Comparison of Several Selective Media for Isolation and Differentiation of Coagulase-Positive Strains of Staphylococcus aureus

Six coagulase-positive strains ofStaphylococcus aureus which had been cultivated in Brain Heart Infusion broth, milk, and brine were plated on seven isolation media. A significant difference in the growth patterns of the individual strains was found as well as a significant effect resulting from the previous cultivation history before plating. Brine and, to a lesser extent, milk were found to reduce maximal cell concentrations attained, but strains grown in brine and milk showed greater ability to withstand the selective action of the isolation media. Fibrinogen applied to the surface of five of the media allowed the formation of characteristic halos by coagulase-positive strains ofS. aureus . Only half of the strains studied produced a zone of precipitation on SM110-Egg Yolk agar. The isolation medium containing cycloheximide and a high level of polymxin B was most inhibitory to the organisms.