Phylogenetic and Functional Analysis of Gut Microbiota of a Fungus-Growing Higher Termite: Bacteroidetes from Higher Termites Are a Rich Source of β-Glucosidase Genes

Abstract

Fungus-growing termites, their symbiotic fungi, and microbiota inhibiting their intestinal tract comprise a highly efficient cellulose-hydrolyzing system; however, little is known about the role of gut microbiota in this system. Twelve fosmid clones with β-glucosidase activity were previously obtained by functionally screening a metagenomic library of a fungus-growing termite, Macrotermes annandalei. Ten contigs containing putative β-glucosidase genes (bgl1-10) were assembled by sequencing data of these fosmid clones. All these contigs were binned to Bacteroidetes, and all these β-glucosidase genes were phylogenetically closed to those from Bacteroides or Dysgonomonas. Six out of 10 β-glucosidase genes had predicted signal peptides, indicating a transmembrane capability of these enzymes to mediate cellulose hydrolysis within the gut of the termites. To confirm the activities of these β-glucosidase genes, three genes (bgl5, bgl7, and bgl9) were successfully expressed and purified. The optimal temperature and pH of these enzymes largely resembled the environment of the host's gut. The gut microbiota composition of the fungus-growing termite was also determined by 454 pyrosequencing, showing that Bacteroidetes was the most dominant phylum. The diversity and the enzyme properties of β-glucosidases revealed in this study suggested that Bacteroidetes as the major member in fungus-growing termites contributed to cello-oligomer degradation in cellulose-hydrolyzing process and represented a rich source for β-glucosidase genes.