FIG 1 .

Simplified model of bacterial population dynamics during VRE colonization, infection, and selection of relA mutants in an immunocompromised patient. Bacteria, including VRE, are first ingested from the patient-proximal hospital environment. Transit through the upper GI tract coupled with broad-spectrum antibiotic treatment kill off nearly all other microbes, allowing VRE to grow to very high densities in the lower GI tract. A small number of bacteria from the GI tract population seed the bloodstream, where the population expands again. Once a relA mutant bacterium occurs in the bloodstream VRE population, it is able to grow to a high enough density to be detected, because it provides a survival advantage against antibiotic treatment, nutrient limitation, and/or oxidative stress.