Behaviour of L. monocytogenes in an artificially made biofilm of a nisin-producing strain of Lactococcus lactis

The survival of Listeria monocytogenes in a binary biofilm with a bacteriocin producer (Lactococcus lactis CNRZ 150) was investigated. Two situations were simulated: in the first, L. monocytogenes was deposited on a 1-day biofilm of Lactococcus lactis (deferred adhesion); in the second, L. monocytogenes was simultaneously mixed with Lact. lactis (simultaneous adhesion). Biofilms were cultivated in tryptic soy broth supplemented with 6 g l−1 of yeast extract (TSB-YE) and L. monocytogenes counts were followed for 48 h, both in co-culture with Lact. lactis and in pure culture. The influence of the mode of inoculation of L. monocytogenes (deferred or simultaneous adhesion) into the Lact. lactis biofilm, the size of the L. monocytogenes inoculum and the replacement of the culture medium at 20–24 h on the survival of L. monocytogenes was studied. Results showed that the antilisterial activity of the Lact. lactis started within the first 6 h of the deposition of L. monocytogenes. The log cycle reduction rate in number of L. monocytogenes in the mixed biofilm (compared to the pure biofilm) was greatly dependent on the inoculum size: when the smallest inoculum was used to colonise stainless steel coupons (106–107 CFU ml−1), the log cycle reduction was greater and L. monocytogenes was not detected after t=10 h (simultaneous adhesion) and t=24 h (deferred adhesion) in the adherent population as well as in the planktonic population. On the other hand, in the case of a greater supply of L. monocytogenes (108 CFU ml−1), the results showed that the early reduction of L. monocytogenes counts was relatively slow and was followed by a stabilisation of the population, leading to the establishment of a great number of resident cells in the biofilm (105 to 106 CFU cm−2). This population level was maintained during the 48 h of experimentation and replacement of the culture media with fresh medium at t=22 h (simultaneous adhesion) or t=24 h (deferred adhesion) did not modify the level of the population of L. monocytogenes within the biofilm.