Short communication: Coliform Petrifilm as an alternative method for detecting total gram-negative bacteria in fluid milk

摘要

Postpasteurization contamination (PPC) of fluidmilk remains a challenge for some dairy processors.Pseudomonas is the most common contaminant of fluidmilk after pasteurization, and therefore methods todetect PPC should be inclusive of Pseudomonas andother gram-negative contaminants (e.g., coliform bacteria).Our objective was to compare the ability of 3M(St. Paul, MN) coliform and Enterobacteriaceae (EB)Petrifilm to detect total gram-negative bacteria withthat of the standard method, crystal violet tetrazoliumagar. To that end, we evaluated coliform Petrifilm, EBPetrifilm, and crystal violet tetrazolium agar to detectgram-negative bacteria in naturally contaminatedsamples of fluid milk. A total of 92 observations derivedfrom shelf-life testing of 33 milk samples from 5 differentprocessing facilities were evaluated for (1) presenceof coliforms on coliform Petrifilm at both 24 and 48 hof incubation; (2) presence of any growth, regardless ofgas production, on coliform Petrifilm at both 24 and48 h of incubation; (3) presence of EB on EB Petrifilmat both 24 and 48 h of incubation; (4) presence of anygrowth, regardless of gas or acid production, on EBPetrifilm at both 24 and 48 h of incubation; and (5)presence of gram-negative bacteria on crystal violet tetrazoliumagar after 48 h of incubation. Sensitivity andspecificity analysis of results indicated that comparedwith the standard method (i.e., crystal violet tetrazoliumagar), the method that performed the best, basedon balanced accuracy (i.e., the average of sensitivityand specificity), was coliform Petrifilm evaluated forthe presence of any growth after 48 h of incubation(sensitivity = 0.787; specificity = 0.839). This methodcan be easily adopted by the dairy industry as manyprocessing facilities already test for coliforms using coliformPetrifilm. Improving the ability of processors todetect PPC will improve the quality of the fluid milksupply.