E. coli O157
Steel blue, colourless or inhibited
Escherichia coli (E. coli) are bacteria commonly found in the gut of humans and warm-blooded animals. Most strains of E. coli are harmless. Some strains, however, such as Verocytotoxigenic E. coli (VTEC), also known as Shigatoxigenic E. coli (STEC) can cause severe foodborne diseases. Enterohaemorrhagic E. coli (EHEC) are a subset of VTEC, which can cause severe disease in humans such as Haemolytic Uraemic Syndrome (HUS). VTEC have been isolated from the gut contents of many animals, including cattle and sheep. VTEC are mainly transmitted to humans primarily through consumption of contaminated foods, but can also be transmitted through handling animals carrying these bacteria.
The E. coli serotype O157:H7 or its non-motile variant O157:H- is the most common VTEC serotype in relation to public health. Its significance was recognized in 1982, following two outbreaks in the USA. Since then, more than 180 outbreaks have been reported worldwide, with an estimated W.H.O figure of 70,000 infections per year.
1. Easier detection compared to SMAC: E. coli O157 is detected by a characteristic mauve colour after only 24 h of incubation, while most other E. coli are blue. The conventional medium for the detection of E. coli O157 is Sorbitol MacConkey (SMAC) Agar, which has very poor specificity, thus exhibiting an abundance of false positives (Proteus, E. hermanii, etc.). Sorbitol MacConkey Agar is also difficult to read because there is a change of colouration in the case of prolonged incubation.
*Sensitivity from scientific study: K.A. Bettelheim, 1998. Reliability of CHROMagar™ O157 for the detection of enterohaemorrhagic E. coli (EHEC) O157 but not EHEC belonging to other serogroups. J.Appl.Microbiol.85:425-428.