Abstract
Shiga toxin-producing Escherichia coli (STEC) are diverse bacteria, with seven serogroups (O26, O45, O103, O111, O121, O145, O157; “Top 7”) of interest due to their predominance in human disease. Confirmation of STEC relies on a combination of culturing, immunological and molecular assays, but no single gold standard for identification exists. In this study, we compared analysis of STEC between three independent laboratories (LAB) using different methodologies. In LAB A, colonies of Top 7 were picked after serogroup-specific immunomagnetic separation of feces from western-Canadian slaughter cattle. A fraction of each colony was tested by PCR (stx1, stx2, eae, O group), and Top 7 isolates were saved as glycerol stocks (n = 689). In LAB B, a subsample of isolates (n = 171) were evaluated for stx1 and stx2 using different primer sets. For this, approximately half of the PCR were performed using original DNA template provided by LAB A and half using DNA extracted from sub-cultured isolates. All Top 7 isolates were sub-cultured by LAB A and shipped to LAB C for traditional serotyping (TS) to determine O and H groups, with PCR-confirmation of virulence genes using a third set of primers. By TS, 76% of O groups (525/689) matched PCR-determined O groups. Lowest proportions (p < 0.05) of O group matches between PCR and TS (62.6% and 69.8%) occurred for O26 and O45 serogroups, respectively. PCR-detection of stx differed most between LAB A and LAB C. Excluding isolates where O groups by PCR and TS did not match, detection of stx1 was most consistent (p < 0.01) for O111 and O157:H7/NM. In contrast, for O45 and O103, stx1 was detected in >65% of isolates by LAB A and <5% by LAB C. Stx2 was only detected by LAB C in isolates of serogroups O121, O145, and O157:H7/NM. LAB B also detected stx2 in O26 and O157:H12/H29, while LAB A detected stx2 in all serogroups. Excluding O111 and O157:H7/NM, marked changes in stx detection were observed between initial isolation and sub-cultures of the same isolate. While multiple explanations exist for discordant O-typing between PCR and TS and for differences in stx detection across labs, these data suggest that assays for STEC classification may require re-evaluation and/or standardization.
Highlights
Among zoonotic pathogens, Shiga toxin-producing Escherichia coli (STEC) are a diverse group of bacteria, with seven serogroups (O26, O45, O103, O111, O121, O145, O157) of particular interest due to their predominance in human disease [1]
O group determined by PCR and traditional serotyping (TS) showed less agreement (p < 0.05) for O26 and O45 as compared to other serogroups
The majority of O group mismatches between PCR and TS resulted in serotypes with only one or two isolates, 11.2% of PCR-determined O45 isolates were attributed to serotype O110:H31, while 2.8% of O157 isolates were attributed to O71:H32
Summary
Shiga toxin-producing Escherichia coli (STEC) are a diverse group of bacteria, with seven serogroups (O26, O45, O103, O111, O121, O145, O157) of particular interest due to their predominance in human disease [1] Of these “Top 7”, serotype O157:H7/NM is the most characterized and can be detected in concentrations as low as 25 CFU/g by direct plating on selective media after immunomagnetic separation (IMS) [2]. Traditional serotyping (TS) based on O- and H-antigens and agglutination of antisera has been invaluable for distinguishing strains during outbreaks [13] This process necessitates the generation of specific sera, is labor-intensive and is only performed by a few specialized laboratories. Classification of E. coli is continuously being refined, as illustrated by DeBroy et al [16] advocating merging a number of E. coli O groups due to TS cross reactivity and homology between gene sequences, while others have proposed that there are likely undiscovered O groups [18]
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