Positive interactions between lactic acid bacteria promoted by nitrogen-based nutritional dependencies
Interactions positives entre bactéries lactiques favorisées par les dépendances nutritionnelles fondées sur l’azote
Résumé
Lactic acid bacteria (LAB) are often studied in association with yeasts or propionibacteria in various fermented food products and the mechanisms underlying their interactions are being well characterized. Nutritional dependencies, especially those regarding nitrogen sources, has been shown to govern numerous of these microbial positive interactions. However, interactions solely between LAB have rarely been investigated. Understanding how they can positively interact could be useful in multiple food-related fields: production of fermented food products with enhanced functional properties or fermentation of new food matrices. This study investigates the exploitation of the proteolytic activity and amino acid auxotrophies of LAB strains to promote positive interactions between proteolytic and non-proteolytic strains. A chemically defined medium was thus developed to specifically allow the growth of six LAB strains used, three proteolytic and three non-proteolytic. Each of the proteolytic strains, Enterococcus faecalis CIRM-BIA2412, Lactococcus lactis NCDO2125, and CIRM-BIA244, was co-cultured with each one of the non-proteolytic LAB strains: L. lactis NCDO2111, Lactiplantibacillus plantarum CIRM-BIA465 and CIRM-BIA1524. Each proteolytic strain induced different types of interactions: either strongly positive, weakly positive, or no interactions, with E. faecalis CIRM-BIA2412, L. lactis NCDO2125 and L. lactis CIRM-BIA244, respectively. Strong interactions were associated with the production of higher concentrations in peptides, and in five free amino acids which are essential in both human and microbial nutrition: tryptophan, valine, phenylalanine, leucine and isoleucine. They also led to faster acidification rates, lower final pH, higher raffinose utilization, and higher concentrations in five volatile compounds. These results suggest nevertheless that proteolytic LAB do not equally stimulate non-proteolytic LAB and that the stronger the interactions between LAB are, the more functional outputs we can expect. Thus, this study gives insight into how to create new associations of LAB strains and to guaranty their positive interactions.
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