The objective of this study was to screen Lactobacillus plantarum strains isolated from the traditional Slovak raw sheep milk cheese for their inhibitory potential. Seventy-two strains were obtained from samples of raw sheep milk and raw sheep milk cheeses collected from April 2017 to September 2018, in 23 geographical areas of Eastern Slovakia, by inoculation of de Man, Rogosa, and Sharpe agar plates (Oxoid, Basingstoke, UK). Using both the genus- and species-specific PCR methods, 43 strains were identified as Lactobacillus spp., and 10 strains were confirmed as Lb. plantarum. First, the whole bacterial cultures of Lb. plantarum strains were tested by disc diffusion assay. All showed very good antibacterial activities against 6 selected foodborne pathogens, including Escherichia coli, Salmonella Enteritidis, Pseudomonas aeruginosa, Listeria monocytogenes, Staphylococcus aureus, and Bacillus cereus. Then, cell-free neutralized supernatants and partially purified bacteriocins were prepared from the 4 Lb. plantarum strains that exhibited the best antibacterial potential, and they were tested the same way as the whole bacterial cultures. Seven of the 10 Lb. plantarum strains harbored the plnEF gene, 3 strains harbored the plnD gene, and 2 strains possessed both the plnA and plnC genes that encode the production of the respective plantaricins. The presence of both plnR and plnL genes was only detected in a single Lb. plantarum isolate. Based on the results of this study, 4 strains of Lb. plantarum appeared to be suitable candidates for further testing in the dairy manufacturing sector, particularly in the production of raw sheep milk products.
Keywords: Lactobacillus plantarum; antimicrobial activity; plantaricin; sheep cheese.
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