Improved Isolation and Differentiation of Enterococci in Cheese

Abstract

Further documentation of an enterococcus selective differential (ESD) medium was obtained in isolations from eight different cheeses. An improved differentiation of tetrazolium salt (2, 3, 5-triphenyl tetrazolium hydrochloride [TTC])-reducing strains of Streptococcus faecalis from TTC-nonreducing or TTC-faintly-reducing Streptococcus faecium was attained. The sensitivity of the medium was evaluated in comparison with that of KF streptococcal, Pfizer selective enterococcus (PSE), the medium of Reinbold, Swern, and Hussong (RSH), and the medium of Saraswat, Clark, and Reinbold (SCR). Selective counts, rate of colony formation, and ease of isolation and differentiation of colonies were examined. The specificity of the medium was also investigated. ESD supported the fastest rate of growth and the maximum size of colonies; counts in this medium were in most cases possible with 17 h of incubation, whereas the other media required 24 to 48 h. A presumptive identification of 1,077 isolates by four biochemical tests disclosed that SCR, RSH, and ESD selected high, comparable percentages of strains that approximated most closely the typical description of enterococci (66, 60.1, and 58%, respectively). Low percentages (21.1 and 30.7%) were yielded by KF and PSE. The utility of ESD for a rapid, presumptive identification of enterococci was confirmed by serological and biochemical testing of color TTC-differentiated colonies isolated from 18 cheeses.