A Simple Medium for Isolation of Coagulase-Positive Staphylococci in a Single Step

An agar medium containing NaCl, egg yolk and tellurite for selective quantitative isolation of coagulase-positive staphylococci from food was developed. Isolation and identification of the staphylococci was achieved in a single step. A properly diluted food sample was spread over the medium and incubated for 24 h at 42 C. Coagulase-positive staphylococci appeared as small grey to dark-grey colonies surrounded by a dense white opacity. Coagulase-negative bacteria which, at times, grow on this medium, did not produce this reaction. The identification on this selective medium of isolates from 683 different food samples as coagulase-positive staphylococci was subsequently confirmed by the coagulase test. Comparative titrations of 29 various coagulase-positive staphylococcus strains on both the selective medium and nutrient agar yielded nearly identical titers. The growth of heat-stressed staphylococci was inhibited by the selective medium. Complete reversal of the inhibition was achieved by a 3-h pre-incubation in brain heart infusion at 37 C.