Evaluation of various selective media for the detection of Pseudomonas species in pasteurized milk

Pseudomonas spp. are common gram-negative, post-pasteurization contaminants that contribute to spoilage of pasteurized dairy products. This study evaluated 5 common selective media for detecting Pseudomonas spp. in pasteurized milk. The performance of each selective medium for recovering 12 different Pseudomonas isolates (selected to represent a diversity of pasteurized milk isolates) was compared with that of standard plate count agar pour plates. Pseudomonas isolates showed varying abilities to produce colonies on different selective media. For 2 of 12 isolates, a 48-h incubation time was required for colony formation on any of the media tested. Violet red bile agar and coliform Petrifilm (3M, St. Paul, MN) were less effective than standard plate count agar pour plates at recovering Pseudomonas, regardless of incubation time, and MacConkey agar showed poor detection efficiency compared with SPCP after a 48-h incubation (R-2 = 0.26). Therefore, the use of violet red bile agar, MacConkey agar, or coliform Petrifilm may not be sufficient for detecting common Pseudomonas spp. in milk. The methods showing the highest detection efficiencies were crystal violet tetrazolium agar (CVTA) pour plates (R-2 = 0.95) and CVTA plates inoculated by spiral plating (R-2 = 0.89) incubated at 32 degrees C for 48 h. Overall, plating milk samples on CVTA followed by a 48-h incubation at 32 degrees C was the most effective selective method for recovering a diversity of Pseudomonas spp. from milk.