1.6 Disinfection & Sterilisation

Question 1

Person-to-person transmission of microorganisms is extremely common. What are the routes of transmission?

Answer 1

1. Direct contact with contaminated tissues (blood, body fluids etc)
2. Indirect contact with contaminated instruments / surfaces (dental probe)
3. Airborne droplets

Question 2

What are the common sites of entry for transmission?

Answer 2

1. Body orifices
2. Breakage of skin
3. Inhalation
4. Mucosal surfaces

Question 3

What are the 4 infection control strategies, with examples?

Answer 3

1. Immunisation (Hep B vaccine)
2. Barrier (gloves)
3. Isolation (dirty + clean areas)
4. Disinfection / sterilisation (handwashing)

Question 4

What is the difference between antisepsis, sanitisation, disinfection, and sterilisation?

Answer 4

Antisepsis = the destruction of microorganisms on living tissue

Sanitisation = the reduction of microbe population on an object to safe by public health standards

Disinfection = the process of eliminating microorganisms on objects, except bacterial spores

Sterilisation = the physical or chemical process that completely destroys ALL forms of viable microorganisms from an object

Question 4

What is the difference between biocidal and biostatic?

Answer 4

Biocidal = action that kills microorganisms (irreversible)

Biostatic = action that inhibits growth (reversible on removal of agent)

Question 5

Microbes have varying resistances to biocides. Which are the least and most resistant and what are their disinfection/sterilisation requirements?

Answer 5

1. Low level disinfection = Vegetative bacteria, lipid or medium sized viruses, and fungi
2. Intermediate level disinfection = Nonlipid or small viruses
3. High level disinfection = Mycobacteria
4. Sterilisation = bacterial spores (most resistant)

Question 6

What are the mechanisms of microbicidal activity?

Answer 6

1. Cell membrane disruption
2. DNA damage (degradation, oxidation, alkylation, thymidine dimer)
3. Protein damage (denaturation, oxidation, crosslinking, alkylation)
4. Lipid oxidation

Question 7

What are physical sterilisation methods?

Answer 7

1. Dry heat (160C for 2 hours)
2. Wet heat (121C for 10-12 mins + heating OR 132C for 3 mins + heating)
3. Radiation (260nm gamma irradiation)
4. Filtration

Question 8

Chemical sterilisation can be either gas or liquid. What are the different gases and liquids that can be used?

Answer 8

Gases = Ethylene oxide or Hydrogen peroxide vapour

Liquids = 2% glutaraldehyde, 37% formaldehyde (fromalin), OR hydrogen peroxide (7.5-25% for 1 hr to 30 mins)

Question 9

What are the 3 ways to monitor the sterilisation process?

Answer 9

1. Mechanical = temperature + pressure sensing and timing instruments
2. Chemical = assess conditions via chemicals
3. Biological = best method, directly measures surviving microorganisms

Question 10

What are the factors affecting efficacy of disinfection and sterilisation procedures?

Answer 10

Number, resistance, and access of microorganisms

Concentration + potency of agent

Phsycial + chemical factors (temp, pH etc)

Duration of exposure

Question 11

What are the two physical disinfection methods?

Answer 11

1. Boiling (10 mins)
2. Pasturisation (72C for 16s)

Question 12

What are chemical disinfection methods

Answer 12

1. Ethylene oxide gas
2. Aldehydes
3. Stabilised H2O2
4. Halogens
5. Phenolics
6. Alcohols eg ethanol
7. Quaternary ammonium compounds
8. Bis Biguanides (chlorhexidine)
9. Heavy metals