The Role of Earthworm Gut-Associated Microorganisms in the Fate of Prions in Soil

The effect of earthworm gut (species Lumbricus terrestris, Aporrectodea caliginosa, and Eisenia fetida) on transiting bacterial community was studied using 16S rRNA-based techniques. CFB bacteria (classes Flavobacteria and Sphingobacteria) were considered as facultative gut-associated. Newly detected family Lumbricoplasmataceae within the class Mollicutes (Firmicutes) was proposed to be obligate earthworm-associated bacteria. Firmicutes and Gammaproteobacteria strongly reduced their diversity and relative number upon the passage being gut-sensitive bacterial groups, although the genus Pseudomonas (Gammaproteobacteria) were gut-resistant members of community. Other bacterial groups have varied number; unclassified Spingomonadaceae (Alphaproteobacteria) and Alcaligenes faecalis (Betaproteobacteria) represented the gut-resistant part of their populations. No earthworm host-specific effect was observed. Up to 20% of bacterial species isolated from the soil and earthworm sources were able to digest recPrP in pure culture; Gammaproteobacteria, Actinobacteria, and Bacilli were the taxa with the biggest potential to deplete the recPrP. Most of studied fungal isolates degraded recPrP. The prion was demonstrated to be depleted in vitro in aqueous extracts of the soil and the cast within 2-6 days. Non-specific proteolytic activity strongly increased from soil substratum to the cast through the trypsin- and chymotrypsin-like proteases released by the earthworm. However, the passage through the gut did not promote any enhanced recPrP digestion. Thus, under applied conditions the microbial-earthworm gut systems do not produce proteases de novo, which notably affect the prion proteolysis.