Understanding phage dynamics and their potential roles during soy sauce fermentation using metagenome-assembled genomes.

The composition and functional roles of phages in fermented foods have been gaining increasing attention. However, their ecological functions and underlying mechanisms in high-salt soy sauce fermentation remain largely unexplored. In this study, we investigated phage communities, their potential functions, phage-host interactions, and host defense mechanisms in two different soy sauce fermentation processes (Cantonese-type process, CP; Japanese-type process, JP) using shotgun metagenomics. A total of 823 phage species (viral operational taxonomic units, vOTUs) were identified, with the majority exhibiting a temperate lifestyle (89.19%). The most abundant family was Straboviridae (CP, 9.95%-11.39%; JP, 12.04%-13.73%), followed by Salasmaviridae (CP, 6.92%-7.94%; JP, 5.70%-7.02%). Although the phage composition differed between the two processes, the number of vOTUs was positively correlated with prokaryotic species richness, total acidity, and amino acid nitrogen content, and negatively correlated with pH. A comparative genomic analysis revealed that 91 phages were associated with 26 bacterial genomes (metagenome-assembled genomes, MAGs), with Lactococcus petauri (MAG16) and Halomonas elongata (MAG51) hosting the most phage species. An analysis of host defense mechanisms showed that all 45 bacterial MAGs harbored CRISPR-Cas type I systems, and 95.6% encoded restriction-modification systems. Functional annotation using the KEGG and CAZymes databases indicated that the phages predominantly encoded genes related to cell growth, replication, and metabolism of amino acids, carbohydrates, and nucleotides, with more genes enriched in the JP than in the CP. Additionally, auxiliary metabolic genes (e.g., pfkA, ldh, adhP, ilvE, and arcA) were identified in 29 phages. These genes are potentially involved in metabolic pathways that may be linked to flavor compound production. Together, these findings provide novel insights into the ecological and potential functional roles of phages during soy sauce fermentation.