MODIFIED PORK PLASMA AGAR FOR THE ENUMERATION OF STAPHYLOCOCCUS-AUREUS IN FOODS

The coagulase reaction of S.aureus on the PPSA (pork plasma for S.aureus) agar of Devoyod et al. was found to be fibrinogen-deficient. By including bovine fibrinogen (BFG) in the medium, the fibrin halos around S.aureus colonies became more distinct, preparations of pork plasma previously unacceptable for inclusion in the original PPSA agar were performing well, and the amount of pork plasma required in PPSA agar could be reduced by nearly 90%. In the modified medium, designated PPF (pork plasma fibrinogen) agar, the agar base (Baird-Parker agar without egg yolk) was unchanged. After surface plating, the base was covered with 8 mL of a modified overpour agar: 2.5% pork plasma 0.38% BFG, and 0.0015% soy trypsin inhibitor in 0.7% Bacto agar. Most S.aureus strains could be enumerated after 24 hr of incubation at 35°C: the others required 44 hr. Without soy trypsin inhibitor, a number of strains showed considerable fibrinolysis between 24 and 44 hr of growth; this activity was neutralized by the inhibitor. The S. aureus counts of 27 food samples on PPF agar were essentially the same as the confirmed S.aureus counts obtained by the Baird-Parker-method.