The incidence of non-O157 Shiga toxin producing Escherichia coli in the Canterbury region - a pilot study

Abstract

Shiga-toxin producing E. coli (STEC) are increasingly recognized as agents of sporadic and outbreak associated gastroenteritis. Although E. coli O157 has traditionally been the most commonly identified STEC serotype, infections caused by non-O157 STEC are on the rise worldwide. Difficulties in their isolation and identification contribute to underestimation of their true virulence. Existing molecular methods can be used for rapid toxin gene detection; however a major limitation is that the bacterial strain is not recovered. This research has compared molecular and culture-based methods for the detection and isolation of STEC from clinical samples. STEC multiplex PCR, real-time PCR and Multiplex Ligation-dependent Probe Amplification were compared for their ability to detect STEC, while the following enrichment broths and selective agar were compared for enrichment and isolation of STEC: tryptic soy broth, modified tryptic soy broth supplemented with vancomycin and cefsulodin, acid enrichment broth, Rainbow® Agar O157, Statens Serum Institut enteric agar, MacConkey sorbitol agar (CT-SMAC), CHROMagar™ O157 and EHEC agar. A pilot study, consisting of screening 102 faecal samples from patients presenting with diarrhoea and/or haemolytic uremic syndrome (HUS) was conducted to establish the incidence of non-O157 STEC in the Canterbury region. The outcome of the clinical trial was the detection and isolation of eight STEC, including four non-O157 STEC. Two of the non-O157 STEC isolates failed to grow on CT-SMAC, the selective media of choice for the isolation of STEC by many clinical laboratories. Acid enrichment is an effective method for the enrichment of STEC and qPCR is a rapid and a sensitive method of detecting STEC. Evidence from this study strongly suggests that incidence of non-O157 STEC in New Zealand is likely to be under reported.