The determination of sulfite-reducing clostridia in foods - critical comments on the new ISO 15213 method

Besides the detection of the pathogenic resp. toxinogenic clostridia specie s (Clostridium perfringens, Clostridium botulinum) the determination of the group of sulfite-reducing clostridia which are able to cause strong spoila ge of foods or meals could be important. For practical reasons a method for detecting sulphite-reducing clostridia instead of sulphite-reducing bacter ia is necessary, because the detection of sulphite-reducing bacteria is not suitable to evaluate the hygienic quality of most of the foods examined. O n the other hand, in many product groups (e.g. fresh meat, meat products, f ishery products, bakery products, creame- and pudding-powder) the detection of sulfite-reducing clostridia gives valuable information for the detectio n of the microbial-hygienic quality of foods. The new ISO-method 15213 "Hor izontal method for the enumeration of sulphite-reducing bacteria growing un der anaerobic conditions" is in our opinion not suitable to detect sulphite -reducing clostridia as a group, because the sodium-sulphite concentration of the proposed iron-sulphite-medium is much to high. It is necessary to re duce the sulphite content to 0.05 % for cultivating Clostridium, sporogenes , most of the Clostridium bifermentans strains and some other sulphite-redu cing clostridia, which are - besides Clostridium perfringens predominant in most foods. Furthermore, it has to be pointed out that it is not suitable to use non-selective media (e.g. iron-sulphite-agar) for the cultivation an d enumeration of sulphite-reducing clostridia from most of the foods especi ally of animal origin. It rather depends on the original microbial load of the substrate to be examined, whether it is useful to apply media with or w ithout selective components for cultivation. Therefore the new horizontal I SO-method can be applied only for a limited number of product groups (e.g. cream- and pudding-powder, flavours), if sulphite-reducing clostridia have to be detected. In this paper the reasons for the declining of the new ISO- method are documented with facts from literature including own experimental results. Moreover it will be presented basic information on cultivation of clostridia in foods.