Low-temperature survival of Salmonella spp. in a model food system with natural microflora.

The United States Department of Agriculture requires chilled poultry carcass temperature to be below 4°C (40°F) to inhibit the growth of Salmonella and improve shelf life. Post-process temperature abuse of chicken leads to proliferation of existing bacteria, including Salmonella, which can lead to the increased risk of human infections. While models predicting Salmonella growth at abusive temperatures are developed using sterile media or chicken slurry, there are limited studies of Salmonella growth in the presence of background microflora at 4-10°C. Experiments in this study were conducted to determine the growth of Salmonella Typhimurium and Heidelberg at 4-10°C in brain heart infusion broth (BHI) and non-sterile chicken slurry (CS). Nalidixic acid-resistant Salmonella Typhimurium and S. Heidelberg (3 log CFU/mL) were inoculated separately in CS and sterile BHI in a 12-well microtiter plate and incubated at 4°C, 7°C, and 10°C, following which samples were taken every 24 h for up to 6 days. Samples from each well (n=5) were spread plated on XLT4 agar+nalidixic acid and incubated at 37°C for 24 h. Bacterial populations were reported as CFU/mL. No significant differences (p>0.05) were observed in the survival of both strains in CS and BHI over the period of 6 days at all temperatures except S. Heidelberg at 7°C. Survival populations of both strains at 4°C were significantly different (p ≤ 0.05) than at 7°C and 10°C in both media types. S. Heidelberg showed a maximum growth of 2 logs in BHI at 10°C among all the treatments. Growth patterns and survival of Salmonella at near refrigeration temperatures during carcass chilling can be useful to develop models to predict Salmonella growth post-processing and during storage, hence assisting processors in improving process controls.