The enteric microogranisms Salmonella, Escherichia coli and Shigella flexneri prefer to grow in neutral pH environments. They nevertheless experience dramatic pH fluctuations in nature and during pathogenesis. In response to environmental encounters with acid, these organisms have evolved complex, inducible acid survival strategies. Regulatory features include an alternative factor (sigma S), 2- component signal transduction systems (PhoP/Q; MviA/?) and the major iron regulatory protein Fur. Specific survival mechanisms include emergency pH homeostasis by inducible amino acid decarboxylases and probable roles for DNA repair, chaparonins, membrane biogenesis as well as others that remain poorly defined. Continued study of acid survival in these organisms will provide insights regarding stress management and will have a direct impact on our understanding of pathogenesis.