Principles involved in the detection and enumeration of clostridia in foods

The clostridia are a group of anaerobic bacteria that vary considerably in their biochemical and physiological properties. Not surprisingly, attempts to develop a single isolation medium for all species that occur in foods have not been entirely successful, and the problem is compounded by the need to recover both vegetative cells and spores, some of the latter being unable to germinate without heat activation. Most available isolation media, except some of those used in the dairy industry, include sulphite and an appropriate iron salt, so that blackening due to sulphite reduction can serve as a differential test for clostridia. The limitations of this test in solid agar media are discussed and some advantages described in relation to its use in liquid media for Most Probable Number determinations. A medium favoured for the purpose is the Differential Reinforced Clostridial Medium of Gibbs and Freame (1965). An unresolved issue is whether or not special precautions are needed to exclude oxygen during food sample preparation and dilution, preparation of media, and in conditions used for anaerobic incubation. Although such stringency may be required for maximum recovery of sub-lethally damaged cells or spores, practical constraints in food control laboratories necessitate use of relatively simple procedures for detecting clostridia routinely.