Culture media for enterococci and group D-streptococci

Abstract

Lancefield group D-streptococci are contaminants of various food commodities, especially those of animal origin. They encompass the new genus Enterococcus comprising 13 known species and some species of streptococci which have their habitat in the intestine of animals, e.g. Streptococcus bovis, suis and equinus. The serologically based grouping may no longer constitute the best definition for streptococci from the food chain. Food hygiene monitoring systems using enterococci as indicators need reliable methods for selective cultivation and identification of marker strains. Up to now more than 100 modifications of selective media have been described for isolating streptococci or enterococci from various specimens. The selection of a medium requires either experience or consultation. It depends on the kind of specimen, the method of cultivation (plate count or membrane filter) and whether or not the habitat is heavily contaminated with other organisms. The choice of media is made more difficult as commercial versions of the same culture medium may vary in recipe and/or performance from producer to producer. Therefore, reviewing the literature may help in the choice of medium and confirmation tests. The selectivity and productivity of some commonly used or cited media are reported here, partly based on our own experience: citrate azide tween carbonate agar (CATC), kanamycin aesculin azide agar (KAA) and M-enterococcus agar (ME) including earlier results with aesculin bile azide agar (ABA), and thallous acetate tetrazolium glucose agar (TITG). No medium was completely selective for all group D-streptococci or for all enterococci but some media were highly selective for a single Enterococcus species, e.g., for E. faecalis which serves as indicator of human pollution. Confirmatory tests must be carried out when experience in the evaluation procedure is limited. Selective media for enterococci should be used only after or while checking in parallel their selectivity and productivity against appropriate test organisms.