Culture Media Used in the Detection and Enumeration of Coagulase-positive Staphylococci

Abstract

The availability of highly productive and selective media for the detection and enumeration of coagulase-positive staphylococci (CPS) is of great importance in routine food surveillance programmes. Staphylococcus aureus has a prominent place among the CPS. Thus, selective media for detecting CPS primarily detect S. aureus. In this chapter, selective and diagnostic media for S. aureus and other CPS are reviewed. Baird-Parker agar is widely accepted as the most satisfactory medium for the enumeration of CPS in foods. Productivity is high, especially in the case of stressed cells. However, tellurite reduction and egg yolk reaction are poor diagnostic systems when testing many foodstuffs and the medium is not very selective. This may give unreliable results in samples with high levels of competitive flora. Attempts have therefore been made to replace the egg yolk by animal plasma in order to differentiate CPS directly on the plate from other competing microorganisms. Commercially available rabbit plasma fibrinogen agar is a suitable medium for enumerating CPS in foods without the need for cumbersome and time-consuming confirmatory tests. Baird-Parker agar and rabbit plasma fibrinogen medium are recommended by the International Organization for Standardization (ISO). Baird-Parker agar is used also as the official method by the Food and Drug Administration (FDA) in the United States. Selective liquid media that use MPN techniques are available for detecting low numbers of CPS (<100 cfu g−1). Studies have shown that in most samples recoveries of CPS are similar after enrichment in tryptone soya broth with 10% NaCI and 1% sodium pyruvate (PTSBS), Giolitti–Cantoni broth with Tween 80 (GCBT) and liquid Baird-Parker (LBP). However, significant differences in selectivity have been observed. Highest numbers of competing flora were observed in PTSBS, leading to problems in identifying CPS when streaked onto Baird-Parker agar. Selectivity was superior in LBP than in GCBT. While PTSBS is recommended by the FDA, the ISO favours the use of GCBT.