SELECTION OF A MEDIUM FOR THE ISOLATION AND ENUMERATION OF ENTEROCOCCI IN DAIRY PRODUCTS1

Abstract

Ten media, commonly used for the detection and isolation of streptococci of sanitary significance in water, dairy and other food products, were compared to establish a plating medium for the enumeration of enterococci in dairy products. To make all media suitable for comparison by the agar plate method, agar was added to those media which initially were recommended for use as broths. Criteria used in selecting the medium were high recovery, selectivity but not undue inhibition of enterococci and ease in obtaining and interpreting results. The recovery data of three different platings of enterococcus cultures were statistically analyzed. In this manner, one medium was eliminated on the basis of low recovery. Six of the remaining nine media were eliminated because they permitted the growth of non-enterococcus cultures. Two of the three media then remaining were eliminated because they allowed one S. bovis culture to grow. In addition, these media showed considerable variation in size and color of enterococcus colonies. The medium selected, the Citrate azide medium of Reinbold, Swern and Hussong (13), was modified by increasing the azide concentration. This did not result in undue inhibitory effects. It was further tested by obtaining recovery data for 158 known enterococcus cultures. High selectivity was demonstrated by showing that 408 colonial isolates from plates of raw milk, cheese and butter could be identified as enterococci.