Role of Nonhost Environments in the Lifestyles of Salmonella and Escherichia coli

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Abstract
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 com- monly known as a commensal of the lower intestine of mam- mals, 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 medi- ate resistance to acid pH as well as growth on lactose, which is critical for a commensal of mammals (11, 57). In this review, we discuss whether Salmonella and E. coli live in stable, dividing populations in nonhost environments and whether such environments constitute dead ends for these species (e.g., as a consequence of residing in the vertebrate lower intestine, whose contents are regularly excreted (86, 102)). In addition, we examine the role that genes specific to Salmonella and E. coli play in the different abilities of these species to proliferate outside animal hosts.