Mitomycin‐supplemented washed blood agar for the isolation of Shiga toxin‐producing Escherichia coli other than O157:H7

Aims: Isolation and recognition of the prominent Shiga toxin (Stx)‐producing strains of Escherichia coli (STEC) serovar O157:H7 can be confirmed easily by their late fermentation of sorbitol and lack of β‐glucuronidase activity, but there has been no culture method of choice for detecting non‐O157 STEC strains because of their biochemical diversity. Apart from Stx, many STEC strains produce enterohaemolysin (Ehly) regardless of their serovars. Methods and Results: Although washed blood agar media, with or without the addition of antibiotics (vancomycin, cefixime, and cefsulodin) (WBA and WBVCCA), have been used to detect Ehly, a proportion of STEC strains consistently failed to produce haemolysin on these media. Washed blood agar medium was therefore studied further in order to increase the yield of strains producing Ehly. Conclusions: It was found that the addition of 0·5 μg ml–1 of mitomycin C to the agar medium (WBMA) markedly increased the number of such strains. Thus, of 185 STEC strains comprising 95 O157 and 90 non‐O157 STEC consisting of 34 serovars. Ninety‐seven per cent of these strains produced haemolysis on WBMA, compared with only 76% and 83%, respectively, on WBA and WBVCCA. Significance and Impact of the Study: The appearance of the Ehly zone of haemolysis that was easily distinguishable from that of α‐haemolysin was enhanced by the incorporation of mitimycin C into washed‐blood medium.