A medium for the isolation, enumeration and rapid presumptive identification of injuredClostridium perfringensandBacillus cereus

摘要

A blood‐free egg yolk medium (BCP) containing pyruvate, inositol, mannitol and a bromocresol purple indicator in a nutrient agar base has been developed to initiate the growth ofClostridium perfringens. It is comparable to blood agar for the growth of normal, chilled stored vegetative cells and heat‐injured spores ofCl. perfringensandBacillus cereus. It has the advantage over blood agar in exhibiting presumptive evidence ofCl. perfringens(production of lecithinase and inositol fermentation) after an overnight incubation at 43°C‐45°C. Pyruvate, catalase and other hydrogen peroxide degraders were found to remove toxins rapidly formed in media exposed to air and light. Free radical scavengers of superoxide, hydroxyl ions and singlet oxygen were ineffective. Without scavengers the formation of 10–20 μg/ml hydrogen peroxide in the exposed medium was indicated and found lethal to injuredCl. perfringens.The BCP medium has been used successfully for the rapid identification and enumeration ofCl. perfringensin foods and faeces from food poisoning outbreaks and cases of suspected infectious diarrhoea. Greater recovery of severely injured vegetativeCl. perfringenscould be obtained by pre‐incubation at 37°C of inoculated media for 2–4 h followed by overnight incubation at 43°C‐45°C. Tryptose‐sulphite‐cyclo‐serine and Shahidi‐Ferguson‐perfringens agar base were found to inhibit the growth of several strains of injured vegetativeCl. perfringens. This was not completely overcome by the addition of pyruvate. The inclusion of mannitol also allows the medium to be used for the presumptive identification ofB. cereus. Growth and lecithinase activity are profuse on BCP. Heat‐injured spores are recovered equally well on BCP and blood agar. A scheme for the identification of some other