Agar Medium for the Selective Enumeration of Coagulase-positive Staphylococcus from the Rat Alimentary Tract

Abstract

A selective agar medium (medium J 1 ) is proposed for the quantitative enumeration of egg yolk-positive (EYP) and egg yolk-negative (EYN) Staphylococcus pyogenes from the digestive tract and feces of the rat. This medium, buffered at p H 5.0, is composed of acid casein hydrolysate and yeast extract with 7.5% sodium chloride, 1.6% sodium pyruvate, 0.0008% 2,3,5-triphenyltetrazolium chloride (TTC), and 6% egg yolk emulsion. Inoculation is by the pour-plate method and incubation is at 38 C in a water-jacketed incubator for 36 hr. Colonies of S. pyogenes reduce TTC; EYP strains are surrounded by a halo of opacity; and EYN strains may be surrounded by a red halo, but no opacity. Small, white colonies of S. epidermidis may develop, but Micrococcus , and all other groups of Staphylococcus recognized in the rat intestinal flora, are inhibited. Other bacterial genera, notably Bacillus, Corynebacterium, Proteus , and Streptococcus , are also inhibited.