Disinfection

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 10C
  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 72C for 20 seconds
• Heating to 80C 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 (75C 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 121C _at_ 15 psi for 15 min. or
  121C _at_ 30 psi for 3 min
• Need to maintain autoclaves and monitor
  effectiveness
• Temperature pressure charts
• Chemical indicators (Brownes 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

18
(No Transcript)
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 lt24 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

24
(No Transcript)
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 (gt 130 cases)
• implantation of contaminated grafts from humans
  (cornea, 3 cases dura mater, gt 110 cases)

26
The Problem of CJD and TSEs

• Current advice on decontamination
• Incinerate instruments used on known cases
• Quarantine instruments used on suspected cases
• Improved need for instrument tracking

27
Summary

• Definitions
• Physical methods
• heat (wet heat with pressureautoclaving),
  filtration, irradiation
• Chemical methods
• Halogens, alkylating agents, EtOxide, alcohols,
  etc.
• Problem of CJD