The dense bacterial consortium called the “microbiota” that inhabits the intestinal tract is recognized increasingly as playing a major role in human health and disease. The microbiota generally influences the host in a beneficial fashion by shaping gastrointestinal and immune functions, exerting protection against pathogens, and contributing to metabolic pathways. Escherichia coli is a consistent member of the humanintestinal microbiota, colonizing the intestine within a few days after birth and persisting throughout the life of the host. The E. coli strain population can be categorized in at least four major phylogenetic groups, each group being more specifically associated with certain ecological niches. E. coli strains belonging to group B2 are recovered from the environment less frequently but can persist longer in the colon than other groups and represent 30–50% of strains isolated from the feces of healthy humans living in high-income countries.
Up to 34% of commensal E. coli strains of the phylogenetic group B2 carry a conserved genomic island named the “pks island”. This gene cluster codes for genes that allow production of a putative hybrid peptide-polyketide genotoxin, Colibactin. In vitro infection with these strains induces DNA double-strand breaks (DSBs) in cultivated human cells, but the pks island was not proved to cause DNA damage in vivo. In this study, the authors explore whether those bacteria were able to induce genetic damage in vivo on the colonic mucosa and to characterize the consequences of this damage on mammalian cells in relation with the number of infecting bacteria. They report that pks+ E. coli induced DSBs in vivo. In addition, infection of various mammalian cells with pks+ E. coli induced, at very low multiplicity of infection, reversible DNA damage response that did not repair all DSBs, leading to chronic mitotic and chromosomal aberrations together with increased frequency of gene mutation and anchorage-independent growth. Taken together, these findings strongly suggest that these pks+ strains are genotoxic in vivo and provide insights into mechanisms by which common E. coli strains may contribute to cellular transformation and possibly sporadic colorectal cancer tumorigenesis.
Escherichia coli induces DNA damage in vivo and triggers genomic instability in mammalian cells. PNAS USA June 7 2010 doi: 10.1073/pnas.100126110
Escherichia coli is a normal inhabitant of the human gut. However, E. coli strains of phylogenetic group B2 harbor a genomic island called “pks” that codes for the production of a polyketide-peptide genotoxin, Colibactin. Here we report that in vivo infection with E. coli harboring the pks island, but not with a pks isogenic mutant, induced the formation of phosphorylated H2AX foci in mouse enterocytes. We show that a single, short exposure of cultured mammalian epithelial cells to live pks+ E. coli at low infectious doses induced a transient DNA damage response followed by cell division with signs of incomplete DNA repair, leading to anaphase bridges and chromosome aberrations. Micronuclei, aneuploidy, ring chromosomes, and anaphase bridges persisted in dividing cells up to 21 d after infection, indicating occurrence of breakage–fusion–bridge cycles and chromosomal instability. Exposed cells exhibited a significant increase in gene mutation frequency and anchorage-independent colony formation, demonstrating the infection mutagenic and transforming potential. Therefore, colon colonization with these E. coli strains harboring the pks island could contribute to the development of sporadic colorectal cancer.