Enterohemorrhagic Escherichia coli in human infection:: In vivo evolution of a bacterial pathogen

Background. Enterohemorrhagic Escherichia coli (EHEC) cause most cases of the hemolytic uremic syndrome (HUS) worldwide. To investigate genetic changes in EHEC during the course of human infection, we analyzed consecutive stool samples and shed isolates from patients with HUS, focusing on the genes encoding Shiga toxin (stx) and intimin (eae). Methods. Sequential stool samples from 210 patients with HUS were investigated for the persistence of E. coli strains harboring stx and/or eae. Initial stool samples were collected during the acute phase of HUS, and subsequent stool samples were collected 3 - 16 days later (median interval, 8 days). Results. Organisms that were stx and eae positive (stx+/eae+ strains, N = 7) or stx negative and eae positive (stx-/eae+ strains, N = 5) were detected in the initial stool samples from 142 patients. Subsequently, the proportion of those who shed stx+/eae+ strains decreased to 13 of 210 patients, whereas the proportion of those who shed strains that were stx-/eae+ increased to 12 of 210 patients. Seven patients who initially excreted strains that were stx+/eae+ shed, at second analysis, stx-/eae+ strains of the same serotypes; they had no free fecal Shiga toxin at follow-up. Comparison of the initial and follow-up isolates from these patients with use of molecular-epidemiological methods revealed loss of stx genes and genomic rearrangement. Conclusions. We demonstrate the loss of a critical bacterial virulence factor from pathogens during very brief intervals in the human host. These genetic changes have evolutionary, diagnostic, and clinical implications. Generation of stx- mutants might contribute to subclonal evolution and evolutionary success.