Comparison of Baird-Parker agar and rabbit plasma fibrinogen medium for the enumeration of Staphylococcus aureus in raw milk and raw milk products

Baird-Parker agar (BPA) is widely accepted as the most satisfactory medium for the enumeration of Staphylococcus aureus in foods. However, the medium is not without disadvantages, especially when investigating raw milk and ra w milk products. It is not completely selective and therefore detection of S. aureus is limited if the sample is heavily contaminated with competing m icro-organisms. When analyzing raw milk and fresh cheese made from raw milk also floccose colonies of Absidia corymbifera can occur on low dilution pl ates. Reduction of tellurit and egg yolk reaction do not function satisfact orily as elective markers in BPA. In Rabbit Plasma Fibrinogen medium (RPFA) the egg yolk reaction is replaced by coagulase activity as a diagnostic sy stem. 56 raw milks and raw milk cheeses were tested for S. aureus by surfac e plating using BPA and a commercially:available RPFA, respectively. On BPA a considerably higher amount of accompanying flora was observed than on RP FA. Consequently, with RPFA S. aureus was detected in more samples than wit h BPA. The differences were not related to the absence of egg yolk in RPFA, but to different qualities of the Baird-Parker base. Therefore differences in selectivity can occur when using different brands of Baird-Parker agar (egg yolk tellurite glycine pyruvate agar). In cases where S. aureus counts were obtained with both media no significant differences in counts could b e evaluated. RPFA gave more reliable results and is less labor- and time co nsuming. Therefore the pour-prate method using RPFA is recommended for the enumeration of S. aureus in samples where high counts of competing micro-or ganisms are expected, e.g. raw milk and raw milk products. Counting of colo nies with halos should be done after 24 and 48 hours, respectively. Mold gr owth can be suppressed by adding 0,125 mg pimaricin to the plate.