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Listeria monocytogenes, recognized as one of the major human pathogens transmitted by food, cause abortion and neonatal infections, and it is responsible for contagious diseases whose clinical course is usually characterized by several and often septicemic manifestations of meningo-cerebral phenomena. L. monocytogenes is widely present in ready-to-eat-products, soil, water and animal samples. Official standards include zero tolerance for L. monocytogenes in all ready-to-eat products, since L. monocytogenes has been involved in known food-borne outbreaks of listeriosis, particularly in high-risk populations including neonates, children, immunocompromised hosts and pregnant women. Antagonistic microorganisms may be used as protective cultures to control pathogens or to extend shelf life of food products: lactic acid bacteria (LAB), often associated with dairy products and with fermented foods, should be considered as the ideal choice for application as protective cultures as they are naturally present in food products, have a long history of safe use and form part of the gut microflora of humans and animals. Their antagonism involves growth inhibition of other bacteria and fungi through competition for nutrients or production of antimicrobial active metabolites, such as organic acids, hydrogen peroxide, diacetyl, antimicrobial enzymes, bacteriocins and reuterin.
In this work, an in vitro screening of LAB strains was carried out to assess their antimicrobial properties against different isolates of Listeria monocytogenes, for their possible technological use in food production. Antimicrobial activity of both culture supernatants and discs with viable cells of 12 different LAB isolates, 7 isolated from Guirra sheeps milk by the “Comunidad de Valencia” and 5 from the Spanish collection CECT, was studied by assessing their ability to inhibit the growth of Listeria isolates by agar diffusion assays. Data obtained suggested that the disc method produced better results compared with the well method, using culture supernatant. In fact, many isolates belonging to Lactobacillus pentosus, Lactococcus lactis subsp. lactis, Lactobacillus paracasei subsp. paracasei, Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacillus acidophilus inhibited growth of Listeria isolates in the disc method, whereas a reduced number of LAB strains was active when tested by the well assay.