Effect of Earthworm Digestion on Abundance, Composition and Diversity of Bacterial Pathogens in Sewage Sludge from Wastewater Treatment Plants

The increased production of sewage sludge is a major environmental concern as the sludge contains hazardous components, particularly human bacterial pathogens (HBPs). Transit of sewage sludge through the earthworm gut reduce or even eliminate HBPs and modify bacterial taxonomic and functional composition. However, it is unclear whether the effect is general or dependent on the type of sewage sludge involved. We characterized the taxonomic and functional profiles of bacterial assemblages in sewage sludge from different wastewater treatment plants (WWTPs), before (sludge) and after earthworm gut transit (casts). We found that composition and diversity of both taxonomic and functional bacterial communities of sludge and casts were significantly different. However, these differences varied among WWTPs with both increases and decreases in composition and diversity after gut transit. Interestingly, most bacterial taxa present in earthworm casts were not detected in the original sewage sludge. All sludge samples initially contained low levels of HBPs, which were significantly reduced or eliminated in earthworm casts. Nevertheless, gut transit increased the abundance of some HBPs. Further studies should determine whether vermicomposting effectively eliminates these HBPs and whether the differences in earthworm cast bacterial communities, which are dependent on the sewage sludge source, persist in the final vermicompost.