Gut-associated bacteria and their roles in wood digestion of saproxylic insects: The case study of flower chafer larvae

Abstract

Protaetia acuminata (Fabricius, 1775) (Coleoptera: Scarabaeidae) is widely distributed throughout Southeast Asia and plays a significant role in nutrient cycling by facilitating the decomposition of woody materials, a process that likely relies heavily on the contribution of symbiotic bacteria within their digestive system. However, their gut bacteria have not been thoroughly studied. By using V3-V4 amplicon sequencing, it was revealed that the midgut (MG) of Pr. acuminata larvae and fermented sawdust after rearing (FSD) share a similar microbial community, predominantly composed of Proteobacteria and Actinobacteriota, as well as functional genes associated with cellulolysis, nitrogen respiration, nitrate reduction and aerobic chemoheterotrophy. In contrast, the bacterial community in the hindgut (HG) was distinctly different, with anaerobic respiration being the dominant metabolic process. Agromyces, Altererythrobacter, Bacillus, Cellulomonas, Lysinibacillus, Pseudoxanthomonas and the family Promicromonosporaceae were the most common genera in MG, HG and FSD samples. The culture-based isolation method yielded 67 isolates from the larvae, with gram-positive bacteria predominating in HG and MG, whereas gram-negative bacteria were primarily found in the FSD. These microorganisms produce a range of lignocellulolytic enzymes including β-endoglucanase, laccase and xylanase that enable the beetles to digest their plant-based diet efficiently and also involve many biochemical pathways relating to biogeochemical cycling. Our results provide valuable insights into the gut-associated Pr. acuminata flower chafer larvae and could serve as a basis and reservoir for future studies on lignocellulolytic enzyme-producing bacteria.