Evaluation of the Petrifilm SM and VRB Dry Media Culture Plates for Determining Microbial Quality of Poultry

Abstract

A self-contained sample-ready system, Petrifilm, which has been developed as an alternative method to the standard aerobic plate count (SPC) and coliform counts as determined by violet red bile (VRB) pour plates, was evaluated for the first time with poultry samples. Swab samples were taken of 109 broiler carcasses at various degrees of freshness, and SPC and VRB pour plates were compared to Petrifilm counts. The correlation coefficient of log ]0 SPC and logi 0 Petrifilm SM count was 0.926 with a regression line slope of 1.009 and an intercept of-0.106. A correlation was not determined with coliform counts because when the total number of colonies on a plate is greater than 300 and the coliforms make up less than 10% of the total population of microorganisms capable of growing on the VRB nutrients it is difficult to count the gas producing colonies. The frequency of countable isolates producing gas from brilliant green lactose bile broth was 44% from standard VRB and 86% from Petrifilm gas producers. This study suggests that for poultry products, the Petrifilm SM provides an adequate alternative to the SPC, and that the Petrifilm VRB gives a higher predictive value for the number of coliforms present , but would not be suitable under all conditions. Petrifilm plates (3M, St. Paul, Minnesota) are the consequence of a new technology where a nutrient medium has been coated on a film. The result is a convenient, sample-ready system for enumerating bacteria. Ginn et al. (3) compared the Petrifilm SM plates for determining the total aerobic plate count of raw milk samples with the standard plate count (SPC) method and found the Pet-rifilm SM method to be a suitable alternative to the SPC. Petrifilm VRB plates gave equivalent coliform counts to the APHA approved procedure for determining coliform counts from violet red bile agar (VRB) (5). Petrifilm plates have received AOAC (4) and APHA approval for raw and pasteurized milk. In previously reported studies with non-milk samples, Smith et al. (6) found the Pet-rifilm SM method to be a feasible alternative to the traditional aerobic plate count method for enumerating mesophilic aerobic bacteria from fresh ground beef, and Fung et al. (2) determined total counts from seafood and found a correlation coefficient of 0.99 between SPC and Petrifilm SM. The objective of our study was to compare the effectiveness of the Petrifilm SM and VRB dry media culture plates with traditional standard methods for enumerating total aerobic plate counts and coliform counts from poultry carcasses at various degrees of freshness. MATERIALS AND METHODS A total of 109 broiler carcasses was obtained from local consumer outlets and held at refrigeration temperatures for 0 to 8 d. Samples from each carcass were obtained by swabbing a 12.3-cm 2 area of breast skin with a sterile cotton swab. The swab was then broken into 9.9 ml of a sterile saline (.85%) solution blank from which appropriate serial dilutions were made. The same diluted samples were used for a paired comparison of Petrifilm and standard plate counts. Petrifilm counts were made using Petrifilm SM plates for total counts and Pet-rifilm VRB plates for coliform counts. For comparison, aerobic plate counts were made by the pour plate method with standard methods agar (DIFCO) and coliform counts were made by the overlayed pour plate method (/) with violet red bile agar (DIFCO). Isolates from Petrifilm VRB and conventional col-iform plates were randomly selected and inoculated into brilliant green lactose bile broth (BGLB) to determine the ability of the isolates to produce gas from lactose. All total count plates were incubated at 21°C for 72 h, and all coliform plates were incu-bated at 37°C for 24 h. The Petrifilm SM plate contains a cold water-soluble gelling agent (guar) and a tetrazolium indicator dye in the top film and guar with standard methods agar in the bottom film. The Pet-rifilm VRB plate is constructed in much the same way with violet red bile nutrients substituted for the standard methods nutrients. The Petrifilm plates are inoculated by lifting the top film up and pipetting 1 ml of sample onto the center of the bottom film. The top film is then returned to its original position with a careful, rolling motion. A plastic spreader is used to evenly distribute the sample over a 20-cm 2 circular growth area. After allowing 30 s for the guar to set up the plates can be freely moved to the incubator without fear of spilling the inoculum. The data were examined by a regression analysis of