Profiling the composition and metabolic activities of microbial community in fermented grains for the Chinese strong-flavor Baijiu production by using the metatranscriptome, high-throughput 16S rRNA and ITS gene sequencings

The composition and function of microbial community obtained by sequencing 16S rRNA/ITS gene amplicons (DNA level) were compared with taxonomy derived from metatranscriptomes (RNA level) from the same fermented grains (FG) sample, which obtained from the key fermentation time point during the Chinese strong-flavor Baijiu (CSFB) production process. The results showed that the fungi with the highest relative abundance were Saccharomyces (RNA: 83.15%, DNA: 89.74%). The Kroppenstedtia (37.09%), the most abundant bacterium at the DNA level, was not detected at the RNA level, and the highest content genus was Streptococcus (93.75%) at the RNA level, indicating that there was an obvious difference between the composition of DNA-level and RNA-level prokaryotic communities observed in the targeted FG. For the microbial function in KEGG pathway database, most genes of microorganisms in FG were both related to "Metabolism" pathway, and enriched in "Carbohydrate metabolism" and "Amino acid metabolism" at two levels. While the functional categories enriched in other pathways were different, such as "Replication and repair", "Membrane transport" and "Cell motility", which were rich at the DNA level but low at the RNA level. Furthermore, Saccharomyces cerevisiae was the most active microbe in the top15 pathways, followed by Torulaspora dellbrueckii. During the conversion of starch to ethanol, S. cerevisiae showed high metabolic capacity, and cooperated with other microorganisms to convert pyruvate to acetaldehyde through acetyl-CoA and acetic acid, and then acetaldehyde to ethanol. Our findings could provide useful insights for further determining the active microbial function and metabolic mechanism in the whole fermentation process of Chinese Baijiu.