  Effective biological utilization of wood biomass is necessary worldwide.
Since several insect larvae can use wood biomass as a nutrient source, studies
on their digestive mechanism are expected to speculate a novel rule in wood
biomass processing. Here, the relationships of inhabitant bacteria involved in
carbon and nitrogen metabolism in the intestine of beetle larvae, an insect
model, are investigated. Bacterial analysis of larval feces showed enrichment
of members of which could include candidates for plant growth promotion,
nitrogen cycle modulation, and/or environmental protection. The abundances of
these bacteria were not necessarily positively correlated with the abundance in
the habitat, suggesting that they might be selectively enriched in the
intestines of larvae. Further association analysis predicted that carbon and
nitrogen metabolism in the intestine was affected by the presence of the other
common bacteria, the populations of which were not remarkably altered in the
habitat and feces. Based on hypotheses targeting these selected bacterial
groups, structural estimation modeling analyses statistically suggested that
their metabolism of carbon and nitrogen and their stable isotopes, {\delta}13C
and {\delta}15N, may be associated with fecal enriched bacteria and other
common bacteria. In addition, other causal inference analyses, such as causal
mediation analysis, linear non-Gaussian acyclic model (LiNGAM), and
BayesLiNGAM, did not necessarily affirm the existence of prominent bacteria
involved in metabolism, implying its importance as the bacterial groups for
metabolism rather than a remarkable bacterium. Thus, these observations
highlight a multifaceted view of symbiotic bacterial groups utilizing carbon
and nitrogen from wood biomass in insect larvae as a cultivator of potentially
environmentally beneficial bacteria.
