Acid resistance in enteric bacteria.

Shigella species require a uniquely small inoculum for causing dysentery. One explanation for the low infective dose is that Shigella species are better able to survive the acidic conditions encountered in the stomach than are other enteric pathogens. We have tested Shigella species, Escherichia coli, and Salmonella species for the ability to survive at pH 2.5 for at least 2 h. Most isolates of Shigella and E. coli survived this treatment, whereas none of the Salmonella isolates were able to do so. The ability of Shigella species to survive at low pHs does not require the presence of the large virulence plasmid or growth at 37 degrees C but is strikingly dependent on growth phase. We have also found that Shigella isolates exposed to acid lose the ability to invade epithelial cells.