Introduction
Sterilization is a critical process in pharmaceutical, medical and microbiology laboratories. It ensures that all forms of microorganisms, including spores, are completely destroyed from instruments, culture media, dressings and sterile products. Proper sterilization prevents contamination, stops the spread of infections and maintains the safety of pharmaceutical preparations.
Several physical and chemical methods are used to kill, remove or inhibit microorganisms. The choice of method depends on the type of material, heat sensitivity and purpose of sterilization.
Important Terms
- Sterilization: Complete destruction of all microorganisms including spores.
- Disinfection: Destruction of pathogens but not spores.
- Sanitization: Cleaning and disinfection of surfaces.
- Antiseptics: Chemicals applied on living tissues to prevent infection.
- Germicide: Agent that kills microorganisms.
- Preservatives: Substances added to prevent microbial growth in products.
Classification of Sterilization Methods
Physical Methods
- Dry heat sterilization
- Moist heat sterilization
- Radiation (UV, X-ray, Gamma rays)
- Filtration methods
Chemical Methods
- Gaseous sterilization
- Liquid disinfectants and antiseptics
1. Dry Heat Sterilization
Dry heat kills microorganisms by oxidative damage and protein denaturation. It requires higher temperature and longer exposure than moist heat because dry air has poor penetration.
a) Sunlight and Drying
Sunlight has UV rays which kill microorganisms to some extent. Drying removes moisture required for microbial survival. This method is natural but unreliable.
b) Red Heat
Used for sterilizing metal instruments like inoculating wires by heating till red hot. Very effective but applicable only to small metallic items.
C) Flaming
Used for glass slides, mouth of culture tubes, scalpels etc. The article is passed through flame but not heated to red hot. Kills only vegetative cells.
d) Incineration
Used for destroying contaminated materials like dressings, animal carcasses and soiled disposables. Very effective for waste disposal.
e) Hot Air Oven
The most widely used dry heat method. Sterilization is done at 160°C for 2 hours or 180°C for 1 hour. Used for glassware, oils, fats, powders and metal instruments.
Advantages
- Suitable for heat-stable materials
- No moisture-related corrosion
Disadvantages
- Slow process
- Not suitable for heat-labile products, plastics or rubber
2. Moist Heat Sterilization
Moist heat is more effective than dry heat as steam penetrates better and coagulates proteins rapidly.
a) Below 100°C — Pasteurization
Used for milk and heat-sensitive liquids. Examples include holder method (63°C for 30 minutes) and flash method (72°C for 20 seconds).
b) At 100°C — Boiling and Tyndallization
Boiling kills vegetative cells but not spores. Tyndallization (heating at 100°C for 20 minutes on three days) is used for media that cannot withstand autoclaving.
c) Above 100°C — Autoclaving
The most reliable sterilization method. Steam under pressure (121°C at 15 psi for 15 minutes) kills all microorganisms including spores.
Uses
- Culture media
- Glassware
- Dressings, rubber items
- Heat-stable injections
Advantages
- Fast and efficient
- Good penetration
Disadvantages
- Not suitable for powders, oils or heat-sensitive items
- Corrosive for some metals
3. Radiation Sterilization
a) Ultraviolet (UV) Rays
Non-ionizing radiation used for surface sterilization. Causes DNA damage (thymine dimers). Used in laminar air flow units and operating rooms.
b) Ionizing Radiation
Includes X-rays, gamma rays and electron beams. Highly penetrating and used for medical equipment, pharmaceuticals and food.
Advantages
- Cold sterilization suitable for heat-sensitive materials
Disadvantages
- Requires special equipment
- Expensive
4. Filtration Methods
Used for heat-sensitive liquids. Filters remove microorganisms without heating.
Types of filters
- Asbestos filters (Seitz)
- Sintered glass filters
- Ceramic filters (Berkefeld)
- Membrane filters (Millipore) – most widely used
Advantages
- Suitable for antibiotics, vaccines, enzyme solutions
- No heat damage
Disadvantages
- Filters may clog
- Not suitable for suspensions
5. Chemical Methods
1) Gaseous Sterilization
Used for materials sensitive to moisture and heat.
Common gases
- Formaldehyde – used for room sterilization
- Ethylene oxide – very effective, used for plastics, catheters, syringes
- Beta-propiolactone – highly active but carcinogenic
Merits
- Penetrates well
- Suitable for delicate equipment
Demerits
- Toxic and irritant
- Long aeration time needed
2) Disinfectants and Antimicrobial Agents
a) Alcohols
70% ethanol or isopropyl alcohol used for skin disinfection.
b) Aldehydes
Formaldehyde and glutaraldehyde (Cidex) act by protein cross-linking.
c) Phenolics
Effective surface disinfectants (cresol, hexylresorcinol).
d) Halogens
Iodine and chlorine compounds oxidize cellular components.
e) Heavy Metals
Mercury, silver and copper salts act by protein denaturation.
f) Surface Active Agents
Cationic detergents (quaternary ammonium compounds) destabilize membranes.
g) Dyes
Acridine and triphenylmethane dyes inhibit bacteria.
Key Points
- Moist heat is more effective than dry heat for microbial destruction.
- Radiation is ideal for heat-sensitive materials.
- Filtration is the method of choice for heat-labile liquids.
- Gaseous sterilization is effective but toxic and needs careful handling.
- Each method has specific uses, advantages and limitations.
Detailed Notes:
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