Bacillus spp. increase the Paracin 1.7 titer of L. paracasei HD1.7 in sauerkraut juice: Emphasis on the influence of inoculation conditions on the symbiotic relationship

Abstract Bacteriocins from lactic acid bacteria are natural preservatives that inhibit foodborne pathogenic microorganisms and have recently attracted much attention. Lactobacillus paracasei HD1.7 was previously isolated from sauerkraut fermentation broth in the laboratory and has been shown to produce the bacteriocin Paracin 1.7. In this study, L. paracasei HD1.7 was cocultured with three Bacillus species (B. subtilis, B. licheniformis, and B. pumilus). Bacillus spp. promoted the growth of L. paracasei HD1.7 and production of Paracin 1.7. Adjusting the inoculation time (ITII) and inoculation ratio (IR), L. paracasei HD1.7 affected the pH of the fermentation broth and interspecific symbiosis. The inoculum conditions and symbiotic relationships differed between strain combinations at the highest Paracin 1.7 titer. L. paracasei HD1.7 showed commensalism with B. subtilis but collaboration with B. licheniformis and B. pumilus. Gene expression results showed that coculture accelerated nutrient transfer and information exchange by upregulating the relative expression level of glucose transmembrane protein gene (ptsG) and quorum-sensing-related genes (prcK, prcR, and luxS). In contrast, spore formation-related genes (sigE, sigF, sigG, and spo0A) were inhibited. Structural equation modeling (SEM) was used to verify the feasibility of changing the symbiotic relationship by controlling the ITII and IR to ultimately increase the titer of bacteriocin.