Surveillance for Shiga toxin--producing Escherichia coli, Michigan, 2001-2005.(DISPATCHES)(Disease/Disorder overview)

A surveillance system used different detection methods to estimate prevalence of Shiga toxin-producing Escherichia coli during 2003-2005 and 2001-2002. More non-O157 serotypes were detected by enzyme immunoassay than by evaluation of non-sorbitol-fermenting E. coli isolates. We therefore recommend use of enzyme immunoassay and culture-based methods.

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Infection with Shiga toxin-producing Escherichia coli (STEC) is a frequent cause of gastrointestinal disease, particularly among children and elderly persons (1). Detection of O157 STEC by culture relies primarily on sorbitol MacConkey agar (SMAC) (2) because O157:H7 strains cannot rapidly ferment sorbitol (3). In contrast, using culture to detect sorbitol-fermenting O157 (4) and non-O157 serotypes is problematic because on SMAC these strains are indistinguishable from other E. coli. Consequently, whether the predominance of STEC O157 in disease reflects actual differences in pathogen prevalence or a bias associated with detection is unclear. We therefore sought to determine whether STEC prevalence, particularly of non-O157 serotypes, increased when enhanced detection methods were used.

The Study

The Michigan Department of Community Health implemented a sentinel surveillance system to evaluate blood-containing stool samples from 20 laboratories during April 2003-October 2005 and all stool samples from 2 hospitals during July 2004-October 2005. All suspect non-sorbitol-fermenting E. coli from the remaining laboratories were also examined.

The samples, transported in C&S transport medium (Medical Chemical Corporation, Torrance, CA, USA), were screened for Shiga toxin (Stx) by enzyme immunoassay (EIA) (Meridian BioScience, Cincinnati, OH, USA) after enrichment with gram-negative broth (Remel, Lenexa, KS, USA). EIA is sensitive and specific but cannot detect the Stx2e variant (5), and Pseudomonas aeruginosa can produce false-positive results (6). Samples were cultured on SMAC (Remel) and cefixime-tellurite SMAC (7), and samples from the 2 hospitals were tested for occult blood (Beckman Coulter, Fullerton, CA, USA) before EIA testing. Serotyping (Statens Serum Institute, Copenhagen, Denmark; BD Difco, Franklin Lakes, NJ, USA) and real-time PCR for stx1,2 genes (8) were performed on strains that had positive EIA results, suspect non-sorbitol-fermenting E. coli, and multiple colonies of sorbitol-fermenting (SF) strains that had positive EIA results. For some samples, the EIA result was negative but NSF stx-positive colonies were detected on SMAC, which indicated a false-negative EIA result. Epidemiologic data were obtained for STEC-positive patients.

During the 5 years studied, 438 STEC were isolated; 401 (92%) were O157. Prevalence over time did not differ ([chi square]= 4.14, degrees of freedom [df] = 4, p = 0.39). Similarly, overall prevalence of non-O157 serotypes during 2001-2002 and 2003-2005 did not…