A modified pork plasma agar for the enumeration of Staphylococcus aureus in foods

Abstract

The coagulase reaction of Staphylococcus 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 h of incubation at 35 degrees C; the others required 44 h. Without soy trypsin inhibitor, a number of strains showed considerable fibrinolysis between 24 and 44 h 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.

A note on the isolation of Staphylococcus aureus from raw minced meat

“…Although BP medium itself is not expensive, the confirmatory tests needed are time-consuming or costly. Baird-Parker Pig Plasma Agar (BPP) proposed by Devoyod et al (1976) and Stadhouders et al (1977) and its modifications containing bovine fibrinogen and pig plasma (Hauschild et al 1979) or rabbit plasma (Beckers et al 1984) appear to be the most appropriate media for enumeration of Staph. aureus from foods.…”

Variations in growth of Staphylococcus aureus 234 after heat stress in milk

“…Since the colonies showing variations in their characteristics examined in this investigation were picked from a non-selective medium and if the same can be extrapolated to colonies from selective media Iike BPA, then it would have serious consequences, for instance a colony of very small size or non-coagulating type mightgive misleading results du ring their interpretation. This could be even more serious with BPA supplemented with plasma for coagulase activity (Hauschild et al, 1979) or in direct coagulase-TNase tandem test (Lachica, 1980) commonly used during the isolation and identification of enterotoxigenic S aureus strains from different foods. Hence, great caution should be exercised while interpreting the results based on these activities.…”