In vitro genotoxicity analyses of colibactin-producing E. coli isolated from a Japanese colorectal cancer patient

Colibactin is a polyketide-peptide genotoxin produced by enteric bacteria such as E. coli, and is considered to contribute to the development of colorectal cancer. We previously isolated E. coli strains from Japanese colorectal cancer patients, and in the present study we investigated the genotoxic potency of the colibactin-producing (clb+) E. coli strains that carry the polyketide synthases "pks" gene cluster (pks+) and an isogenic clb- mutant in which the colibactin-producing ability is impaired. Measurement of phosphorylated histone H2AX indicated that DNA double strand breaks were induced in mammalian CHO AA8 cells infected with the clb+ E. coli strains. Induction of DNA damage response (SOS response) by crude extract of the clb+ strains was 1.7 times higher than that of the clb- E. coli in an umu assay with a Salmonella typhimurium TA1535/pSK1002 tester strain. Micronucleus test with CHO AA8 cells revealed that infection with the clb+ strains induced genotoxicity, i.e., the frequencies of micronucleated cells infected with clb+ strain were 4-6 times higher than with the clb- strain. Since the intestinal flora are affected by dietary habits that are strongly associated with ethnicity, these data may contribute to both risk evaluation and prevention of colorectal cancer in the Japanese population.