In vitro evaluation of Lactobacillus animalis SB310, Lactobacillus paracasei subsp. paracasei SB137 and their mixtures as potential bioprotective agents for raw meat

Lactobacillus animalis SB310 and Lactobacillus paracasei subsp. paracasei SB137, isolated from the gut of veal calves, were investigated in terms of antimicrobial activity for a possible application as bioprotective agents in vacuum-packed raw meat. In the first trial, cultures of single strains and their mixture obtained adding the two strains before the incubation, their cell-free supernatants and buffered cell-free supernatants were tested in vitro against a wide range of spoilage or potentially pathogenic bacteria. In the second trial different mixtures were evaluated for the same tests (L. animalis: L. paracasei rates = 1:2, 1:5, 1:10, 2:1, 5:1, 10:1 and 1:1 obtained mixing the two cultures grown separately). An evident inhibitory activity exerted by the single Lactobacillus strains and the mixtures was observed for all the target bacteria tested. In almost all the cases, the inhibition halos produced by the mixtures were significantly higher than those produced by the single strains, especially if L. paracasei subsp. paracasei SB317 was predominant in the mixture. Among the target bacteria, Pseudomonas fluorescens and Pseudomonas putida were clearly the most susceptible; a high variability in Enterobacteriaceae was detected, depending on the species. Serratia marcescens and Escherichia coli O157:H7 resulted as the less susceptible strains. A very limited activity of the cell-free supernatants was found for all the 12 strains tested, compared with the action of viable Lactobacilli, highlighting that the antimicrobial action originated from a combination of bacterial competition and the production of extracellular compounds. The absence of effects by the buffered cell-free supernatants suggests that these compounds are organic acids. Further studies are necessary to clarify the effects of these strains when applied to raw meat.