TAR-DEPENDENT AND TAR-INDEPENDENT PATTERN-FORMATION BY SALMONELLA-TYPHIMURIUM

When Salmonella typhimurium cells were allowed to swarm on either a mi nimal or complex semisolid medium, patterns of cell aggregates were fo rmed (depending on the thickness of the medium). No patterns were obse rved with nonchemotactic mutants. The patterns in a minimal medium wer e not formed by a mutant in the aspartate receptor for chemotaxis (Tar ) or by wild-type cells in the presence of alpha-methyl-D,L-aspartate (an aspartate analog), thus resembling the patterns observed earlier i n Escherichia coli (E. O. Budrene and H. C. Berg, Nature [London] 349: 630-633, 1991) and S. typhimurium (E. O. Budrene and H. C. Berg, Abstr acts of Conference II on Bacterial Locomotion and Signal Transduction, 1993). Distinctively, the patterns in a complex medium had a differen t morphology and, more importantly, were Tar independent. Furthermore, mutations in any one of the genes encoding the methyl-accepting chemo taxis receptors (tsr, tar, trg, or tcp) did not prevent the pattern fo rmation. Addition of saturating concentrations of the ligands of these receptors to wild-type cells did not prevent the pattern formation as well. A tar tsr tcp triple mutant also formed the patterns. Similar r esults (no negative effect on pattern formation) were obtained with a ptsI mutant (defective in chemotaxis mediated by the phosphoenolpyruva te-dependent carbohydrate:phosphotransferase system [PTS]) and with ad dition of mannitol (a PTS ligand) to wild-type cells. It therefore app ears that at least two different pathways are involved in the patterns formed by S. typhimurium: Tar dependent and Tar independent. Like the Tar-dependent patterns observed by Budrene and Berg, the Tar-independ ent patterns could be triggered by H2O2, suggesting that both pathways of pattern formation may be triggered by oxidative stress.