The genetic structure of Escherichia coli populations in primary and secondary habitats

Escherichia coli were recovered from the members of two two-person households and their associated septic tanks. The E. coli were isolated using selective and non-selective isolation techniques, characterized using the method of multi-locus enzyme electrophoresis and screened for the presence of virulence factors associated with extra-intestinal disease by using PCR. The growth rate-temperature relationships of strains from the two habitats were also determined. Temporal variation explained 25% of the observed electrophoretic type (ET) diversity in the humans. Among-host variation accounted for 29% of the observed allelic diversity. In one household, ET diversity of the E. coli population in the septic tank was significantly lower than ET diversity in the humans providing the inputs to the septic tank. Molecular analysis of variance revealed that, on average, strains recovered from the septic tank of this household were genetically distinct from strains recovered from the humans providing the faecal inputs to the septic tank. Further, the growth rate-temperature response of strains differed between strains isolated from the septic tank and strains isolated from the humans. Septic tank isolates grew better at low temperatures than strains isolated from humans, but more slowly at high temperatures compared to the human isolates. By contrast, no real differences in ET diversity, allelic diversity, or the growth characteristics of strains could be detected between strains from the humans and strains from the septic tank of the other household. The results of this study suggest there are strains of E. coli that are better 'adapted' to conditions found in the external environment compared to strains isolated from the gastrointestinal habitat. Further, the finding that the numerically dominant clones and clonal diversity in secondary habitats can differ substantially from those found in the source populations will confound efforts to identify the sources of faecal pollution in the environment.