Comparison of the Baird–Parker agar and 3MTMPetrifilmTMrapid S. aureus count plate methods for detection and enumeration of Staphylococcus aureus Original Research Article

This study compared Baird–Parker (B–P) agar plating with the 3MTMPetrifilmTMrapid S. aureus count plate method (PFRSA) for detection and enumeration ofStaphylococcus aureus . Sampling of deli-sliced meats (n=11) and cheeses (n=39), meat sandwiches (n=7) and raw bovine milk (n=14) revealed B–P to be insufficiently selective. Even with narrow identification criteria for presumptive S. aureus, 73% of 84 isolates from B–P were not S. aureus. None of the meat, cheese or sandwich samples tested positive using the PFRSA method. All 14 raw bovine milk samples tested positive for S. aureus using the PFRSA method and confirmed S. aureus isolates were recovered from 12 samples on B–P plates. Results of two storage studies using inoculated Swiss and mozzarella cheeses showed that the two enumeration methods were essentially equivalent and that increases in S. aureus numbers of more than 2 log cfu are unlikely on Swiss and mozzarella cheeses stored at ≤25°C for ≤20 h. Despite a high-temperature incubation step that prevented isolate confirmation, the PFRSA method was found to be a suitable alternative to B–P for detecting and enumerating S. aureus. Because of the relative speed of the PFRSA method, analysts may consider using it as an initial screen with positive samples re-tested using the B–P method, with subsequent testing, e.g. coagulase, of isolates.