Culture Media for Enterococci

Abstract

Enterococci, formerly classified as faecal streptococci, belong with few exceptions to the Lancefield group D streptococci. They are contaminants of various foods, especially those of animal origin. The genus Enterococcus comprises to date 40 known species. Most of them have their habitat in the intestine of man and animals. Serologically based grouping does not constitute a reliable definition for these bacteria. The same applies to the term “faecal streptococci” and it seems reasonable therefore to look for enterococci sensu stricto. Food hygiene monitoring systems use enterococci as indicators for faecal contamination and such use requires reliable methods for selective cultivation and identification of marker strains. More than 100 modifications of selective media have been described in the past for isolating faecal streptococci or enterococci from various specimens. The selection of a medium requires consideration of the specimen type (solid or liquid), the method of cultivation (pre-enrichment/enrichment, plate count or membrane filter technique) and whether the specimen 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 formulation and/or performance from producer to producer. Therefore, reviewing the literature may help in the choice of medium and of confirmatory tests. Glycopeptide-resistant strains of enterococci are a cause of nosocomial infections. Enterococci in the food chain may function as transmitters from animals to man. Hence, the detection and identification of vancomycin-resistant enterococci (VRE) is needed in food microbiology and special media and procedures are required. The selectivity and productivity of some commonly used or cited media are reported here: citrate azide Tween carbonate (CATC) agar, kanamycin aesculin azide (KAA) agar, M-Enterococcus (ME) agar, aesculin bile azide (ABA) agar, and thallous acetate tetrazolium glucose (T1TG) agar. In clinical literature, ABA is mostly called Enterococcosel (ECS) and ME known as Slanetz and Bartley agar. No medium is completely selective and productive for all strains of enterococci, but some media are more selective for some Enterococcus species, e.g. E. faecalis and E. faecium, which serve as indicators of faecal contamination or which are primarily responsible for nosocomial infections. Confirmatory tests must be performed where the selectivity of the medium is not sufficient to exclude the growth of other bacteria like pediococci or Leuconostoc. Selective media for enterococci should be used only after or while simultaneously checking their selectivity (false positive) and productivity (false negative) against appropriate test organisms which are representative of the kind of specimen.