Comparative analysis of extreme acid survival in Salmonella typhimurium, Shigella flexneri, and Escherichia coli

The Salmonella typhimurium has a minimum growth pH of 4.0, which is lower than that of either Shigella flexneri (pH 4.8) or Escherichia coli (pH 4.4), but in a complex medium E. coli and S. flexneri persist when exposed to lower pH levels (2-2.5) compared to S. typhimurium (3.0). S. typhimurium and E. coli, but not S. flexneri, exhibit log-phase and stationary-phase acid tolerance response systems that give protection to pH 3.0. Some additional acid survival systems that require supplementation are used by E. coli and S. flexneri but not S. typhimurium.