motility genes. Clearly that was not the case. In a second of the bacteria on 0.3% agar. S-motile cells on 0.3% agar approach, we tested two strains containing TnS::lacZ inser- moved forward with an accelerating speed, then paused, and tions in A-motility genes to determine whether expression of then moved forward again or reversed their direction. All the reporter genes was affected by the different agar surfaces. reversals of direction were observed after pauses. Further- We found that the expression ofthe 3-galactosidase for these more, S-motile cells were observed to interconnect and glide fusion strains was about the same on both 1.5% and 0.3% agar over each other to quickly fill gaps so that the distribution of plates (Table 4). These results suggest that cells on 0.3% agar cells became more or less uniform (data not shown). may have a potentially functional A-motility apparatus, but Effect of Cell Cohesion on Swarming. What causes S-mo- somehow the soft and wet surface of 0.3% agar prevents the tility to be activated and A-motility to be inhibited on 0.3% motility apparatus from functioning normally. Two strains agar? Previous studies showed (10) that S-motile cells (both with TnS::lacZ insertions in S-motility genes also failed to A+S+ and A-S+ cells) can agglutinate in liquid medium by show variations in /3-galactosidase production when placed using cell surface structures such as pili and fibrils, whereas on different substrates (Table 4). A+S- cells remain suspended mainly as single cells. The Effect of Surface Firmness on Fruiting Body Formation. following data suggest that cellular cohesiveness may play a When myxobacteria are starved, they will aggregate to form role in colony swarming on 0.3% agar. We found the cohe- fruiting bodies. Although some of the A+S- cells failed to siveness of S-motile cells growing on 0.3% CYE agar to be fruit, S-motility is not absolutely required for fruiting (14). extraordinarily strong: the colonies remained intact after The A+S- strain (DK1253) and the A-S+ strain (DK1217) being transferred from 0.3% agar to liquid solution and then showed a certain degree of aggregation and fruiting on CF vortex mixed at maximum speed for 1 min. In contrast, the plates with 1.5% agar (Fig. 3 a and b). When the same cells colonies of A+S- strains were very easy to disperse. Some were put onto the CF plates with 0.3% agar, the A+S- strain chemicals such as EDTA and ethanol were found to inhibit (DK1253) failed to show any aggregation (Fig. 3c); the A-S+ the agglutination of S-motile cells (29). We found that in the strain (DK1217) formed fruiting bodies on a CF plate with presence of these chemicals (1.5 mM EDTA or 2% ethanol), 0.3% agar (Fig. 3d). Similar results were obtained with S-motile cells failed to swarm on 0.3% CYE agar (data not several other A+S- and A-S+ strains (data not shown). Thus shown). We have also isolated many mutants from A+S- the ability of A+S- cells to form fruiting bodies on CF medium appears to be very much affected by the inhibition of Table 3. Effect of agar concentration on motility A-motility on 0.3% agar (Table 3). Motility of individual cells Role of Nutrients and Cell Density in Cell Swarming. Rich medium seemed to be very important for swarming on 0.3% CYE CF agar. When we replaced CYE medium with less-rich medium Strain Motility type 1.5% 0.4% 1.5% 0.4% such as CMM or CF medium, the swarming on 0.3% agar was DZ2 A+S+ + ++ + ++ reduced (data not shown), even though poor nutrient medium DK1622 A+S+ + ++ + ++ did not inhibit S-motility (see examples in Table 3). High cell DZF1 A+S- (leaky) + + + + density promoted swarming on 0.3% agar. A high cell inoc- DK1300 A+S- + + + + ulum (4 x 109 cells per ml) caused swarming to begin DK1253* A+S- - - + + immediately after plating on 0.3% agar, whereas a small DK1217 A-S+ + + + ++ ± Table 4. Effect of agar concentration on expression of A- and DK1218 A-S+ + + ++ S-motility genes DZF4150 A-S- - - - - Motility was observed on 1.5% and 0.4% agar. +, Cells moving at /-Galactosidase activity, unit per a rate of 2-4 ,um/min; ±, cells moving at a rate of <2 ,Lm/min orjust mg of protein per min vibrating in place; + +, cells moving at a rate of >4 Zm/min; -, no CYE CF cell movement. Due to the softness of 0.3% agar, the video images of the cells were very sensitive to vibration. To obtain more stable Strain Motility type 1.5% 0.3% 1.5% 0.3% video pictures, we often used 0.4% or 0.5% agar instead of0.3% agar MXH1216 S+, A::lacZ 152 153 180 164 for these studies; control experiments showed that these media gave MXH1273 A::lacZ 113 116 144 141 results similar to cells on 0.3% agar. S+, *Usually, A- or S-motile cells move better on CMM or CF medium MXH1651 A+, S::lacZ 113 102 137 115 than on CYE medium. DK1253 was basically nonmotile when it MXH1226 A+, S::lacZ 88 76 90 88 was plated on CYE medium with 1.5% agar; however, it moved well /-Galactosidase activities were assayed. The numbers listed are on CF medium with 1.5% agar. the average of duplicate samples. The error is within 10%. Downloaded by guest on September 26, 2021 3382 Microbiology: Shi and Zusman Proc. Natl. Acad. Sci. USA 90 (1993) A+ S (DK1253) A- S + (DK1 21 7) connect and glide over each other, thereby creating transient surfaces from cell-cell contact that enable the myxobacteria to overcome the softness and move forward on 0.3% agar. The cohesiveness ofthe S-motility apparatus is caused by cell 1.5% agar surface pili or fibrils that enable cells to contact each other. CF However, these very same phli or fibrils may also provide cohesiveness to firm surfaces to slow down cell movement that may be responsible for reduced S-motility on 1.5% agar. In flagellated bacteria with dual motility systems, such as a 0) V. parahaemolyticus, the two flagellar organelles, which consist of independent motor-propeller structures, are di- rected by a common chemosensory control system (31). We found that frz genes (11), homologs to chemotaxis genes in enteric bacteria, affected the frequency of reversal of the 0.3% agar direction of movement of both A- and S-motility systems CF (unpublished data). It is therefore likely that the frz genes control both A- and S-motility movements in a pattern similar to that found in flagellated bacteria. We thank Drs. Dale Kaiser and Patricia Hartzell for strains. We c a thank Drs. Gonzalo Acuna, Karen Smith, Mark McBride, and Mr. FIG. 3. Effect of agar concentration on fruiting body formation of Sohail Tavazoie for their contributions to this work. We thank Drs. A+S- and A-S+ cells. All CF plates were inoculated with 2 x 108 Julius Adler, Kathy O'Connor, Thilo Kohler, and Eldie Berger for cells and incubated at 32°C for 2 days. Strains used are indicated in very helpful discussions. This work is supported by Public Health the figure. The bright spots in the colonies are fruiting bodies; they Science Grant GM 20509 from the National Institutes of Health. are 0.1-0.2 mm in diameter. 1. Houwink, A. L. & van Iterson, W. (1950) Biochim. Biophys. Acta 5, 10-44. inoculum (5 x 107 cells per ml) delayed swarming until after 2. Leifson, E. & Hugh, R. (1953) J. Bacteriol. 65, 263-271. the cells had grown to a higher density (data not shown). 3. Sneath, P. H. A. (1956) J. Gen. Microbiol. 94, 331-339. 4. Smith, D. G. (1972) Sci. Prog. Oxford 60, 487-506. 5. Ulitzur, S. (1975) Arch. Microbiol. 104, 67-71. DISCUSSION 6. Baumann, P. & Baumann, L. (1977) Annu. Rev. Microbiol. 31, In this study, we demonstrated that A- and S-motility systems 39-61. 7. Shinoda, S. & Okamoto, K. (1977) J. Bacteriol. 129,1266-1271. in M. xanthus are different not only genetically and morpho- 8. Belas, R., Mileham, A., Simon, M. & Silverman, M. (1984) J. logically but also functionally: cells that show only A-motility Bacteriol. 158, 890-896. (A+S- cells) move better than cells that have only S-motility 9. McCarter, L., Hilmen, M. & Silverman, M. (1988) Cell 54, (A-S+ cells) on relatively firm and dry surfaces like 1.5% agar, 345-351. whereas cells that show only S-motility move much better than 10. Shimkets, L. J. (1990) Microbiol. Rev. 54, 473-501. cells that show only A-motility on relatively soft and wet 11. Zusman, D. R. & McBride, M. J. (1991) Mol. Microbiol. 5, surfaces like 0.3% agar. We do not know what differences 2323-2329. cause behavior. 12. Hodgkin, J. & Kaiser, D. (1977) Proc. Natl. Acad. Sci. USA 74, between 1.5% and 0.3% agar the changed 2938-2942. Since these observations were also made on two agar substi- 13. Hodgkin, J. & Kaiser, D. (1979) Mol. Gen. Genet. 171,167-176. tutes (agarose and Gel-Gro), we conclude that the firmness and 14. Hodgkin, J. & Kaiser, D. (1979) Mol. Gen. Genet. 172,177-191. wetness of the substrate are probably responsible for the 15. Burchard, R. P. (1970) J. Bacteriol. 104, 940-947. different properties of A- and S-motility. M. xanthus naturally 16. Sodergren, E. & Kaiser, D. (1983) J. Mol. Biol. 43, 28-29. lives in the soil or on animal dung that changes its firmness and 17. Stephen, K., Hartzell, P. & Kaiser, D. (1989)J. Bacteriol. 171, wetness day by day. Having two different motility systems 819-830. with should enable M. xanthus 18. MacRae, T. H., Dobson, W. J. & McCurdy, H. W. (1977) Can. different selective advantages J. Microbiol. 23, 1096-1108. to adapt to a variety of physiological and ecological environ- 19. Dobson, W. J. & McCurdy, H. D. (1979) Can J. Microbiol. 25, ments in a similar way to the dual motility systems of flagel- 1150-1158. lated bacteria (see Introduction). Similar multiple motility 20. Kaiser, D. (1979) Proc. Natl. Acad. Sci. USA 76, 5952-5956. systems with selective advantages may also exist in some 21. Fink, J. S. & Zissler, J. F. (1989) J. Bacteriol. 171, 2028-2032. other species of gliding bacteria, since many of them exhibit 22. Behmlander, R. M. & Dworkin, M. (1991) J. Bacteriol. 173, swarming patterns similar to A- and S-motility (30). 7810-7821. What makes A- and S-motility function differently on 23. Campos, J. M. & Zusman, D. R. (1975) Proc. Natl. Acad. Sci. not yet know the answer to USA 72, 518-522. different agar surfaces? We do 24. Campos, J. M., Geisselsoder, J. & Zusman, D. R. (1978) J. that question. We think that it is unlikely to involve the Mol. Biol. 119, 167-178. regulation of gene expression of the motility genes since 25. Hagen, D. C., Bretscher, A. P. & Kaiser, D. (1978) Dev. Biol. TnS-lac transcriptional fusions in two A- and two S-motility 64, 284-296. genes did not show different levels of f-galactosidase activity 26. Miller, J. H. (1972) Experiments in Molecular Biology (Cold when they were placed on 1.5% or 0.3% agar. We found that Spring Harbor Lab., Cold Spring Harbor, NY). gliding motility of single cells (A-motility) requires a firm and 27. Reichenbach, H. (1966) in Encyclopedia Cinematographica, dry surface. We hypothesize then that the motor for A-mo- ed. Wolf, G. (Institut far den Wissenschaftlichen Film, Gottin- tility must physically interact with the surface and push gen, Germany), film E778/1965, pp. 557-578. the surface to move the cells forward. A+S- cells 28. Kaiser, D. & Crosby, C. (1983) Cell Motil. 3, 227-245. against 29. Shimkets, L. J. (1986) J. Bacteriol. 166, 842-848. on agar; it appears that the softness vibrated in place 0.3% 30. Starr, M. P., Stolp, H., Triiper, H. G., Balows, A. & Schlegel, and wetness of this surface fail to provide sufficient support H. G. (1981) The Prokaryotes (Springer, Berlin), Vol. 1, Sec- for the A-motility motor. Since we found that cellular cohe- tion C. siveness was correlated with swarming on 0.3% agar, it is 31. Sar, N., McCarter, L., Simon, M. & Silverman, M. (1990) J. possible that the S-motility apparatus permits cells to inter- Bacteriol. 172, 334-341. Downloaded by guest on September 26, 2021