Antagonistic effect of Lactobacillus strains against Escherichia coli and Listeria monocytogenes in milk.

The current work studied four types of binary antagonist/pathogen bacterial culture system, in order to determine the effect of interaction between two strains of Lactobacillus plantarum and two food-borne pathogens, Listeria monocytogenes and Escherichia coli, in whole UHT milk at 37°C. To determine the type of interaction between the two bacterial populations in co-cultures and to evaluate the antagonistic activity of the lactic acid bacteria (LAB) on the pathogenic bacteria, the growth curves, the kinetic parameters, and the pH profiles of mono- and co-cultures were compared. The Lb. plantarum strains showed different bacteriocin-like inhibitory substance (BLIS) production, auto- and co-inducible. The antibacterial effect of neutralized supernatants of mono and co-cultures harvested at different times of incubation was assessed in order to establish the presence of bacteriocin-like inhibitory-substances (BLIS) and their possible relation to the growth inhibition of the pathogen. The LAB reduced the growth of Esch. coli and of List. monocytogenes by 4 and ∼5 log cycles, respectively and influenced other growth kinetic parameters, such as μ(max) and lag phase, in the different binary combinations. The growth of the LAB was not relevantly altered by simultaneous growth with the pathogenic strains showing an interaction of amensalism. The pattern of inhibition exerted by the LAB on the pathogens was different; Lb. plantarum LB279 inhibited the growth of List. monocytogenes more effectively than that of Esch. coli. The behaviour of Esch. coli in co-culture with Lb. plantarum WS4174 suggested the presence of metabolic crowding in the mechanism of growth suppression. This exploratory study showed the complexity and specific particularities of the inhibition phenomena between bacterial communities.