Role of nonhost environments in the lifestyles of Salmonella and Escherichia coli

The gram-negative bacterial species Salmonella enterica and Escherichia coli are members of the family Enterobacteriaceae that spend a good part of their lives as residents of animal hosts. S. enterica is the etiologic agent of gastroenteritis and typhoid fever in humans (88), whereas E. coli is most commonly known as a commensal of the lower intestine of mammals, although pathogenic variants also exist (77). The animal host is believed to be the primary habitat of these two enteric species (86), which are genetically endowed to do well in this environment. For example, Salmonella has genes that mediate invasion of and survival within host cells, including genes that promote resistance to different microbicidal host products (62, 88). Likewise, the E. coli genome encodes proteins that mediate resistance to acid pH as well as growth on lactose, which is critical for a commensal of mammals (11, 57). [PUBLICATION ABSTRACT]