Purpose: The purpose of this work was to investigate virulence gene combinations in E. coli from potential positive samples where pSTEC were not confirmed.
Methods: A total of 121 enrichment broths comprising 93 Big6 and 28 E. coli O157 potential positives were analysed using immunomagnetic separation and colony hybridisation targeting E. coli that harbor stx, eae, or were of a pSTEC serotype.
Results: STEC, eae-containing E. coli, and E. coli of a pSTEC serotype that lack stx and eae were found in 43.0, 28.0 and 28.0% of Big6 samples and 21.4, 14.3 and 3.6% of E. coli O157 samples, respectively. Forty-five of 121 (37%) samples were shown to contain multiple isolates of E. coli harboring different genetic markers that would be detected during screening and which could account for the potential positive status of that sample.
Significance: This study indicates that organisms other than pSTEC contribute to the generation of potential positive samples during screening. The presence of these organisms may give rise to an elevated potential positive rate, increase the difference between potential positive and confirmed positive rates, and reduce the perceived effectiveness of currently established confirmation protocols. Whilst ongoing refinement of confirmation protocols is required to ensure maximum isolation rates are achieved from samples containing pSTEC, a focus should remain on identifying genetic and or phenotypic differences of pSTEC that can be exploited during the screening process.