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.

Changes in microbial populations, kinds of lactic acid bacteria and biochemical characteristics of Greek traditional feta cheese during ripening

“…1951) incubated at 30°C for 5 days anaerobically (GasPak); cycloheximide (Sigma Chemical, St Louis, MO, USA) was added (100 mg/L) to prevent the growth of yeasts in MRS agar and M17 agar (Kandler and Weiss 1986). Enterococci were grown on citrate azide agar (CAA; Saraswat et al . 1963) after 3 days at 37°C and Gram‐negative organisms on nutrient agar crystal violet (NACV) at 21°C for 24 h. All media were obtained from Merck, except AcA and CAA, which were made from the individual ingredients.…”

Changes in the microbiological and chemical characteristics of an artisanal, low‐fat cheese made from raw ovine milk during ripening

“…Decimal dilutions were prepared in sterile peptone water (1.0 g/L) and plated in duplicate. Lactic acid bacteria (LAB) were grown anaerobically (Gas‐Pack system; BBL, Cockeysville, MD, USA) on MRS agar (pH 6.2) 4 at 30°C for 5 days; Gram + , catalase‐cocci on M17 agar at 30°C for 48 h, and enterococci on citrate azide agar (CAA) 5 at 37°C for 3 days. Cycloheximide (Sigma chemicals, St Louis, MO, USA) was added (100 mg/L) to MRS and M17 agar to prevent the growth of yeasts 6 .…”

Enterococci in Milk and Milk Products

“…Saraswat et al 48 compared ten selective media and found citrate a i d e medium'59 to be the most effective for isolation of fecal streptococci from dairy products. The efficacy of citrate azide agar to meet the rigid requirements for enumeration of enterococci in milk products has also been reported by other researchers.…”