Characterization of the acid resistance phenotype and rpoS alleles of shiga-like toxin-producing Escherichia coli.

Shiga-like toxin-producing Escherichia coli (SLTEC) strains are an important group of enteric pathogens. In this study we have examined the abilities of 58 SLTEC isolates to survive at pH 2.5 and found 13 of these isolates to be defective in acid resistance. Introduction of rpoS on a plasmid conferred acid resistance to the majority of the acid-sensitive isolates. The rpoS genes from two of these isolates were sequenced; both isolates contained lesions in the rpoS gene resulting in a nonfunctional RpoS. These results show that mutant rpoS alleles exist in natural populations of E. coli. Such mutations may play an important role in determining the infective dose of SLTEC and suggest that isolates may vary in infectivity.