Isolation and Characterization of a Novel Motility Mutant in SALMONELLA TYPHIMURIUM

A new mutant, BT40, isolated from mutagenized cultures of S. typhimurium was deficient in chemotaxis (Che- phenotype) in tryptone semisoft agar, but had random motility with a tumbling frequency (37%) similar to that of the parental strain, ST23 (39%). In spatial assays accumulation of BT40 cells in capillary tubes containing attractants was decreased by 33%-59% compared to ST23, but BT40 had a normal response to attractants in temporal assays. The methylation level of the chemotactic proteins was decreased in BT40. At room temperature, the mean swimming speed was 11 μm/s for BT40 and 20 μm/s for ST23; more BT40 cells wobbled (50%) than ST23 cells (30%), also. The proportion of time that the flagella rotated in the clockwise direction was 5% for BT40 and 28% for ST23. Motile BT40 cells had only 1-3 flagella per cell, and the length of the flagellar filaments was only 40% of the normal. The mutant phenotype was designated as CheG- and conjugational studies located cheG between 12 min and 33 min on the chromosomal map. Transductional crosses with fla mutants indicated that cheG was located in flagellar region I at 23 min, and complementation analysis with recA fla strains mapped cheG in flaFVIII. Transductional analysis of spontaneous revertants indicated that besides intragenic revertants, there were pseudorevertants with at least 4 types of suppressor mutations: type I suppressors, linked to the class I Tn10 by 16%, were located between the Tn10 and cheG; type II suppressors, linked to the class I Tn10. by 7%, were distal to cheG with respect to the Tn10; type III were linked to the class I Tn10 by 1%; type IV suppressors were not linked to the class I Tn10 and included in this type were the suppressor linked by over 60% to the class II Tn10. The CheG- phenotype was due to a defect in a flagellar gene that caused the mutant to have short flagella and fewer flagellar filaments resulting in a decrease in the stability of the flagellar bundle. This instability accounted for occurrence of random motility in BT40 even though the individual flagella had a bias to counterclockwise rotation. It is postulated that the cheG mutation prevented normal expression of operons downstream to flaFVIIl in the flagellar regulatory cascade resulting in the pleiotropic defects observed in BT40 cells