Subterranean synergies: termite bacterial diversity and eugenol-mediated selective dysbiosis

Subterranean termites, Coptotermes formosanus and Reticulitermes flavipes (Isoptera: Rhinotermitidae) rank among the most economically significant wood-feeding pests, relying on complex symbiotic associations with gut microbes to facilitate lignocellulose digestion, nitrogen fixation, and other essential metabolic processes. Although their bacterial communities have been individually described, direct comparisons and the effects of plant-derived bioactive compounds on these symbioses remain poorly understood. Here, we present the first comparative analysis of bacterial communities in these two termite species under phytochemical stress induced by eugenol, a phenolic monoterpenoid with known insecticidal and antimicrobial properties. Using 16S rRNA amplicon sequencing, we demonstrate that although the two species harbor distinct bacterial assemblages, they share a conserved core microbiota dominated by Spirochaetota. C. formosanus harbored a higher relative abundance of Bacteroidota, whereas R. flavipes exhibited prevalence of Firmicutes, Elusimicrobiota, and Actinobacteria. Despite these differences, both species shared a core bacterial community dominated by Spirochaetota. Eugenol exposure resulted in significant termite mortality and induced taxon-specific shifts in bacterial composition without altering overall community diversity, indicating a selective restructuring rather than a broad-spectrum disruption of the termite bacteriome. Specifically, eugenol decreased the abundance of Spirochaetota, particularly the genus Treponema, while enriching Firmicutes and Proteobacteria. This pattern of selective dysbiosis indicates a mechanistic shift away from non-specific antimicrobial effects, underscoring targeted microbial restructuring as a key ecological consequence of eugenol exposure. Moreover, PICRUST2-based predictions indicated that eugenol treatment alters microbial functional potential, including pathways associated with carbohydrate metabolism, fermentation, and amino acid biosynthesis, suggesting that eugenol selectively interferes with key symbiotic functions critical to termite survival. These findings demonstrate species-specific differences in termite-associated bacterial assemblages and highlight the potential of eugenol to selectively disrupt functionally important microbial taxa, providing a foundation for microbiome-targeted, environmentally sustainable termite control strategies.