LIMITATIONS OF FIBRINOGEN-POLYMYXIN MEDIUM IN DETECTING COAGULASE-POSITIVE STAPHYLOCOCCI IN RAW MILK

Agar medium for the selective enumeration of coagulase-positive Staphylococcus from the rat alimentary tract

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 pH 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.

Application of replica plating and computer analysis for rapid identification of bacteria in some foods. I. Identification scheme

A method was devised and tested for a quantitative identification of microbial flora in foods. The colonies developing on the initial isolation plates were picked with sterile toothpicks and inoculated on a master plate in prearranged spacing and order. The growth on the master plates was then replicated on a series of solid-agar plates containing differential or selective agents. The characteristic growth and physiological responses of microbial isolates to penicillin, tylosin, vancomycin, streptomycin, chloramphenicol, neomycin, colistin, and to S S Agar, Staphylococcus Medium No. 110, and Potato Dextrose Agar were recorded, together with Gram reaction and cell morphology. This information was then fed into an IBM 1410 digital computer which grouped and analyzed each isolate into 10 microbial genera, or groups, according to the identification key. The identification scheme was established by use of reference culture studies and from the literature. This system was used to analyze the microbial flora in dover sole (Microstomus pacificus) and ground beef. The method described in this article enables one to examine large numbers of microbial isolates with simplicity.