Prions are unique infectious agents responsible for fatal neurodegenerative disorders known as Transmissible Spongiform Encephalopathies (TSEs), such as Creutzfeldt-Jakob disease in humans and scrapie in sheep. Unlike bacteria or viruses, prions are composed solely of a misfolded protein and contain no genetic material like DNA or RNA. This protein-only structure grants prions extraordinary resistance to standard sterilization and decontamination methods, including chemical disinfectants and routine heat sterilization. Highly specialized and aggressive destruction protocols are necessary to neutralize prions and prevent the accidental transmission of these diseases through contaminated medical instruments and materials.
Understanding Prion Resistance
Prions are difficult to destroy because they lack a metabolism or genome that can be targeted by standard sterilization techniques. Techniques using ultraviolet light, alcohol, or routine autoclaving are designed to disrupt the cell walls or nucleic acids of conventional pathogens, but prions are immune to these effects. The infectious prion protein, known as PrPSc, is an abnormally folded version of a normal protein (PrPC).
This misfolded form is characterized by a high concentration of $\beta$-sheet structures, which provide immense stability and resistance to degradation. This tight structure prevents the protein from being easily broken down by common enzymes (proteases) or unfolded by heat or chemicals. For instance, prions can survive exposure to 70% alcohol and formaldehyde, a chemical often used to preserve biological tissues. The remarkable stability of the PrPSc structure is the central reason why specialized, harsh protocols are required for inactivation.
Established Protocols for Inactivation
Prion destruction requires extreme measures combining aggressive chemical exposure with intense thermal treatment to destabilize the resistant protein structure. Chemical protocols include using sodium hypochlorite (bleach) at 20,000 parts per million (ppm) available chlorine for one hour. Alternatively, a strong alkaline solution of 1 to 2 Normal (N) sodium hydroxide (NaOH) can be used for a minimum of one hour at room temperature.
For heat-resistant items, steam sterilization (autoclaving) must be performed under significantly more severe conditions than those used for other pathogens. A standard method is the porous load steam sterilization cycle at 134°C for 18 minutes in a pre-vacuum sterilizer. Another option is the gravity displacement sterilizer at 121°C to 132°C for up to 60 minutes.
It is often recommended that these thermal processes be combined with an initial chemical treatment, such as soaking in sodium hydroxide, to achieve the highest possible level of inactivation, although this combination can be corrosive to instruments. These protocols require contaminated items to be kept moist, as dried prions that have tightly bound to surfaces are even more resistant to inactivation. The safest method for materials that cannot be reused is complete destruction by high-temperature incineration.
Specialized Application in Clinical and Laboratory Settings
Stringent prion inactivation protocols are implemented in environments where the risk of transmission is highest, such as hospitals and high-level biocontainment laboratories. In clinical settings, the primary concern is reprocessing reusable surgical instruments, especially those used in neurosurgery, ophthalmology, and spinal procedures. These procedures involve contact with high-risk tissues like the brain and spinal cord. Instruments used on patients with known or suspected prion disease must be clearly tagged and subjected to specialized decontamination before reuse.
A significant challenge in reprocessing is the complex design of some surgical tools, which impedes thorough cleaning and prevents chemical and thermal agents from reaching all contaminated surfaces. To mitigate this risk, many facilities use single-use disposable instruments for procedures involving high-risk tissues. For heat-sensitive medical devices that cannot withstand high-temperature autoclaving, alternative processes like hydrogen peroxide gas plasma sterilization are sometimes recommended.
Decision-making processes focus on minimizing the time instruments are allowed to dry. It is often recommended that instruments be immersed in water or a prionicidal detergent immediately after use to prevent prions from binding tightly to metal surfaces.
Advancements in Prion Destruction Research
Current research focuses on developing less corrosive and less energy-intensive alternatives to the harsh, established protocols. The use of enzymes, which specifically target and break down proteins, is a promising area of investigation. For instance, certain keratinase enzymes derived from bacteria have demonstrated the ability to degrade prions under moderate conditions, such as at 65°C for a short duration, especially when combined with a biosurfactant.
Researchers are also exploring novel chemical compounds and detergents that can destabilize the PrPSc structure without damaging delicate medical equipment. One example involves the study of acidic detergents, such as sodium dodecyl sulfate (SDS) in acidic solutions, which have shown potential to reduce infectivity when combined with autoclaving. Other experimental methods include the use of vaporized hydrogen peroxide, which is believed to unfold and fragment the prion protein.
While these novel methods show significant promise for safer and more practical decontamination, they are not yet standard practice. They continue to be rigorously tested to confirm their efficacy against all prion strains.