Characterization of microbial communities during Grifola frondosa (maitake) wood log cultivation

The method used to cultivate the popular Japanese mushroom Grifola frondosa (maitake), called 'wood log cultivation', comprised two steps: (1) the mycelium is grown around a wood log in a plastic bag, and (2) the mycelium that has spread on the wood log is transferred into casing substrates in a forest. This method is still popular in Japan due to its low cost and high-quality crop production. The importance of the microbiome that inhabits mushroom-cultivation surroundings has recently attracted attention, but no study of the microbial communities in maitake cultivation has been published. We investigated how the bacterial communities changed in wood logs in comparison with a control group (without inoculation) and their interaction with maitake during the first to fourth years of maitake wood log cultivation. A maitake biomass was detected by quantitative PCR in wood logs but not in the casings, and we thus decided to investigate the bacterial communities in wood log samples for control and first- to fourth-year cultivation. The results indicate that the phyla Proteobacteria, Firmicutes, and Gemmatimonadota play key roles in changes of the microbiome composition for maitake wood log cultivation. In a functional profile, bacteria communities in the wood logs during maitake cultivation showed higher relative abundance in cellulolysis, glycolysis, TCA cycle, and many biosynthesis pathways, whereas the control group showed higher relative abundance in fermentation. These results suggested that (i) the bacterial communities which inhabited maitake cultivated wood logs may help the maitake degrade wood cellulose, and (ii) part of the glucose from the cellulose degraded by both maitake and bacteria was used for the bacterial TCA cycle instead of fermentation. Bacteria also produce some chemicals that maitake mycelium may need. It is also likely that some potential intracellular parasites dwell with maitake. The different cultivation stages showed different network structures. A network analysis indicated that Class Gammaproteobacteria is a potential keystone taxon for the microbiome network stability of maitake cultivated wood logs. These results contribute to the understanding of the microbiome in maitake-cultivation surroundings and will improve maitake wood log cultivation.