Comparative Analysis of Gut Microbiomes in Parasitic Roundworms Reveals Phylogeny-Associated Community Structure and Functional Adaptation.

Roundworm nematodes are globally distributed zoonotic parasites that inhabit the intestinal tract of various mammals. Although these parasites reside in the host's guts, their own intestinal ecosystems remain poorly understood. Recent evidence suggests that helminths may harbor distinct gut microbiomes that contribute to their physiology and host interactions, yet cross-species comparisons are lacking. Here, we performed full-length 16S rRNA sequencing to characterize and compare the gut microbiomes of four major roundworm species-Ascaris suum (As), Baylisascaris schroederi (Bs), Toxocara cati (Tc), and Toxocara vitulorum (Tv). Across 38 individual worms, we identified 359 bacterial taxa dominated by Enterobacteriaceae, with Escherichia coli, Salmonella enterica, and Klebsiella pneumoniae forming a conserved core community. Despite this compositional similarity, beta-diversity and hierarchical clustering analyses revealed that microbial community structure was primarily determined by parasite phylogeny and roundworm sex, not host diet. Functional prediction using PICRUSt2 indicated clear species-specific enrichment in metabolic pathways, such as carbohydrate metabolism in Bs and xenobiotic metabolism in As, reflecting adaptive divergence of microbial functions. Collectively, these findings demonstrated that roundworm gut microbiomes exhibited taxonomic conservation but functional specialization, shaped by the evolutionary history of the parasites themselves. This study established a conceptual framework viewing the parasite as the primary host of its microbiome and provided new insights into the co-evolutionary relationships between helminths and their symbiotic bacteria.