Disinfection and Sterilisation
⭐ The four terms ★★★
The lecture's summary slide lists these four and nothing else by name. Learn them verbatim — the discriminating clause in each is what earns the marks.
| Term | Definition |
|---|---|
| Sterilisation | The process of killing or removing ALL living microorganisms, including endospores. The criterion of sterilisation is to kill spores. |
| Disinfection | The process that reduces the number of pathogenic microorganisms — but not necessarily bacterial spores and other resistant microorganisms — on objects or in materials, to a level which is not harmful to health. |
| Asepsis | A condition in which living microorganisms are absent. |
| Aseptic technique | Technique to prevent bacteria from entering the body or other materials. |
Sterilisation and disinfection are not two points on a scale of thoroughness. They are separated by one organism-state: the endospore.
Sterilisation is absolute — the criterion is killing spores, because the spore is the most resistant thing in microbiology. Kill it and by definition you have killed everything else.
Disinfection is relative — it reduces pathogens to a level not harmful to health, and explicitly does not promise to touch spores.
So when the paper asks why an autoclave and not boiling: boiling at 100 °C kills vegetative cells but not spores, so it disinfects. The autoclave reaches 121 °C under pressure, kills spores, and therefore sterilises. One word of the definition decides which instrument you may use for surgery.
- Define sterilisation. → The process of killing or removing ALL living microorganisms including endospores; the criterion is to kill spores
- Define disinfection. → Reducing the number of pathogenic microorganisms — not necessarily spores or other resistant organisms — to a level not harmful to health
- What single thing separates them? → Whether spores are killed
- Define asepsis and aseptic technique. → A condition in which living microorganisms are absent; and the technique preventing bacteria from entering the body or other materials
Physical methods — heat ★★★
The three families of antimicrobial method are physical, chemical and biological. The physical methods are heat, radiation, filtration and low temperature.
| Dry heat | Moist heat |
|---|---|
| Incineration | Boiling |
| Flaming | Pasteurisation |
| Hot-air sterilisation | Steam sterilisation at atmospheric pressure (flowing steam) |
| Infrared rays | Intermittent sterilisation |
| Steam sterilisation under high pressure — autoclaving |
⭐ The three sets of numbers — Section II blank material
| Method | Conditions | Significance and application |
|---|---|---|
| Hot-air sterilisation | 160–170 °C for 1–2 hours | Glassware, porcelain, metal objects |
| Pasteurisation — a process using mild heat to reduce the microbial level in heat-sensitive materials | 62 °C for 30 min, or 72 °C for 15 seconds | Kills vegetative pathogens (not spores) · milk, beer, wine |
| Autoclaving | 15 lb/in² (1.05 kg/cm²), 121 °C, 15–20 minutes | Kills both spores and vegetative microbes · heat-tolerant materials |
Why moist heat beats dry heat
- Moist heat is generally more effective than dry heat
- Hot steam more easily denatures proteins
- Hot steam has strong penetration
- Hot steam sends out latent heat
Line the three up and the pattern is unmistakable.
Pasteurisation, 62 °C — kills vegetative pathogens. That is disinfection, and deliberately so: heat the milk any harder and you ruin it. The aim was never sterility.
Autoclaving, 121 °C under pressure — kills both spores and vegetative microbes. That is sterilisation.
Hot air, 160–170 °C for 1–2 hours — also sterilisation, but look at the cost: forty degrees hotter and up to eight times longer, for the same result. That is the four reasons above made quantitative. Dry heat only conducts; steam condenses on the surface, releasing its latent heat exactly where it is needed, and water makes protein denature far more readily.
So why use hot air at all? Because steam ruins glassware, powders and oils. The method follows the material.
- Name the dry-heat methods. → Incineration, flaming, hot-air sterilisation, infrared rays
- Name the moist-heat methods. → Boiling, pasteurisation, flowing-steam sterilisation, intermittent sterilisation, autoclaving
- Autoclave conditions? → 15 lb/in² (1.05 kg/cm²), 121 °C, 15–20 minutes — kills spores and vegetative microbes
- Pasteurisation conditions and purpose? → 62 °C/30 min or 72 °C/15 s; kills vegetative pathogens in milk, beer and wine
- Hot-air sterilisation conditions? → 160–170 °C for 1–2 hours; glassware, porcelain, metal
- Why is moist heat better? → It denatures proteins more easily, penetrates strongly, and releases latent heat
Radiation, filtration and low temperature ★★★
| Radiation | Detail |
|---|---|
| Ionising radiation | Gamma ray — high penetration |
| Ultraviolet (UV) | 240–300 nm; most effective wavelength 265 nm. Mechanism: destroys bacteria by forming thymine dimers. Characteristics: poor penetration; damages eyes and skin. Applications: air disinfection and surface disinfection |
| Microwaves | Kill indirectly by heat; not very effective |
UV damage to DNA depends on three factors: wavelength, intensity and duration.
| Method | Detail |
|---|---|
| Filtration | Mechanism: mechanically removes microorganisms. Application: heat-sensitive liquid materials — serum, toxin, antibiotics. Pore size 0.45 µm and 0.22 µm |
| Low temperature | Preserves microorganisms — it does not kill them |
Filtration does not sterilise against viruses. A 0.22 µm filter removes bacteria and fungi, but most viruses are far smaller and pass straight through — which is why the original definition of a virus was filterable agent, as the virology units will say. It is also why filtration is used for serum and antibiotics: those materials would be destroyed by heat, and the filter is the only option.
UV's weakness is its only real limitation. Poor penetration means it disinfects the surface the light strikes and nothing beneath — no use inside a wrapped instrument, excellent for an empty operating theatre or a laminar-flow cabinet. And because 265 nm damages DNA, it damages your DNA too: the lamp goes off before anyone enters.
Note also that low temperature preserves. Freezing is how the laboratory stores organisms, not how it kills them.
- What is the most effective UV wavelength, and the mechanism? → 265 nm; it forms thymine dimers in DNA
- UV characteristics and applications? → Poor penetration, damages eyes and skin; used for air and surface disinfection
- What three factors determine UV damage? → Wavelength, intensity, duration
- Filtration — mechanism, use, pore size? → Mechanically removes microorganisms; heat-sensitive liquids such as serum, toxin and antibiotics; 0.45 and 0.22 µm
- What does low temperature do? → Preserves microorganisms — it does not kill them
Chemical methods ★★
| Mechanism | Example |
|---|---|
| Destroy cell membranes | Surfactants/detergents — dissolve the lipid bilayer |
| Denature bacterial proteins | Enzymes — interfering with bacterial metabolism |
| Damage genetic material | Formaldehyde — DNA, RNA |
Commonly used chemical agents
- Soaps and detergents
- Phenol
- Alcohols
- Heavy metals — silver nitrate (1%)
- Chlorine
- Iodine
- Aldehydes — 37% formaldehyde (formalin)
- Dyes — crystal violet
- Name the three chemical mechanisms with an example of each. → Destroy cell membranes (detergents) · denature proteins (affecting enzymes and metabolism) · damage genetic material (formaldehyde)
- Name eight chemical agents. → Soaps and detergents, phenol, alcohols, heavy metals (1% silver nitrate), chlorine, iodine, aldehydes (37% formaldehyde = formalin), dyes (crystal violet)
Biological methods ★
- Antibiotic — a microbial product that inhibits or kills other microorganisms
- Bacteriocin — an agent produced by a bacterium that inhibits or kills closely related species, or even different strains of the same species ⭐ 2019 Section I term
- Bacteriophage — a virus that infects a bacterium
All three were defined in earlier units — antibiotic and bacteriocin in Unit 3 §5, bacteriophage in Unit 4 §2. They reappear here because the lecture classes them as antimicrobial methods, which is the framing a Section III question would use.
- Name the three biological antimicrobial methods. → Antibiotic, bacteriocin, bacteriophage
- Define bacteriocin. → An agent produced by a bacterium that inhibits or kills closely related species or strains
Factors influencing antimicrobial activity ★★
- The concentration and kind of agent used
- The length of exposure to the agent
- The temperature and pH at which the agent is used
- The number and kinds of microorganisms
- The organic matter bearing the microorganism — blood, sputum, pus, vomit, excreta, biofilm
The first four are obvious. The fifth is the one that fails in practice.
Organic matter — blood, pus, sputum — physically shields organisms from the agent and chemically inactivates it. Pour disinfectant onto a soiled instrument and much of it is consumed by the soil before it reaches a bacterium.
Biofilm is the worst case: a polysaccharide matrix that the agent cannot penetrate, on catheters, prostheses and endoscopes.
Which is the reason every decontamination protocol in every hospital says the same thing: clean first, then disinfect. Not thoroughness for its own sake — the disinfection step simply does not work otherwise.
- Name the five factors influencing antimicrobial activity. → Concentration and kind of agent · length of exposure · temperature and pH · number and kinds of microorganisms · organic matter bearing them
- Name the organic matters listed. → Blood, sputum, pus, vomit, excreta, biofilm
- Why must instruments be cleaned before disinfection? → Organic matter shields organisms and inactivates the agent; biofilm is impenetrable
Revision
The lecture's own summary slide
- Terms: disinfection, sterilisation, asepsis, aseptic technique
- High temperatures are used to inhibit microbial growth
- Moist heat is more effective than dry heat
- UV light is suitable for surface and air disinfection
- Many factors influence the efficacy of antimicrobial agents
Every number in one place
| Method | Numbers |
|---|---|
| Autoclave | 121 °C · 15 lb/in² (1.05 kg/cm²) · 15–20 min — kills spores |
| Hot air | 160–170 °C · 1–2 h — glassware, porcelain, metal |
| Pasteurisation | 62 °C/30 min or 72 °C/15 s — vegetative pathogens only |
| UV | 240–300 nm; optimum 265 nm — thymine dimers |
| Filter | 0.45 µm and 0.22 µm — heat-sensitive liquids |
| Silver nitrate | 1% |
| Formalin | 37% formaldehyde |
- Define sterilisation and disinfection, and state the difference. → Killing/removing ALL microorganisms including endospores (criterion: killing spores) vs reducing pathogens — not necessarily spores — to a level not harmful to health
- Define asepsis and aseptic technique. → A condition in which living microorganisms are absent; technique preventing bacteria entering the body or other materials
- Give the autoclave and pasteurisation conditions. → 121 °C, 15 lb/in², 15–20 min (sterilises) · 62 °C/30 min or 72 °C/15 s (disinfects)
- Why is UV limited to surfaces and air? → Poor penetration — and it damages eyes and skin
- Name the five factors influencing antimicrobial efficacy. → Agent concentration and kind · exposure time · temperature and pH · number and kinds of organisms · organic matter present