Sterilisation definition type of sterilization methods along with examples.
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Sterilization - Definition, Types & Methods
Definition
Sterilization is the complete destruction or removal of all forms of microorganisms - including vegetative bacteria, bacterial spores, fungi, viruses, and mycobacteria - from a surface, object, or substance. It eliminates around 10⁶ log colony-forming units (a 6-log kill).
"Sterilization represents the total destruction of all microbes, including the more resilient forms such as bacterial spores, mycobacteria, nonenveloped viruses, and fungi."
- Medical Microbiology 9e
Key distinctions:
- Sterilization = total elimination of ALL microorganisms (including spores)
- Disinfection = destroys most organisms, but more resilient microbes may survive
- Antisepsis = reduces microbial numbers on living skin surfaces
- Sanitization = reduces microbial count to "safe" levels (e.g., food industry)
Kinetics of Killing
Killing by heat, radiation, or chemicals follows exponential (first-order) kinetics - a fixed proportion of survivors is killed during each time increment. The rate of killing increases exponentially with arithmetic increases in temperature or concentration of disinfectant. - Sherris & Ryan's Medical Microbiology, 8th Ed., p. 102
Types / Classification of Sterilization
Sterilization is broadly divided into two major categories:
1. Physical Methods
A. Heat Sterilization
Heat is the most effective and widely used method. It works by denaturing proteins and destroying cell constituents and enzymes.
i. Dry Heat
| Method | Conditions | Examples |
|---|---|---|
| Incineration / Flaming | Naked flame | Wire loops in microbiology lab, needle/knife blade emergency sterilization, disposable material |
| Hot-air oven | 160-170°C for 2 hours | Glassware, metals, heat-resistant oils, waxes (cannot be sterilized in autoclave) |
- Dry heat requires higher temperatures and longer times because it lacks reactive water molecules.
- Dry heat is effective for materials immiscible in water (oils, waxes) and for glassware preparation.
ii. Moist Heat
Moist heat (water/steam) is far more rapid and effective than dry heat. Water molecules denature proteins irreversibly by disrupting hydrogen bonds at relatively low temperatures.
| Method | Conditions | Examples |
|---|---|---|
| Autoclave (Steam under pressure) | 121°C / 15 psi for 15-20 min | Surgical instruments, dressings, culture media, liquids - gold standard |
| Boiling | 100°C | Most pathogens, some spores - general use |
| Pasteurization | 63°C for 30 min (holder) OR 72°C for 15-20 sec (flash/HTST) | Milk, beverages, plastic hospital equipment |
| Tyndallization (Fractional sterilization) | 100°C on 3 consecutive days | Heat-sensitive materials - allows spores to germinate between cycles |
"Moist heat in the form of water or steam is far more rapid and effective in sterilization than dry heat because reactive water molecules denature protein irreversibly by disrupting hydrogen bonds between peptide groups at relatively low temperatures." - Sherris & Ryan's Medical Microbiology, 8th Ed., p. 104
- Autoclaving is the most reliable method. Effectiveness depends on: temperature, duration, steam penetration into the load, and load size.
B. Radiation
i. Ultraviolet (UV) Radiation (Non-ionizing)
- Wavelength ~260 nm damages DNA by forming pyrimidine dimers
- Activity level: Sterilizing (all microorganisms) but has poor tissue penetration
- Examples: Air sterilization in operating theatres and laboratories, surface disinfection
ii. Ionizing Radiation (Gamma rays, X-rays, accelerated electrons)
- Damages DNA and generates destructive free radicals
- Activity level: Sterilizing (all microorganisms)
- High penetrating power
- Examples: Sterilization of pre-packaged disposable medical supplies (syringes, catheters, gloves), food preservation
C. Filtration
A mechanical method - uses filters with defined pore sizes to physically remove microorganisms.
| Filter Type | Pore Size | Use |
|---|---|---|
| HEPA filters | 0.3 µm | Air sterilization in operating rooms, laminar flow cabinets |
| Membrane filters (e.g., Millipore) | 0.22 µm | Sterilization of heat-sensitive liquids: sera, vaccines, antibiotic solutions, eye drops |
| Seitz (asbestos) filter | Variable | Older bacteriological use |
| Chamberland (candle) filter | Variable | Water purification |
Filtration removes bacteria and fungi but may not remove viruses unless ultra-filters are used. Steps include sieving, adsorption, and trapping.
D. Mechanical / Other Physical Methods
| Method | Details |
|---|---|
| Microwave | Near-boiling temperatures (moisture required); used for hospital waste disinfection, NOT sterilization |
| Ultrasonication | High-frequency sound waves disrupt cells; mainly used for cleaning, not routine sterilization |
2. Chemical Methods
Chemical sterilization is used for biological specimens, heat-sensitive equipment, and surfaces.
A. Gaseous Chemical Agents
| Agent | Mechanism | Examples of Use |
|---|---|---|
| Ethylene oxide (EtO) | Alkylates DNA/proteins; kills all microorganisms | Heat-sensitive equipment: catheters, endoscopes, plastic items, pre-packaged surgical supplies. Note: potentially explosive, requires aeration afterward |
| Formaldehyde | Alkylation of proteins and nucleic acids | Room/surface fumigation |
| Nitrogen dioxide (NO₂) | Oxidizing agent | Enclosed spaces |
| Ozone | Strong oxidizer | Water treatment, air sterilization |
| Hydrogen peroxide vapor | Oxidizes cell components | Pharmaceutical manufacturing, sterile rooms |
| Peracetic acid | Oxidizing agent | Endoscope sterilization |
"Ethylene oxide sterilization is a slow process influenced by the concentration of the gas, relative humidity, exposure time, and temperature. Effectiveness improves with higher EtO concentration, elevated temperatures, and relative humidity of ~30%." - Medical Microbiology 9e
B. Liquid Chemical Agents (Disinfectants/Sterilants)
| Agent | Mechanism | Activity Level | Examples |
|---|---|---|---|
| Glutaraldehyde (2.5%) | Cross-links and alkylates proteins/DNA | High (Sterilizing with prolonged exposure) | Endoscopes, surgical instruments that cannot be autoclaved |
| Hydrogen peroxide | Oxidizes membrane lipids and cell components | High | Contact lenses, wound care |
| Peracetic acid | Oxidizing agent | High/Sterilizing | Endoscopes |
| Chlorine / Hypochlorite (bleach) | Strong oxidizing agent | High | Water disinfection (swimming pools, drinking water), surfaces contaminated with viruses/spores; used as 5% hypochlorite solution |
| Iodophors (Povidone-iodine) | Iodinates/oxidizes microbial components | Intermediate | Pre-surgical skin preparation, wound care |
| Alcohols (70-95% ethanol, 90-95% isopropanol) | Protein denaturation | Intermediate | Skin decontamination before venipuncture; surfaces. Note: inactive against spores; 100% alcohol is less effective |
| Phenolic compounds | Disrupts lipid-containing membranes, leakage of cellular contents | Intermediate | Surfaces, handwashing, inanimate surfaces. E.g., carbolic acid, o-phenylphenol |
| Quaternary ammonium compounds (QACs) | Denature cell membranes | Low | General cleaning, noncritical instruments. E.g., benzalkonium chloride, cetylpyridinium chloride |
Summary Table - Sterilization Methods
| Category | Method | Spectrum | Key Examples |
|---|---|---|---|
| Physical - Heat | Autoclave | All | Surgical kits, media |
| Dry heat oven | All | Glassware, metal | |
| Incineration/Flaming | All | Wire loops | |
| Boiling | Most | General | |
| Pasteurization | Vegetative bacteria | Milk, beverages | |
| Physical - Radiation | UV | All (poor penetration) | Lab air, surfaces |
| Ionizing (gamma) | All | Pre-packaged supplies, food | |
| Physical - Filtration | Membrane filter | Bacteria, fungi | Heat-sensitive liquids |
| HEPA | Airborne microbes | OR air, laminar flow | |
| Chemical - Gaseous | Ethylene oxide | All | Plastics, catheters |
| Formaldehyde gas | All | Room fumigation | |
| Chemical - Liquid | Glutaraldehyde | All (with time) | Endoscopes |
| Chlorine/Hypochlorite | Viruses, bacteria, fungi | Water, surfaces | |
| H₂O₂ | Bacteria, viruses, fungi | Contact lenses | |
| Alcohols | Vegetative bacteria only | Skin prep | |
| Phenolics | Bacteria, fungi | Surfaces | |
| QACs | Most bacteria, fungi | General cleaning |
Sources:
- Sherris & Ryan's Medical Microbiology, 8th Ed., pp. 102-111
- Medical Microbiology 9e, Chapter 3: Sterilization, Disinfection, and Antisepsis
Sterilisation definition type of sterilization methods along with examples.
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