The chemotaxis system, but not chemotaxis, is essential for swarming motility in Escherichia coli

Abstract

The chemotaxis system plays an essential role in swarm cell differentiation and motility. We show in this study that two (Tsr and Tar) of the four chemoreceptors in Escherichia coli can support swarming individually, but sensing their most powerful chemoattractants is not necessary. Conditions that abolish chemotaxis toward serine (presence of serine concentrations that saturate Tsr, or mutations in Tsr that destroy serine binding) have no effect on swarming. Similar results were obtained for the aspartate and maltose chemoreceptor Tar. We also show that although a mutation in the signaling domain of Tsr that inhibits CheA kinase abolishes swarming, nonchemotactic flagellar switch mutants can swarm. Our results suggest that during swarming, the chemoreceptors signal through the chemotaxis pathway and induce swarmer cell differentiation in response to signals other than their known chemoeffectors.

Figure 1

Tsr or Tar alone supports swarming in E. coli. (A) Swarming motility is observed in wild-type (RP437) bacteria (a), but not in RP437 lacking all four transducers (HCB429) (b). Plasmids expressing Tsr (pJC3) (c) or Tar (pNT201) (d), but not those expressing Trg (pLA1) (e) or Tap (pCG1) (f), restore swarming in HCB429. (B) Swimming motility in the same strains as in A. As described in Methods, 20 μM IPTG was used to induce protein expression from plasmids. Plates in A were incubated for 16 h, whereas those in B were incubated for 12 h at 30°C.

Figure 2

High serine concentrations inhibit chemotaxis but do not inhibit swarming. Effect of increasing serine concentrations (0, 15, and 30 mM, left to right) on swimming (chemotaxis) (A) and swarming (B) motility of E. coli (HCB429) expressing only Tsr (pJC3). Serine concentrations (30–50 mM) inhibit chemotaxis even when all four transducers are present (not shown). Growth conditions are as in Fig. 1, except all plates were incubated at 30°C for 16 h.

Figure 3

Tsr(R64C), a serine-binding mutant, supports swarming. Migration of E. coli (HCB429) expressing Tsr(R64C) in swim (A) and swarm (B) media in the presence of increasing (0–30 mM) serine concentrations. Growth conditions are as in Fig. 2.

Figure 4

Tar(T154I), an aspartate-binding mutant, supports swarming. Swarming motility in HCB429 expressing wild-type (A) or mutant (B) Tar(T154I) proteins. Growth conditions are as in Fig. 2.

Figure 5

Flagellation of representative nonchemotactic mutants. Cells grown on swarm media were stained to visualize flagella as described (7). (a) Nonswarming E. coli HCB429 (Fig. 1A, b). (b) Swarming E. coli HCB429/pJC3-Tsr(R64C) (Fig. 3B). (c) Nonswarming E. coli RP2867 (cheBR) (Table 3). (d) Swarming S. typhimurium MY107 (fliG) (Table 3). The morphology of wild-type E. coli (not shown) is indistinguishable from that of the Tsr mutant shown in b. (Bar = 2.8 μm.)