Abstract
After entering the gastrointestinal (GI) tract on the way to their physiological site of infection, enteric bacteria encounter a remarkable diversity in environmental conditions. There are gross differences in the physico-chemical parameters in different sections of the GI tract e.g. between the stomach, small intestine and large intestine. Furthermore, even within a certain anatomical site, there are subtle differences in the microenvironment e.g. between the lumen, mucous layer and epithelial surface. Enteric pathogens must not only survive passage through the rapidly changing environments encountered at different niches of the GI tract but must also appropriately coordinate expression of virulence determinants in response to environmental cues at different stages of infection. There are some common themes in the responses of enteric pathogens to environmental cues, there are also distinct differences that may reflect differences in basic pathogenesis mechanisms. The role of bile and oxygen concentration in spatiotemporal regulation of virulence genes in selected enteric pathogens has been reviewed.