Evaluation of a dry medium culture plate (3M Petrifilm AC) for laboratory pasteurized counts

In this study Petrifilm AC was evaluated for its ability to enumerate thermoduric bacteria in raw milk by the laboratory pasteurized count (LPC). LPC's were conducted on agar pour plates and Petrifilm plates. Both plating techniques were conducted immediately following laboratory pasteurization, and plating was also conducted after incubations of 2 and 4 h at room temperature. Uninoculated raw milk samples, raw milk inoculated with Bacilluscereus , Micrococcus luteus , and raw milk isolates ( Streptococcus , Corynebacterium , and Micrococcus species) were enumerated. A total of 270 samples was analyzed. Uninoculated raw milk samples and raw milk samples inoculated with B. cereus , Corynebacterium , Streptococcus , and some Micrococcus species showed no significant difference between agar pour plates and Petrifilm AC. Other Micrococcus species were recovered at lower numbers on the Petrifilm after laboratory pasteurization. Some of these were comparable to the agar plates after a 4-h incubation at room temperature. This study indicates that Petrifilm AC could be used in place of agar pour plates to conduct the LPC.