Is microbial diversity an asset for inhibiting Listeria monocytogenes in raw milk cheeses?

This study aimed at determining if microbial diversity can be an asset to guarantee the microbial safety of raw milk cheeses. Our results show that microbial consortia from the surface of raw milk cheeses can self-protect against Listeria monocytogenes . Indeed, 10 complex microbial consortia among 34 tested from the surfaces of raw milk Saint-Nectaire cheeses were particularly effective for reducing the growth of L. monocytogenes on cheese surfaces in comparison of a commercial ripening culture, despite the high pH values on the surfaces. One of these consortia (TR15) was selected and propagated on cheese surfaces to create a collection of strains belonging to lactic acid bacteria, Gram-positive and catalase-positive bacteria, Gram-negative bacteria and yeasts. On the surfaces of uncooked cheeses, defined consortia consisting of combinations of several isolates from this collection displayed weaker antagonist activity against L. monocytogenes than the complex consortium TR15. The results from plate counting and analysis by single strand conformation polymorphism (SSCP) converged to show that microbial dynamics in cheeses TR15 differed from that of the defined consortia. TR15 cheeses had the highest levels of cultivable lactobacilli and leuconostocs. Their SSCP profiles were the richest in peaks and were characterised by the presence of Marinilactibacillus psychrotolerans, Carnobacterium mobile, Arthrobacter nicotianae or A. arilaitensis, Arthrobacter ardleyensis or A. bergerei and Brachybactrerium sp. Further investigation will be necessary to gain a better understanding of the microbial interactions involved in inhibiting L. monocytogenes .