Linda Chui

Alberta experience in the detection and characterization of STEC

Shiga toxin-producing Escherichia coli (STEC) of the O157 and non-O157 serotypes are responsible for gastrointestinal diseases reported in numerous outbreaks around the world. While non-O157 serotypes account for more than half of STEC infections, the majority of the frontline diagnostic laboratories routinely screen only for O157 STEC in Canada. As a result, non-O157 STEC infections are significantly under-reported. Current standards of STEC diagnoses focus on O157-specific screening on chromogenic agar, which fails to detect most non-O157 serotypes. Therefore, to develop better diagnostic surveillance of non-O157 STEC serotypes, we compared several methods focusing on the detection of STEC non-O157. We evaluated conventional culture-based approaches using different chromogenic agars, immunoassay systems and molecular detection using real-time PCR. Evaluation of non-O157 STEC on Colorex-O157 agar, which is routinely used in STEC diagnostics, only detected 7.4% of the strains in our collection. We found that many non-O157 STEC strains are sensitive to tellurite and we were able to improve detection of non-O157 STEC using Colorex-O157 agar lacking tellurite. In addition to culture-base approaches, we then evaluated STEC detection using commercial enzyme immunoassays such as SHIGA TOXIN CHEK and SHIGA TOXIN QUIK CHEK. We found these assays yielded detection rates of 80%-85% after culture enrichment and were closer in sensitivity to real-time PCR than other commercially available systems, such as the ImmunoCard STAT!_® . Finally, in an effort to identify novel markers for the detection of non-O157 STEC, we characterized stx gene subtypes and virulence gene profiles for clinical isolates of O157:H7 and non-O157. We found that non-O157 STEC exhibited markedly distinct virulence gene profiles compared to O157:H7 and a greater prevalence of stx₁ over stx₂ subtypes in contrast to O157 STEC. Together our findings highlight improved strategies for detection of non-O157 STEC using currently available systems and development of novel approaches for future non-O157 surveillance.