A potential protective culture; halophilic Bacillus isolates with bacteriocin encoding gene against Staphylococcus aureus in salt added foods

The research aimed to investigate and isolate halophilic bacteria in salt-fermented foods with active inhibitory effect against Staphylococcus aureus. Bacillus were the main isolates found in salted fish and soy sauce during fermentations. From a total of 124 isolates, 37 isolates with inhibitory effect on S. aureus could be classified into six Bacillus species. Among the 37 isolates, 24.3% of isolates contained the genes encoding subtilin (spaS) and/or subtilosin (sboA) in addition to another unknown gene; 18% contained only subtilin gene, 5.4% contained only subtilosin gene and 5.5% had both subtilin and subtilosin genes. Only subtilin gene expressions of two strains, B. subtilis (P5-6) and B. lichenoformis (P7-2) were observed along with housekeeping gene BA-rpoB when co-cultured with S. aureus. The B. subtilis (P5-6) was selected and made as a skim milk-based protective culture containing 2.5 mg/g salt with average viable count and water activity (a(w)) approximately at 6.25 log CFU/g and 0.49, respectively. The Minimum Inhibitory Concentration (MIC) of the protective culture was at 20 AU/mg and Minimum Bactericidal Concentration (MBC) at 80 AU/mg. After application in a cottage cheese model, the protective culture could reduce S. aureus and significantly helped prolong freshness of the cheese over 16 days without affecting its physicochemical properties, particularly pH. The protective culture was also evaluated as safe.