Enzymatic degradation of prion protein by keratinase producing proteolytic micro-organisms

Prions are highly resistant to common proteases and conventional sterilisation processes. Consequently, prion infectivity is destroyed by methods such as incineration, alkaline and thermal hydrolysis. These harsh, destructive and potentially hazardous methods are unsuitable for processing specified risk materials (SRM) and animal by-products, and in the decontamination of medical and laboratory devices, and prion contaminated environments. Thus an environmentally friendly, enzymatic degradation and
decontamination process is a highly desirable alternative. The structural similarity of prion and feather keratin suggests that feather degrading microorganisms have the potential to degrade prions. The objective of this research was to isolate and characterise microbial keratinase and to investigate its ability to degrade ME7 scrapie prion. Thirty two microbial strains were isolated on feather meal agar from primary effluent and farmyard wastes. One of the isolates, a Grams positive bacterium, demonstrated ignificant keratinolytic activity (11.00 ± 0.71 U/ml), and was investigated further. The isolate was identified by 16S rDNA and designated as Bacillus licheniformis N22, and was deposited in National Collection of Industrial Food and Marine Bacteria (NCIMB). The growth conditions for optimum
keratinase synthesis in a minimal growth medium (MGM) were found to be pH 8.5, 50°C, 1.1 % (w/v) feather meal substrate and at incubation time of 32h. The molecular weight of purified keratinase was �28 KDa as measured by SDS-PAGE and confirmed by MALDITOF-MS. Optimum keratinase activity was obtained at pH 8.5 and 50°C. This keratinase
fully degraded recalcitrant melanised feather in 48h, and also digested ME7 scrapie prion at 65°C in 2h to levels of PrPSc undetectable by western blot analysis. In a remarkable synergistic enzymatic preparation composed of keratinase and biosurfactant derived from Pseudomonas aeruginosa NCIMB 8626, ME7 scrapie prion was degraded to undetectable levels at 65°C in 10 min. Interestingly biosurfactant alone showed no detectable activity on ME7 scrapie prion. Time-course degradation analysis showed progressive attenuation of PrPSc signal at 50°C over time. Test of residual infectivity by standard cell culture assay
showed that this enzymatic method completely destroyed standard sheep scrapie prion (SSBP/1) at 65°C in 1h. The mean survival time of mice challenged with enzyme-
digested inocula significantly increased from 278 ± 9 days to 334 ± 42 days compared to those inoculated intraperitoneally with neat ME7 scrapie (p = 0.008 at 95% confidence interval). Furthermore, 47% of all the mice in enzyme-digested group lacked detectable levels of PrPSc . These results suggest a substantial reduction in the infectious titre or complete destruction of ME7 prion infectivity by the enzymatic preparation. Therefore, this mild enzymatic treatment method has potential applications for prion decontamination.

Thesis submitted to Middlesex University London, UK in partial fulfilment of the award of Doctor of Philosophy (PhD) Degree in Environmental Science.