Enumeration of sulphite‐reducing clostridia occurring in foods

For the presumptive enumeration of sulphite‐reducing clostridia in foods, a modification of the Wilson & Blair type agar, containing no dextrose, 0.05% w/v Na 2 S0 3 , 7 H 2 O (SO 2 content about 25%) and rendered more selective by incorporating 10 p.p.m. of polymyxin B sulphate, has been found effective. It is recommended that the medium is incubated in oval cross‐section (‘Miller‐Prickett’) tubes at 32° for up to 3 days. The medium permits the growth of faecal streptococci and staphylococci which form a background of white colonies which do not interfere unduly. However, many Proteus species produce black colonies in this medium and in many instances confirmation of a representative selection of the black colonies obtained is necessary. Where the cultures produce spores, purification for further study can be attained by heating the colonies for 1 min. at 80° and subculturing in poured plates of sulphite‐polymyxin‐agar. If the strains are non‐sporulating conventional purification methods should be applied to the unheated cultures, using this medium. Black colonies reisolated in one of these ways can be confirmed by examining their morphology and growth under aerobiosis and anaerobiosis, including catalase reaction, and, if required, further identified by studying fermentation of glucose, lactose, saccharose, milk and mannitol, the reduction of nitrate, the liquefaction of gelatin, and the formation of indole.