Synergistic Effect of Formate and Cell-Free Supernatant Fermented by Two Probiotics, Bifidobacterium animalis spp. lactis HN019 and Lacticaseibacillus rhamnosus HN001, against Shigella flexneri Growth

The interplay between probiotic metabolites, host health, and inhibition of pathogens has increasingly attracted interest but remains unresolved due to the complex molecular interactions among these factors. We investigated the action of cell-free supernatants (CFSs) from two probiotic bacteria, Bifidobacterium animalis spp. lactis (B. lactis) HN019 and Lacticaseibacillus rhamnosus (L. rhamnosus) HN001 and their effect against the bacterial pathogen Shigella flexneri 2457T (S. flexneri). The CFSs from B. lactis HN019 exhibited a higher antibacterial effect against S. flexneri  growth than the CFSs from L. rhamnosus HN001, independent of the carbon source utilized. This effect correlated with higher formate within the CFSs from B. lactis HN019. As expected, the antimicrobial effects of CFSs were stronger against free S. flexneri cells than for S. flexneri infecting host cells. A synergistic effect of the CFSs with additional small organic molecules such as indole-3-lactate and formate was found. Such interplay between CFSs, indole-3-lactate, and formate was reflected by altered metabolic rates by S. flexneri in the presence of B. lactis HN019 CFSs both in the solution and under biofilm-forming conditions. The synergistic effect between different components acting on S. flexneri gives reasons to believe that suitably designed mixtures of probiotic metabolic products and small-molecule effectors bear promise for successfully combating pathogens.