Microbial and enzymatic inactivation of prions in soil environments

Transmissible spongiform encephalopathies (TSEs) are fatal, infectious neurodegenerative diseases that affect humans and other mammals. The etiological agent in TSEs is the prion and is thought to be composed of aggregated, β-sheet-rich conformers of the prion protein (PrPTSE) derived from misfolding of a benign cellular form of the same protein. Prions are remarkably resistant to inactivation and can persist and remain infectious in the environment for years. Environmental routes of TSE transmission are implicated in chronic wasting disease (CWD) of deer, elk and moose and in scrapie of sheep and goats. Soil is thought to be an important reservoir of CWD and scrapie infectivity. The extent to which prions are inactivated by natural processes in soils is unclear, and methods for inactivating TSE agents in soil are currently lacking. This critical review discusses current knowledge on the degradation of PrPTSE and inactivation of prions by individual bacterial species, mixed microbial consortia, and isolated bacterial and archaeal enzymes, and highlights areas warranting further research. Research conducted to date indicates that few microorganisms are able to degrade PrPTSE. Fungi have not been evaluated for their ability to inactivate TSE agents. Of proteases reported to degrade PrPTSE, most are serine proteases and many require harsh conditions (e.g., elevated temperature, high pH, detergent) for optimal PrPTSE-degrading activity. Declines in the levels of PrPTSE are not always accompanied by equivalent decreases in TSE infectivity. Most biodegradation studies reported to date did not examine effects on TSE infectivity and must therefore be considered inconclusive with respect to the likely impact on prion disease transmission. Improved understanding of the factors affecting prion inactivation is required to discern natural routes of prion inactivation or implement effective practices outside of the laboratory.

► Environmental contamination contributes to the spread of some prion diseases.
► Prions are notoriously difficult in inactivate.
► Relatively few microorganisms are capable of degrading prions.
► High temperatures and pH extrema are often needed for enzymatic prion degradation.
► Serine proteases appear to be leading candidates for prion biodegradation.