Welcome to the house-warming party: social interactions and antagonism against Listeria monocytogenes in multi-species biofilms

ABSTRACT Listeria monocytogenes integrates into multi-species biofilms and persists in food processing facilities. Its persistence depends on the composition of the microbial communities, but those species that accommodate L. monocytogenes , or not, remain to be identified. Moreover, the decision-making on biofilm vs pellicle formation in microbial communities remains poorly understood. It was the aim of this study to identify communities that accommodate Listeria , or not, and to observe biofilm vs pellicle formation in multi-species biofilms. We quantified L. monocytogenes in six reconstituted communities, each consisting of 5–19 strains. A 15-member community was simplified into five species to quantify L. monocytogenes in all 31 combinations of these five strains. Janthinobacterium tructae favored pellicle formation, while Pseudomonas shahriarae convinced others to join a surface-attached biofilm. In the six microbial communities, cell counts of L. monocytogenes were lowest when Carnobacterium maltaromaticum was also present. The antagonistic effect of C. maltaromaticum was confirmed when analyzing all combinations of five strains. The presence of C. maltaromaticum reduced cell counts of L. monocytogenes by about 1 log CFU/cm 2 . Serratia liquefaciens also antagonized L. monocytogenes , but this was observed ( P < 0.05) only in the absence of C. maltaromaticum . Genome analyses of the community identified close to 100 different quorum-sensing systems, including peptides that are also bactericidal against L. monocytogenes . We conclude that the language used to describe human social interactions is also suited to describe microbial interactions in multi-species biofilm communities, but unraveling mechanisms of communication among biofilm-embedded bacteria remains subject to future studies. IMPORTANCE Microbial communication in relation to biofilm vs pellicle formation and on whether to accommodate Listeria monocytogene s or not partially converge with quorum-sensing systems as a key mechanism. Two organisms, Carnobacterium maltaromaticum and Serratia liquefaciens , antagonized L. monocytogenes . Microbial interactions are strongly dependent on the complexity of the microbial community, and different behaviors were observed in biofilms formed by 2, 5, or 15 strains. Results presented in this study document that the use of protective cultures may also reduce persistence of Listeria in food processing facilities but are far from providing an understanding of communication and social rules in complex bacterial communities. The novel perspective on microbial interactions may direct future experiments.