Assessment of phenotypic and genotypic diversity of Escherichia coli shed by healthy lactating dairy cattle.

A study was conducted to assess the diversity among fecal Escherichia coli from healthy lactating cattle. E. coli (n = 100) isolates from 10 healthy lactating dairy cows of a Pennsylvania dairy herd were examined for phenotypic and genotypic characteristics. Results revealed 26, 58, 10, and 6 E. coli isolates belonged to phylogenetic groups A, B1, B2, and D respectively. Overall, 63 serotypes, nine antibiotic resistance profiles, and 65 pulsed-field gel electrophoresis (PFGE) profiles were observed among the 100 isolates. Based on the combination of phenotypic and genotypic characteristics, the 100 E. coli isolates were classified into 76 clonal types. The numbers of different phenotypic and genotypic characteristics of E. coli were observed for each cow at ranges of 2-10, 1- 4, 2-10, and 4-10 for serotypes, antibiograms, PFGE profiles, and clonal types, respectively. The Chao1 estimator was used to estimate diversity among fecal E. coli. It was estimated that a range of 3-55, 1- 4, 2-55, and 8-55 fecal isolates from one cow would be required to include all possible types of E. coli based on serotype, antibiotic resistance profile, PFGE profile, and clonal type respectively. Based on the findings of the study it can be inferred that 1) dairy cattle should be considered as a significant reservoir of genotypically and phenotypically diverse E. coli, and 2) epidemiological investigations that focus on commensal bacteria should take into consideration the diversity within the species being investigated; if not addressed adequately, inappropriate sample size could lead to inaccurate study findings.