Microbiology Methods for ERP Review 3-2020

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Collaborative Study The purpose of this collaborative study was to compare the reproducibility of the Solus One Salmonella assay to the United States Food and Drug Administration (FDA) Bacteriological Analytical Manual (BAM) Chapter 5 Salmonella [5] reference method for instant NFDM (375 g). In this collaborative study instant NFDM was evaluated. The matrix was obtained from a local retailer and screened for the presence of Salmonella by the FDA/BAM Chapter 5 reference method and the Solus One Salmonella test kit . The instant NFDM was artificially contaminated with a lyophilized culture of Salmonella Senftenberg American Type Culture Collection (ATCC) 43845 (antigenic properties: 1,3,19:g,s,t and origin: unknown). The matrix was inoculated at two levels of contamination: a high inoculation level of approximately 2-5 Colony Forming Units (CFU)/test portion and a low inoculation level of approximately 0.2-2 CFU/test portion. A set of uninoculated control test portions (0 CFU/test portion) was also included. The inoculated test portions were held for two weeks at room temperature prior to initiating testing. Twelve replicate samples from each of the three inoculation levels were analyzed by each participant. Due to different test portion sizes, enrichment media and enrichment conditions, unpaired samples were used. A total of 72 samples, 36 for the Solus One method (375 g test portion) and 36 for the reference method (25 g test portion) were sent to each participating technician. Collaborators were also sent an uninoculated test portion for determining the total aerobic plate count (APC) using the 3M ™ Petrifilm ™ Rapid Aerobic Count Plate (AOAC Official Method 2015.13) [6] on the day samples were received. A detailed collaborative study packet outlining all necessary information related to the study including media preparation, test portion preparation and documentation of results was sent to each collaborating laboratory prior to the initiation of the study. Study Design The Salmonella strain used in this evaluation was propagated onto Tryptic Soy Agar with 5% Sheep Blood (SBA) from a Q Laboratories (Cincinnati, OH) frozen stock culture stored at -70°C. The organism was incubated for 24 ± 2 hours at 35 ±1°C. Isolated colonies were picked to 10 mL of Brain Heart Infusion (BHI) broth and incubated for 18 - 24 hours at 35 ±1°C. A lyophilized culture was used to artificially contaminate the instant NFDM. The culture was prepared by diluting the overnight BHI broth culture in a sterile cryoprotectant, reconstituted nonfat dry milk (NFDM), and placing it onto a freeze dry system for 48-72 hours. Afterward, a bulk lot of the instant NFDM was inoculated with the lyophilized culture and mixed thoroughly to ensure even distribution. For the preparation of the Solus One test portions, 25 g of inoculated test product was mixed with 350 g of uninoculated test product to prepare 375 g test portions which were packaged in sterile Whirl-Pak ® bags. For the reference method test portions, a 25 g sample from the bulk lots were directly sampled and packaged in sterile Whirl-Pak ® bags. After inoculation, the test matrix was held for 2 weeks at room temperature (24 ± 2 °C) prior to analysis. Preparation of Inoculum and Test Portions

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