Purpose: In this study, we compared two methods of isolating E. coli O157 to determine which was the most suitable for isolating E. coli O157 from cattle fecal enrichments.
Methods: A total of 1500 cattle fecal samples were tested. For Method A, a 25-g portion of cattle feces was enriched in BAX MP media (DuPont) for 18 - 20 h. Samples were then screened for E. coli O157 using the BAX real-time PCR assay (DuPont) and confirmation was performed on PCR positive broths using immunomagentic separation (IMS). Method B involved enriching a 25-g portion of feces in Buffered Peptone Water for 6 h prior to performing IMS. For each method, cefixime-tellurite sorbitol MacConkey agar was used as the primary culture media, though IMS beads from Method A enrichments were also plated onto three additional media.
Results: E. coli O157 was isolated from 96 of 1500 samples, with 37 (2.5%) samples positive using Method A and 90 (6.0%) samples positive using Method B. Overall, 6 (6.3%) samples were exclusively positive using Method A, 31 (32.3%) samples were positive by both methods and 59 (61.5%) samples were solely positive by Method B. Many of the samples that yielded an isolate using Method B did not proceed to confirmation in Method A, as the PCR screen returned a negative result for O157.
Significance: Method B proved superior to Method A at isolating E. coli O157 from cattle feces. However, as the PCR screen test used in Method A has been validated for beef trim but not fecal enrichments, further work is required to understand the factors which lead to screen test negative results in complex fecal enrichments.