Comparison of selective culture media to enumerate coagulase-positive staphylococci in cheeses made from raw milk

Baird-Parker (BP) medium is the selective medium most commonly used to enumerate coagulase-positive staphylococci in food. However, it seems often not sufficiently selective for the analysis of raw foodstuffs which present a high level of concomitant flora. An alternative medium, rabbit plasma fibrinogen agar (RPFA) medium, is now available but has not yet been tested extensively with cheeses made from raw milk. Therefore, a comparison of both media was carried out with 77 samples from four types of raw-milk cheeses. Two brands of RPFA medium were compared with one brand of BP medium. This last medium was tested after two different shelf lives at +4 degrees C, 1 day and 4 months, to evaluate the possible influence of the storage delay on its stability. Enumeration results from 20 (26%) of the 77 samples could not be used for statistical analysis because of reading problems observed on one or several of the media. These problems, due to a high level of concomitant flora, were more frequently observed using BP medium (18 samples) than using RPFA media (two to three samples). Enumeration means obtained for the 57 (74%) samples retained for statistical purposes were not significantly different between the two types of medium. This study shows that BP medium can be used after a storage delay at at +4 degrees C of 4 months as ready-to-use commercialized medium. However, as RPFA media are much easier and faster to read than BP medium, and as they give similar or even more precise enumeration results than BP medium, the use of RPFA media is recommended to enumerate coagulase-positive staphylococci in cheeses made from raw milk. (C) 1998 Academic Press.