The proteolytic profiles of lactic acid bacteria drive positive interactions in co-culture in a food-mimicking medium

International audienceMicrobial interactions govern natural and artificial environments. So-called positive interactions result in the fitness enhancement of at least one community member without harming the others. Positive interactions between microorganisms in co-culture are accompanied with an increase in the production or degradation of targeted molecules, as observed in fermented food products such as sourdough, kefir, and yogurt. In these products, the available peptides and free amino acids (FAA) are involved in the interactions but the effect of their nature has not been studied yet. This study aims to investigate how different proteolytic (donor) lactic acid bacteria (LAB) strains can stimulate or not the growth of non-proteolytic (receiving) LAB strains in a food-like medium. An increase in organic acids and aroma compounds and a decrease in sugars inducing intestinal discomfort are expected with the co-cultures compared to the mono-cultures of donor strains.A synthetic medium, mimicking a food product, was first developed in order to provide all the necessary nutrients, but proteins as sole nitrogen source. It only allowed the growth of the proteolytic strains. Three donor strains were selected with distinct proteolytic profiles, in terms of released FAA and peptides, and overall degree of proteolysis. Nine pairs of donor/receiver strains of different species (Enterococcus faecalis, Lactococcus lactis, and Lactobacillus plantarum) were co-cultured in compartment chambers that physically separated the bacteria, and in direct co-cultures. LAB growth and metabolism was compared in co-cultures and mono-cultures in terms of bacterial counts, acidification rates, amount in FAA and peptides, residual sugars, and volatile compounds. Receiving strains grew differentially depending on the donor strains they were co-cultured with. The results suggest that the changes in growth of the receiving strain may be influenced by the size, nature, and/or concentration of nitrogen compounds produced by the donor strain. The co-cultures presenting positive interactions showed higher acidification rates, decreased final concentrations of peptides, FAA and residual sugars.This study gives insight into the mechanisms that rule microbial interactions in fermented food products. It also gives strategic knowledge on how to assemble novel bacterial communities that gather multiple targeted functions, leading to better or new food products