Assessment of the BP+ Agar for the Enumeration of S. aureus in Cheeses with Edible Rind

Introduction: When analyzing cheese samples for the enumeration of Staphylococcus aureus, the edible rind is not typically included in the analysis as it contains significant microflora capable of forming a lawn of growth on Baird Parker agar (BP). This practice makes it impossible to obtain an accurate quantitative assessment of S. aureus if present in the sample. One solution to obtain isolated colonies is to further dilute the analytical sample; however, by diluting the competitor flora, the concentration of S. aureus is also diluted and the probability of detecting this microorganism decreases.

Purpose: BP+ agar has already demonstrated its equivalence to BP agar for the enumeration of S. aureus in raw milk cheese without the rind while significantly inhibiting background microflora. In this project, BP+’s effectiveness for the enumeration of S. aureus in raw milk cheese with the rind was assessed.

Methods: The laboratory analyzed more than 100 cheeses with edible rind using Health Canada’s MFHPB-21 method - Enumeration of Staphylococcus aureus in food. BP and BP+ agars were plated in parallel for the analysis of cheese sample, which were each analyzed with and without the rind. The evaluation criteria for validation were 1) equivalence of S. aureus counts, when the microorganism is present, 2) inhibition of the cheese microflora, 3) number of downstream analytical confirmations that could be avoided by using the BP+. 

Results:  In 10% of the analyzed samples, S. aureus was detected only in those containing the cheese’s rind. This observation supports the need to include the rind in the analytical portion to adequately evaluate the risk to consumers.

Significance: Given the observed performance of BP+, its use is an added value for the analysis of a representative portion of edible rind cheeses. Food microbiology laboratories will benefit from a method capable of analyzing the higher risk portions of these types of cheese, thus increasing the probability of detecting S. aureus.