Selective media for group D streptococci

Abstract

Group D streptococci are contaminants of different food commodities, especially those of animal origin. They encompass the physiological subgroups of enterococci, non-enterococci and enterococcus-like organisms. The first and the last subgroups may be included in the proposed new genus Enterococcus in the future. The serologically base grouping may be no longer constitute the best definition of streptococci which are important in food hygiene. Up to the present day more than 100 modifications of selective media have been described for group D streptococci or enterococci. Therefore it is impossible to recommend a universal medium. The choice of a medium depends on whether group D streptococci in total or enterococci are to be detected and whether or not the habitat is highly contaminated. The selectivity and productivity of the following media were compared using a set of test strains: citrate-azide-Tween-carbonate-agar (CATC), kanamycin-aesculin-azide-agar (KAA) and M-enterococcus-agar (ME). Earlier results [Reuter (1978) Arch. Lebensmittelhyg. 29, 84–91] with these and some other media, e.g. aesculin-bile-azide-agar, thallous-acetate-agar, crystal violet-azide-agar, streptococci-selective-agar were considered as well. No medium was completely selective for group D streptococci or for enterococci. Either growth of non-wanted strains occurred or growth rate of single wanted strains was reduced. The parallel use of two media, one highly, the other moderately selective is a reasonable way to obtain best results from a food habitat. Choice of media must be a result of personal experience on particular habitats to be investigated. Confirmatory tests have to be performed in many cases. Commercially prepared media may be used after checking their selectivity and productivity against appropriate test organisms.