Selective Medium for Isolating Staphylococci

Abstract

Summary A selective medium was developed for isolating coagulase-positive and coagulase-negative staphylococci. The selective agents employed were sodium azide, potassium thiocyanate, lithium chloride and glycine. Addition of lithium chloride and glycine is only necessary if high counts of streptococci are expected. All type strains of the various staphylococcal species and other staphylococci tested revealed good growth on this new medium. S. intermedius was the only exception since its recovery was significantly lower than on control medium. However, normal growth of S. intermedius is possible at a lower sodium azide concentration (15–20 mg/litre). Growth of all micrococci and other bacteria such as various bacilli, arthrobacters, lactobacilli, streptococci, Escherichia coli, Enterobacter cloacae and Proteus mirabilis is inhibited: these organisms do not grow at all or at most as pinpoint-sized colonies. Addition of egg yolk or pork plasma to the medium may provide a basis for distinguishing S. aureus from coagulase-negative staphylococci. The new medium is not only useful for the selective isolation of staphylococci but also represents a new routine method fur the separation of staphylococci from micrococci. The great specificity of this new medium could be demonstrated in detecting staphylococci in various foods at levels as low as 100/g.