Earthworms Drastically and Differentially Modify the Bacteriomes and Mycobiomes of Sewage Sludge

Sewage sludge management poses major environmental challenges due to increasing production and concerns about contaminants and microbial risks. Vermicomposting offers a sustainable biological treatment, yet the extent to which different earthworm species shape microbial outcomes remains poorly understood. Here, we examined how gut transit by three epigeic (Eisenia andrei, E. fetida, and Dendrobaena hortensis) and two anecic (Lumbricus friendi and L. terrestris) earthworm species alters bacterial and fungal communities in fresh sewage sludge. Using 16S rRNA and ITS amplicon sequencing combined with multivariate, differential-abundance, and functional prediction analyses, we compared sludge and earthworm cast bacteriomes and mycobiomes. Earthworm gut transit caused pronounced species-specific restructuring of bacterial and fungal community composition, diversity, and functional profiles, with clear separation between sludge and cast communities. Functional analyses indicated coordinated shifts in bacterial metabolic potential and fungal trophic modes consistent with enhanced biosynthetic and decomposer functions. Pathogen profiles were reshaped in a host-dependent manner, with low overall abundances and selective changes rather than uniform suppression. These findings demonstrate that vermicomposting outcomes depend strongly on earthworm species and microbial kingdom, highlighting the importance of earthworm lifestyle diversity when evaluating the ecological safety and agronomic potential of sludge-derived amendments.