Regulatory Mechanisms of Issatchenkia orientalis Y1 on Microbial Community Assembly and Potential Functions in Fermented Grains of Strong-Flavor Baijiu

Issatchenkia orientalis Y1 is an important functional microorganism during the fermentation of strong-flavor Baijiu and may influence microbial community structure, community assembly mechanisms, and microbial functional potential predicted from sequencing data. In this study, a mixed-culture fermentation system was established using fermented grains and Huangshui from strong-flavor Baijiu production. I. orientalis Y1, isolated from fermented grains, was inoculated into this system as a perturbation treatment to investigate its effects on microbial diversity, community composition, and functional profiles, as well as its influence on microbial community assembly pathways. The results showed that inoculation with I. orientalis Y1 significantly altered microbial diversity and increased community complexity. The relative abundances of Paenibacillus, Aspergillus, and Acetobacter decreased, with the peak abundance of Paenibacillus declining from 62.8% to 40.2% and that of Acetobacter decreasing from 27.8% to 19.1%, while Aspergillus remained consistently less abundant than in the control group throughout fermentation. In contrast, the relative abundances of Issatchenkia and Lactobacillus increased, with their peak abundances rising from 15.0% to 23.9% and from 7.6% to 27.0%, respectively. In addition, it increased the contribution of deterministic processes in community assembly, with the deterministic proportion rising from 31.1% to 35.5% in bacterial communities and from 28.4% to 48.8% in fungal communities, while the contribution of stochastic processes decreased. These changes suggest that the microbial community became more controllable and functionally more stable after inoculation. Meanwhile, the overall predicted metabolic activity of the microbial community declined. In conclusion, the addition of I. orientalis Y1 reshaped microbial community structure, influenced microbial community assembly processes, and altered the correlations between dominant microorganisms and metabolic pathways.