CHARACTERIZATION OF THE ACID RESISTANCE PHENOTYPE AND RPOS ALLELES OFSHIGA-LIKE TOXIN-PRODUCING ESCHERICHIA-COLI

Shiga-like toxin-producing Escherichia coli (SLTEC) strains are an imp ortant 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 th ese isolates to be defective in acid resistance. Introduction of rpoS on a plasmid conferred acid resistance to the majority of the acid-sen sitive isolates. The rpoS genes from two of these isolates were sequen ced; both isolates contained lesions in the rpoS gene resulting in a n onfunctional RpoS. These results shaw that mutant rpoS alleles exist i n natural populations off. coli. Such mutations may play an important role in determining the infective dose of SLTEC and suggest that isola tes may vary in infectivity.