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Survival and growth dynamics of Listeria monocytogenes and Salmonella spp. on artificially contaminated cooked ready-to-eat meat products

The aim of the study was to evaluate the potential of survival and growth dynamics of Listeria monocytogenes strains and Salmonella enterica subsp. enterica serovar Enteritidis and Typhimurium, inoculated artificially (individually and in mixture) on two ready-to-eat (RTE) cooked smoked meat products. For the purpose of the study 120 slices of cooked smoked sausage and 40 slices of cooked smoked loin were purchased and inoculated with the strains. Two storage temperatures were selected: 6 oC and 10 oC for 8 days. The study was performed as challenge tests in a secondary contamination scenario to investigate the presence and/or absence of pathogenic bacteria during the shelf life of the products. The inoculum levels at the start of the experiment were 4.46 log10 CFU/g and 2.88 log10 CFU/g for the L. monocytogenes strains at the surface of the cooked smoked loin and cooked smoked sausage respectively. Using the same inoculation method, but adding Salmonella enterica serovars to the mixture, the inoculum levels were 4.15 log10 CFU/g at the surface of the cooked smoked loin and 2.94 log10 CFU/g at the surface of the cooked smoked sausage. L. monocytogenes was detected at all sampling days on both storage temperatures. It showed an average increase by 0.5–1.0 log10 CFU/g on the cooked smoked sausage and by 2.0–3.3 log10 CFU/g for the cooked smoked pork loin for the duration of the study. Salmonella enterica serovars were also present at each sampling day on both storage temperatures. Typical colonies were isolated and serotyped, confirming the survival of these pathogenic bacteria. Salmonella enterica subsp. enterica serovar Typhimurium was the predominant serovar at almost every sampling day. The results from our study showed no competitive relationship in the presence of Salmonella and L. monocytogenes in contaminated meat products. The two types of microorganisms were successfully adapted and developed independently under appropriate conditions, including temperature, humidity, water activity and pH.