Salmonella typhimurium flhE, a conserved flagellar regulon gene required for swarming

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Abstract
The Salmonella typhimurium gene flhE is located at the end of a large flagellar locus in at least 10 peritrichously flagellated Gram-negative bacterial genera, but it shares no significant similarity with other genes. This study shows that flhE is transcribed as part of an flhBAE flagellar operon, under the control of the flagellar master regulator FlhD2C2. Deletion of the chromosomal flhE gene did not affect swimming motility, but it abolished swarming motility across solid agar. Swarming was restored to the ΔflhE mutant by the 130 aa putative envelope protein FlhE, but not by a truncated version lacking the N-terminal signal peptidase I recognition sequence. The ΔflhE mutant was indistinguishable from the wild-type parent in number and distribution of flagella, secretion of flagellin subunits, and flagellar gene expression, and there were no obvious differences in cell-surface LPS and extracellular polysaccharide. The ΔflhE mutant was able to swarm when non-ionic surfactant was included in agar medium, and it showed differences to the wild-type in binding calcofluor and Congo red dyes, and in biofilm production. The data show that the flhE gene is part of the flagella regulon but that it has no role in flagella biogenesis. It appears, nevertheless, to act at the cell envelope to influence flagella-dependent swarming.