Survival of food-borne pathogens on natural black table olives after post-processing contamination

The aim of this study was to evaluate the survival of Salmonella, Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus on natural black olives during aerobic storage without brine and to conclude about the safety of this fermented food. Fermented black olives (pH 3.95; NaCl 6.02%) were artificially inoculated with 4 strains of Salmonella enterica ser. Enteritidis, 4 strains of S. enterica ser. Typhimurium, 3 strains of E. coli O157:H7, 3 strains of L. monocytogenes, and 3 strains of S. aureus either in monoculture or in mixed culture by submerging the fruits in a solution containing ca. 7.0 log CFU/ml of each target pathogen. The olives were subsequently stored aerobically at 4 and 20 °C, and the pathogens were recovered on selective media for a time period of 15 days. Enrichment was employed to define the presence/absence of the target pathogens when their counts were below the detection limit of direct plating. Results showed that the population of all pathogens presented a rapid decline within the first 2 days of storage depending on storage temperature, type of inoculation (monoculture or cocktail) and strain. Storage at 4 °C seemed to prolong the survival of certain strains of pathogens as compared to 20 °C by ca. 1 day. Enrichment revealed the absence (< 1 CFU/25 g) of Salmonella and E. coli O157:H7 from olive samples with the exception of all strains of L. monocytogenes at 4 °C and one strain at 20 °C that were detected by selective enrichment until the end of storage. The results of this study indicate that natural black olives are not a favorable environment to support the growth of the investigated pathogens after post-processing contamination provided that the olives have been subjected to proper fermentation process.