Purpose: This project compared various selective media with the goal of inhibiting the growth of competing microorganisms and reducing the time required to obtain a final result.
Methods: A selectivity study was conducted using 35 S. aureus and 22 non-S. aureus strains isolated from various dairy products. Linearity of BP, HiCrome™, HardyCHROM™ and BPm agars was evaluated on samples of Emmental cheese spiked at concentrations close to the regulatory limit for S. aureus in raw-milk cheeses. The performance of each agar was also assessed using a total of 116 contaminated cheeses. Each agar was incubated for 24 and 48 h in various conditions: temperature of 35 and 42°C in regular or modified atmosphere.
Results: BPm was the only agar to produce performance results comparable to BP (P = 0.46), while significantly inhibiting competing flora, when incubated at 35°C for 48 h. Although the incubation time of BPm is the same as BP, its use improved the limit of quantification of the method in presence of microorganisms showing morphological characteristics similar to S. aureus that were found to be overgrowing on BP. Use of BPm also reduces the turnaround time by avoiding the need to repeat analyses and to confirm non-S. aureus colonies unnecessarily.
Significance: The addition of BPm as a reference agar will allow regulatory organizations to accurately report the amount of S. aureus as an indicator of the potential presence of enterotoxin or a lack of adherence to good manufacturing practices.