Facultative anaerobic halophilic and alkaliphilic bacteria isolated from a natural smear ecosystem inhibit Listeria growth in early ripening stages

In vitro and in situ anti-listerial properties of 3 strains of Facultative Anaerobic Halophilic and Alkaliphilic (FAHA) species, i.e. Alkalibacterium kapii ALK 6, Marinilactibacillus psychrotolerans ALK 9 and Facklamia tabacinasalis ALK 1, were investigated. The 3 strains were isolated from a smear ecosystem originating from a commercial Raclette type cheese and exhibiting strong anti-listerial activity in situ on cheese surface.In a first step, strains were tested in vitro for production of antimicrobial compounds against Listeria innocua 81000-1 and Listeria ivanovii HPB 28. M. psychrotolerans ALK 9 inhibited both indicator strains in spot-on-the-lawn tests while A. kapii ALK 6 showed no inhibiting effect. F. tabacinasalis ALK 1 exerted an in vitro inhibition on L. ivanovii HPB 28, but induced the formation of dense ball-shaped microcolonies of L. innocua 81000-1 in the soft agar, a typical biofilm microstructure. The extent of the biofilm zone was enhanced when F. tabacinasalis ALK 1 and M. psychrotolerans ALK 9 were tested together.In a second step, different combinations of strains were applied on Raclette cheeses ripened at pilot scale and contaminated with 50 cfu/cm2L. innocua at day 7. A control flora of 6 strains, isolated from ecosystem F and corresponding to species commonly found on smear cheeses, was applied on control and test cheeses. In test cheeses, we investigated the impact on Listeria growth of the addition of the 3 FAHA strains, applied as single or mixed cultures. A 1-log inhibition was obtained at day 15 on cheeses treated with FAHA strains applied either as single or mixed cultures. This 1-log inhibition was correlated with the development of FAHA species that reached their maximal count at day 15.This study suggests that the development of FAHA species in early ripening likely contributes to the initial part of the in situ inhibition exerted by the complex cheese surface ecosystem investigated.

Research Highlights
► Two of three FAHA species inhibited Listeria in vitro.
► Growth on cheese surface of each FAHA species was correlated with Listeria inhibition.
► Synergies between the FAHA species were observed in vitro but not in situ.