Evidence for proteolysis of a recombinant prion protein in a lamb brain-amended loamy soil

Soils contaminated by prions, the infectious agents responsible for transmissible spongiform encephalopathy diseases, remain infectious to grazing animals for many years. In this study, the ability of enzymes produced by soil microbes to degrade a recombinant prion protein (recPrP) was investigated in a loamy soil. A 15N‐labelled recPrP was added to soil in which microbial biomass and soil proteolytic activity had been increased by either simultaneous or prior amendment with lamb brain, and distribution of 15N among soil solid particles, soluble molecules and bacterial biomass was determined. After 1 day the proportions of recovered recPrP‐N associated with microbial biomass and soluble molecules were 6–9 and 15–19%, respectively, which is consistent with the hypothesis of degradation. A greater incorporation of 15N‐derived β‐sheeted recPrP into the microbial biomass pool occurred when the soil proteolytic activity was pre‐stimulated by a lamb brain amendment, suggesting that the recPrP degradation in soil is mediated by the activity level of proteolytic enzymes produced by the microbial biomass. The majority (35–87%) of the recovered recPrP‐N was associated with the soil particles. An observed partial degradation of recPrP deposited on a mica surface by soil soluble enzymes indicated a sorption‐related resistance to proteolysis. In conclusion, integration of the stimulation and turnover of the soil microbial component, after an input of a large amount of animal organic matter with the sorption properties of prion protein, is required to model and predict prion survivability, transformation and transmissibility in soil.