Listeria monocytogenes Colonizes Pseudomonas fluorescens Biofilms and Induces Matrix Over-Production

In food facilities, biofilms or their debris might act as for attracting free floating microorganisms. In this sense, , a dense biofilm producer frequently isolated from food contact surfaces, could be a good candidate for sheltering other microorganisms, such as . The main objective of this work was to evaluate the ability of to colonize pre-established biofilms. For this, the movement throughout mature biofilms of a green fluorescent protein (GFP) - tagged strain of was tracked for 24 h by confocal laser scanning microscopy (CLSM). Moreover, in order to check the effect of the incorporation of on the overall matrix production, attached populations of both microorganisms and total biomass (cells + matrix) of the resulting biofilms were measured over time. Planktonic cells of efficiently migrated to preformed biofilms. Moreover, they moved preferentially toward the bottom layers of these structures, suggesting some kind of tropism. When preformed biofilms were conditioning the surfaces, the attached population was on average, 1-2 Log higher than when this organism grew on bare coupons. Furthermore, the arrival of to the already established biofilms led to a matrix over-production. Indeed, biomass values [optical density (OD )] of the resulting biofilms were double those of the ordinary mixed biofilms (1.40 vs. 0.6). The fact that cells accumulate in the bottom layers of preformed biofilms provides this microorganism an extra protection toward physical-chemical damages. This might partly explain why this microorganism can persist in food industry environments.