l-Cysteine is required for induced antibiotic resistance in actively swarming Salmonella enterica serovar Typhimurium

Swarm-cell differentiation in Salmonella enterica serovar Typhimurium (S. typhimurium) results in a biosynthetic mode of growth, despite growing on a rich medium, and cells that have elevated antibiotic resistance. These phenotypes are not a prerequisite for swarm motility. By blocking the switch to anabolic growth using amino acid auxotrophs and screening for the presence of elevated antibiotic resistance in the swarm state, we found that cysteine biosynthesis is crucial for complete swarm-cell differentiation. Mutants were made in each cys biosynthetic operon and all had decreased antibiotic resistance in the swarm state, while swim-cell resistance remained the same as that of wild-type cells. This swarm-state-specific decreased resistance in Deltacys strains could be restored to wild-type levels by the addition of cysteine to swarm medium. Two regulatory mutants, DeltacysB and DeltacysE, failed to swarm unless cysteine was supplemented to the medium. We show that all CysB-responsive operons involved in cysteine biosynthesis are upregulated in the swarm state, even though swarm cells are cultivated on a medium that represses cysteine biosynthesis in the swim state. While swarm medium has sufficient cysteine for growth of S. typhimurium, it does not contain enough for swarm-cell differentiation. We hypothesize that in these cells, the additional cysteine requirement is for use in pathways not directly related to cell growth.