Lecture 1, 2, 3 Sterilisation and Disinfection

Question 1

What is sterilisation

Answer 1

Complete elimination or destruction of all forms of life

Question 2

What is disinfection

Answer 2

Reduces or eliminates most microbes but not spores or other resistant bacteria, fungi or viruses

Question 3

What is disinfectant

Answer 3

Destroys or inhibits growth of organisms on inanimate surfaces

Question 4

What is decontamination

Answer 4

Removal of organisms

Question 5

Properites of ideal disinfectant (7)

Answer 5

1. Rapid activity against wide range of microbes
2. Low/no toxicity
3. No damage to surfaces or unwanted toxic residues
4. Soluble in water
5. Not affected too much by organic materials
6. Safe and environmentally friendly
7. Stability yet readily abilty to be broken down

Question 6

Meaning of death for bacteria and factors which affect its measurement

Answer 6

Death means irreversible loss of ability to reproduce, factors include medium (solid/liquid), defined (minimal or complex), incubation period (time and temperature)

Question 7

Define sterilization by moist heat

Answer 7

Moist heat (autoclave) is safer than dry heat (oven), in autoclaving the water disrupts the H-bonds between peptide groups, requires a lower T than dry heat and kills more efficiently. Uses steam to sterilise

Question 8

Define sterilzation by dry heat

Answer 8

Requires a higher T than moist heat. Sterilzation is by an oxidative process and not recommended apart from materials that can’t be autoclaved. (powders)

Question 9

Describe the 4 parameters of autoclaving

Answer 9

1. Steam - must displace all air and requires time to heat up before timing of autoclaving commences
2. Pressure - requires high pressure
3. Temperature - 121 degrees celcius
4. Time - 15 minutes

Question 10

Define sterilisation by plasma

Answer 10

Plasma consists of H2O2 whose molecules have been excited by additional energy (charged nuclei, free radicals and free electrons.
Plasma can’t be used with molecules that absorb, decompose or react with H2O2.

Question 11

Define ionising radiation

Answer 11

X-rays cause substances to ionise (free radicals) which inactivate many molecules (DNA), however it takes hours, penetrates glass and other materials and suitable for large scale use only.

Question 12

Define chemical sterilisation by ethylene oxide or peracetic oxide

Answer 12

Ethylene oxide inactivates microbes by alkylation of proteins, DNA or RNA. However, it is toxic, doesnt inactivate prions, is a gas at RT and is flammable (mixed with CO2)

Question 13

Define disinfection through pasteurisation

Answer 13

Pasteurisation is disinfection by heat (milk, wine). Decreases count by 97% but doesnt sterilise. Effective because milk pathogens don’t form spores. Pasteurisation is safe, reliable, cheap and no microbial resistance

Question 14

Define disinfection through filtration (membrane or depth)

Answer 14

Filtration is used for heat sensitive liquids, prevents particles entering with liquid

Question 15

Define disinfection through UV irradiation

Answer 15

UV light is non-ionising which alters electron orbits within atoms. Used to disinfect water supplies. Kills cells by forming pyrimidine dimers in bacterial DNA. However bacteria have enzymes which can repair UV damage. UV light has low penetrating power and organic matter can protect bacteria

Question 16

Define disinfection by alchohol

Answer 16

Alcohol (usually ethanol) used to wipe down benches, kills in 30s, has no activity against spores and is flammable

Question 17

Define disinfection by halogens

Answer 17

Halogens (bleach) used to clean blood spills, fast acting, non-flammable but inactivated by organic matter and unstable and corrosive.

Question 18

Define disinfection by peroxygen compounds

Answer 18

Peroxygen compounds (virkon, peracetic acid and H2O2)

Question 19

Define refrigeration and freezing for control of bacteria

Answer 19

Refrigeration is used to extend shelf life of substances and some bacteria can replicate in refrigeration
Freezing is used to preserve substances and bacteria, repeated freeze/thaw cycles reduces bacteria count. Glycerol is used to protect bacteria during freezing

Question 20

Define hand-washing for control of bacteria

Answer 20

Hand-washing can terminate outbreaks of bacteria out of lab, reduces overall infection rates and plain soap used.

Question 21

What are the 6 factors that affect efficacy of disinfectants

Answer 21

1. Concentration of disinfectant - higher conc more sterile
2. Organic matter - can block disinfectant
3. Temperature - Activity increases with increased T
4. pH - Anionic compounds more active at low pH, cationic compounds more active at low pH
5. Drying - May encase bacteria in crystals and protect them
6. Organism - Spores more R than vegetative cells, prions most difficult to inactivate

Question 22

What are the two sterility check procedures

Answer 22

Monitor product status by microbiological assessment of sterilized item or material itself
Monitor sterilization process by mechanical, chemical and biological means

Question 23

Define product testing

Answer 23

Product testing - A negative culture result only indicates that no organisms were detected

Question 24

Define process monitoring

Answer 24

Process monitoring - either physical monitoring of process (thermometers, pressure gauges and gas analysis devices) or chemical which uses indicators to monitor conditions in dry heat, steam, ethlyene oxidiation and ionizing radiation.