The mouse enteric pathogen Citrobacter rodentium, like its human counterpart, enteropathogenic Escherichia coli, causes attaching and effacing lesions in the intestinal epithelium of its host. This phenotype requires virulence factors encoded by the locus for enterocyte effacement (LEE) pathogenicity island. For timely expression of these virulence determinants at the site of infection and for efficient delivery of some virulence factors into epithelial cells, C. rodentium utilizes a positive regulatory loop involving the LEE-encoded regulatory proteins Ler, GrlA and GrlR to control LEE expression. Several transcription factors not encoded by LEE, some of which respond to specific environmental signals, also participate in this regulatory loop. Recently, we identified a non-LEE encoded, AraC-like regulatory protein, RegA, which plays a key role in the ability of C. rodentium to colonize the intestine. RegA functions by activating the transcription of a number of horizontally acquired operons encoding virulence-associated factors, such as autotransporters, fimbriae, a dispersin-like protein and its transporter. In addition, RegA represses transcription of a number of housekeeping genes. Importantly, RegA requires a gut-specific environmental signal, bicarbonate, to exert its effects on gene expression. In our proposed model, when C. rodentium senses bicarbonate ions in the gastrointestinal tract, RegA directs the bacterium to reduce the production of proteins involved in normal cellular functions, while enhancing the production of factors required for colonization and virulence.