Parasite-associated microbiomes in Ascaris and Trichuris nematodes: current evidence, challenges, and future directions

Soil-transmitted helminth (STH) parasites, particularly Ascaris lumbricoides and Trichuris trichiura, co-exist and co-evolve with a complex human microbiota within the gut ecosystem. Most research has focused on how parasitic nematodes affect the host gut microbial structure and its consequences. In contrast, studies on the parasites' own associated microbiota remain underexplored. Understanding this gap is crucial, as it adds another layer of complexity to holobiont-holobiont interactions. Driven by the Parasite Microbiome Project, this review summarizes current evidence on Ascaris- and Trichuris-associated microbiomes from both natural and experimental studies, with a focus on their compositions, potential functional roles, challenges, and future directions. Existing findings are largely based on microbiomes associated with the gut of the parasites and are often characterized by transient, host-derived communities acquired through horizontal transmission, which may contribute to parasite survival and host interaction. We therefore emphasize the need to expand investigations to other specific tissues, particularly reproductive organs, as potential sites of a stable and conserved microbiota in STH parasites. Understanding this aspect is essential for advancing our knowledge of parasite-microbiome-host interactions in STH infections and may uncover the potential of microbiome-based therapeutic interventions for STH control and prevention.