RESUMEN
Despite the widespread use of coliforms as indicator bacteria, increasing evidence suggests that the Enterobacteriaceae (EB) and total gram-negative groups more accurately reflect the hygienic status of high-temperature, short-time pasteurized milk and processing environments. If introduced into milk as postpasteurization contamination, these bacteria may grow to high levels and produce a wide range of sensory-related defects. However, limited information is available on the use and survival of bacterial hygiene indicators in dairy products outside of pasteurized fluid milk and cheese. The goal of this study was to (1) provide information on the survival of a diverse set of bacterial hygiene indicators in the low pH environment of Greek yogurt, (2) compare traditional and alternative detection methods for their ability to detect bacterial hygiene indicators in Greek yogurt, and (3) offer insight into optimal hygiene indicator groups for use in low-pH fermented dairy products. To this end, we screened 64 bacterial isolates, representing 24 dairy-relevant genera, for survival and detection in Greek yogurt using 5 testing methods. Before testing, isolates were inoculated into plain, 0% fat Greek yogurt (pH 4.35 to 4.65), followed by a 12-h hold period at 4 ± 1°C. Yogurts were subsequently tested using Coliform Petrifilm (3M, St. Paul, MN) to detect coliforms; Enterobacteriaceae Petrifilm (3M), violet red bile glucose agar and the D-Count (bioMérieux, Marcy-l'Étoile, France) to detect EB; and crystal violet tetrazolium agar (CVTA) to detect total gram-negative bacteria. Overall, the non-EB gram-negative isolates showed significantly larger log reductions 12 h after inoculation into Greek yogurt (based on bacterial numbers recovered on CVTA) compared with the coliform and noncoliform EB isolates tested. The methods evaluated varied in their ability to detect different microbial hygiene indicators in Greek yogurt. Crystal violet tetrazolium agar detected the highest portion of coliforms, whereas EB Petrifilm detected the highest portion of EB, as well as highest portion of total gram-negative bacteria. Additionally, the D-Count method allowed for faster detection of EB in yogurt by generating results in approximately 13 h rather than the 24 h required when using EB Petrifilm and violet red bile glucose agar. Results from this study indicate that the coliform and EB groups encompass a broad range of dairy-relevant gram-negative bacteria with the ability to survive in Greek yogurt, supporting their use as microbial hygiene indicator groups in low-pH fermented dairy products.
Asunto(s)
Enterobacteriaceae/aislamiento & purificación , Microbiología de Alimentos , Yogur/microbiología , Animales , Recuento de Colonia Microbiana , Grecia , Leche/microbiologíaRESUMEN
It is estimated that 19% of the total food loss from retail, food service, and households comes from dairy products. A portion of this loss may be attributed to premature spoilage of products due to lapses in sanitation and postpasteurization contamination at the processing level. Bacterial groups including coliforms, Enterobacteriaceae (EB), and total gram-negative organisms represent indicators of poor sanitation or postpasteurization contamination in dairy products worldwide. Although Petrifilms (3M, St. Paul, MN) and traditional selective media are commonly used for the testing of these indicator organism groups throughout the US dairy industry, new rapid methods are also being developed. This project was designed to evaluate the ability of different methods to detect coliforms, EB, and other gram-negative organisms isolated from various dairy products and dairy processing environments. Using the Food Microbe Tracker database, a collection of 211 coliform, EB, and gram-negative bacterial isolates representing 25 genera associated with dairy products was assembled for this study. We tested the selected isolates in pure culture (at levels of approximately 15 to 300 cells/test) to evaluate the ability of 3M Coliform Petrifilm to detect coliforms, 3M Enterobacteriaceae Petrifilm, violet red bile glucose agar, and an alternative flow cytometry-based method (bioMérieux D-Count, Marcy-l'Étoile, France) to detect EB, and crystal violet tetrazolium agar to detect total gram-negative organisms. Of the 211 gram-negative isolates tested, 82% (174/211) had characteristic growth on crystal violet tetrazolium agar. Within this set of 211 gram-negative organisms, 175 isolates representing 19 EB genera were screened for detection using EB selective/differential testing methods. We observed positive results for 96% (168/175), 90% (158/175), and 86% (151/175) of EB isolates when tested on EB Petrifilm, violet red bile glucose agar, and D-Count, respectively; optimization of the cut-off thresholds for the D-Count may further improve its sensitivity and specificity, but will require additional data and may vary in food matrices. Additionally, 74% (129/175) of the EB isolates tested positive as coliforms. The data obtained from this study identify differences in detection between 5 microbial hygiene indicator tests and highlight the benefits of EB and total gram-negative testing methods.