Genomic and Metabolic Features of an Unexpectedly Predominant, Thermophilic, Assistant Starter Microorganism, Thermus thermophilus, in Chinese Inner Mongolian Cheese

Abstract

Inner Mongolian cheese is a traditional dairy product in China. It is produced without rennet, using naturally acidified milk that is simmered to achieve whey separation. In order to analyse the impact of simmering on the microbial community structure, high-throughput sequencing was performed to obtain bacterial 16S rRNA sequences from cheeses from the Ordos (ES), Ulanqab (WS), Horqin (KS) and Xilingol (XS) grasslands of Inner Mongolia. The relative abundance of an unexpected microorganism, Thermus thermophilus, ranged from 2% to 9%, which meant that its dominance was second only to that of lactic acid bacteria (LABs). Genome sequencing and fermentation validation were performed in T. thermophilus N-1 isolated from the Ordos, and it was determined that T. thermophilus N-1 could ingest and metabolise lactose in milk to produce lactate during the simmering process. T. thermophilus N-1 could also produce acetate, propionate, citrate and other organic acids through a unique acetate production pathway and a complete propionate production pathway and TCA cycle, which may affect texture and flavour development in Inner Mongolian cheese. Simultaneously, the large amount of citrate produced by T. thermophilus N-1 provides a necessary carbon source for continuous fermentation by LABs after the simmering step. Therefore, T. thermophilus N-1 contributes to cheese fermentation as a predominant, thermophilic, assistant starter microorganism unique to Chinese Inner Mongolian cheese.