Disinfection Sterilization

1. Disinfection & Sterilization Professor Mark Pallen

2. Learning Objectives At the end of this lecture, the student should be able to Define disinfection and sterilisation Describe the common substances and processes used to achieve these outcomes Evaluate issues influencing choice of method

3. Definitions Cleaning process which physically removes contamination but does not necessarily destroy micro-organisms prerequisite before decontamination by disinfection or sterilisation of instruments organic material prevents contact with microbes, inactivates disinfectants Disinfection using an agent that destroys germs or other harmful microbes or inactivates them, usually referred to chemicals that kill the growing forms (vegetative forms) but not the resistant spores of bacteria

4. Definitions Antisepsis destruction of pathogenic microorganisms existing in their vegetative state on living tissue Sterilization any process, physical or chemical, that will destroy all forms of life, including bacterial, fungi, spores, and viruses

5. Methods Physical Heat Filtration Irradiation Quarantine Chemical Choice of method depends on practical issues such as ease of use or material compatibility Proctoscope need not be as free of contamination as an artificial heart valve Cleaning of objects needed before attempt at sterilization

6. Factors influencing ability to kill microbes Strength of the killing agent Time that the agent has to act Temperature of environment rate of microbe death doubles with every 10�C rise in temp. Type of microbe Environment around the area to be decontaminated Number of microbes to be killed

7. Physical Methods: Heat Advantages Non-toxic Quick Cheap Disadvantages Can only be used on heat-resistant materials No use for many plastics, electronics, tarnishes some metals

8. Physical Methods: Heat Pasteurisation First used with milk: 72�C for 20 seconds Heating to 80�C for 1 minute will kill most vegetative organisms Examples: bed-pan washer, proctoscope Dry heat (hot air oven) used on waxes, oils (wet heat usually preferred) Incineration the ultimate sterilization used for disposal of hospital waste Wet heat Boiling limited use as spores may be resistant, boilers may be misused Low temperature steam disinfection (75�C for 30 mins) Used for e.g. ventilator tubing Autoclaving High-tempoerature steam plus pressure (same principle as pressure cooker)

9. Autoclaving Requires steam penetration Cannot work on sealed containers (which can explode!) Risks of a pressure vessel Steam condenses on contact, releasing latent heat of evaporation Condensation leads to vaccum, draws in more steam

10. Autoclaving Best results if air excluded Downward displacement autoclaves Usually for lab use: steam displaces air through outlet Steam heated jacked used to help drying High pre-vacuum autoclaves Air excluded before steam enters

11. Autoclaving Need to establish correct cycle and hold times Typical settings: 121�C @ 15 psi for 15 min. or 121�C @ 30 psi for 3 min Need to maintain autoclaves and monitor effectiveness Temperature & pressure charts Chemical indicators (Browne�s tubes, Bowie-Dick test) Spore tests

12. Physical Methods Filtration Used on labile fluids and on air supplies Gamma-Irradiation Used on disposable plastics, e.g. in sealed packs Only in specialised centres

13. Chemicals Use depends on spectrum of antimicrobial activity and compatibility with materials Also limited by dangers of chemicals themselves Examples Halogens Alcohols Alkylating agents Ethylene oxide Phenolics cetrimide (QAC) chlorhexidine (diguanide)

14. Halogens Hypochlorites (household bleach) & chlorine Advantages active against viruses, spores, fungi Disadvantages inactivated by organic matter, freshness & pH critical (go off if diluted), corrosive to metals Practical Uses 0.1% hypochlorite used as general disinfectant Strong hypochlorite (0.25%) used in lab & on wounds Extra strong (1%) used on HBV blood spills Chlorine used to treat drinking water and control Legionella

15. Halogens Iodophors & iodine Advantages Some activity against viruses, spores, fungi Disadvantages inactivated by organic matter, can stain skin, irritant, expensive Practical Uses Pre-op skin disinfection Povidone iodine used as surgical scrub, as powder on ulcers

16. Alcohols Isopropanol & ethanol Advantages kill vegetative bacteria on clean surfaces in 30 seconds Disadvantages inactive against spores, fungi Inflammable Need to be at correct %age with water (65-80%) Practical uses Skin antisepsis before venepuncture Hand rubs Disinfection of e.g. trolley tops

17. Alkylating agents Glutaraldehyde and Formaldehyde Advantages Good activity against spores, virues, fungi Disdvantages Glutaraldehyde only moderately active against TB Need long exposure time for full effect (3 hours) freshness & pH critical TOXIC! Practical uses Disinfection of endoscopes Blood spills Fumigation

19. Ethylene oxide Highly toxic flammable gas, kills spores! Used for bulky items such as heart lung machines Can be used on glutaraldehyde-labile endoscopes Use limited by safety issues

20. Phenolics & QACs Clear soluble phenolics (e.g. Hycolin) used as disinfectant on soiled surfaces, relatively inactive against spores and viruses Hexachlorophane used as surgical scrub Quaternary ammonium compounds, e.g. cetrimide usually only used in combination with other agents; good detergent properties.

21. Chlorhexidine (a diguanide) Used as general purpose antiseptic for skin and mucous membranes in many formulations, e.g. Hibiscrub, Hibisol, Savlon Advantages: relatively non-toxic and good against S. aureus Disadvantages: can support growth of e.g. P. aeruginosa

22. Factors determining usefulness of chemical disinfection Spectrum of antimicrobial activity is it the right agent for the job? Used at correct concentration concept of 'in use concentration� diluted down from high concentration stored for <24 hours no topping up of old solutions

23. Factors determining usefulness of chemical disinfection Time of exposure You cannot disinfect an endoscope in 5 minutes glutaraldehyde! Correct pH? Inactivating materials Pus, blood vomit, cork, soaps etc Is disinfectant sterile? Many cases of Gram-negatives living in disinfectants! Microbiological �in-use� testing

25. The Problem of CJD and TSEs Creutzfeld-Jakob syndrome and other transmissible spongiform encephalopathies caused by highly resistant proteinaceous particles, prions can survive 3 years of environmental exposure and are unusually resistant to conventional decontamination methods Iatrogenic CJD documented in three circumstances use of contaminated medical equipment (2 cases) use of extracted pituitary hormones (> 130 cases) implantation of contaminated grafts from humans (cornea, 3 cases; dura mater, > 110 cases)

27. Summary Definitions Physical methods heat (wet heat with pressure=autoclaving), filtration, irradiation Chemical methods Halogens, alkylating agents, EtOxide, alcohols, etc. Problem of CJD