Interpreting the asymmetric interaction between yeast and acetic acid bacteria in kefir grains from a metabolic perspective.

Abstract

As a naturally complex mixed microbial system, the intricate microbial interactions within kefir grains remain poorly understood, particularly regarding the relationships between yeasts and acetic acid bacteria. To elucidate the strain interaction mechanisms in kefir grains, this study systematically investigated the interactions between kefir-derived yeast (Kluyveromyces marxianus Y7) and acetic acid bacteria (Acetobacter fabarum A26) by integrating species-specific primer-based qPCR quantification, growth status analysis, biofilm formation, exopolysaccharide (EPS) measurement, and non-targeted metabolomics. The results demonstrated that co-culture established an asymmetric interaction pattern dominated by A26, with Y7 continuously adapting. Metabolomics and pathway enrichment analyses (KEGG) revealed that the interactions specifically activated core pathways such as ABC transporters, amino acid biosynthesis, and protein digestion and absorption. Dynamic changes in key metabolites elucidated their functional roles in the interaction: Y7 upregulated phenyllactic acid (as an antagonistic and signaling molecule) and hexylglutathione (antioxidant) in response to stress; the riboflavin secreted by Y7 may provide metabolic assistance to A26; the accumulation of (S)-2-hydroxyglutarate suggested energy metabolism remodeling in Y7; while the upregulation of 12-hydroxydodecanoic acid was associated with biofilm formation. This study is the first to discover a unique EPS metabolic cycle during co- culture: early synthesis followed by degradation, accompanied by the re-accumulation of sucrose, which constitutes a key internal carbon resource recycling strategy. In summary, from the perspective of metabolites and pathways, this research reveals that the two strains establish an efficient symbiotic metabolic system by defining functional roles, driving metabolic division of labor, and achieving resource cycling.