CRISPR1 analysis of naturalized surface water and fecal Escherichia coli suggests common origin

Clustered regularly interspaced short palindromic repeats (CRISPRs) are part of an acquired bacterial immune system that functions as a barrier to exogenous genetic elements. Since naturalized Escherichia coli are likely to encounter different genetic elements in aquatic environments compared to enteric strains, we hypothesized that such differences would be reflected within the hypervariable CRISPR alleles of these two populations. Comparison of CRISPR1 alleles from naturalized and fecal phylogroup B1 E. coli strains revealed that the alleles could be categorized into four major distinct groups (designated G6-G9), and all four allele groups were found among naturalized strains and fecal strains. The distribution of CRIPSR G6 and G8 alleles was similar among strains of both ecotypes, while naturalized strains tended to have CRISPR G7 alleles rather than G9 alleles. Since CRISPR G7 alleles were not specific to naturalized strains, they, however, would not be useful as a marker for identifying naturalized strains. Notably, CRISPR alleles from naturalized and fecal strains also had similar spacer repertoires. This indicates a shared history of encounter with mobile genetic elements and suggests that the two populations were derived from common ancestors.