Classify Various Methods of Sterilization with Suitable Examples
Sterilization is a crucial procedure in microbiology, medicine, pharmaceuticals, food technology, and research laboratories. It eliminates all microorganisms, including highly resistant bacterial spores, to prevent contamination and infection. Sterilization ensures the safety of surgical instruments, laboratory media, pharmaceuticals, dressings, and various equipment that come in direct contact with the human body or experimental cultures.
Sterilization techniques are broadly classified into Physical, Chemical, and Radiation methods. Each method is selected based on the type of material, level of contamination, heat sensitivity, and intended application.
Need for Sterilization
• To prevent infections during surgeries and medical procedures.
• To avoid sample contamination in laboratories.
• To maintain asepsis in hospitals and clinics.
• To ensure safety and quality of pharmaceutical and food products.
Classification of Sterilization Methods
Category Method Examples
Physical Methods Heat Sterilization (Dry & Moist), Filtration Autoclave, Hot Air Oven, Membrane Filters
Chemical Methods Liquid Sterilants, Gaseous Sterilants Alcohols, Aldehydes, Ethylene Oxide Gas
Radiation Methods Ionizing and Non-ionizing Radiation Gamma rays, UV rays
Mechanical Methods Filtration HEPA filters, Membrane filters
1. Physical Methods of Sterilization
A. Heat Sterilization
Heat is the most commonly used method. It destroys microorganisms by denaturing proteins and oxidizing cellular components.
Heat sterilization is of two types:
1. Dry Heat Sterilization
Dry heat removes microorganisms by oxidation. It is suitable for materials that can withstand high temperature and do not get damaged by drying.
Technique Temperature / Time Suitable For Example
Hot Air Oven 160°C for 1 hour / 180°C for 30 min Glassware, metal instruments, powders, oils Sterilizing test tubes, Petri dishes, scalpels
Incineration Direct flame Infectious waste, used needles, dressings Hospital waste disposal
Flaming Direct heating Inoculating loops, spatulas Microbiology labs
Advantages
• No moisture → no corrosion
• Suitable for powders and oils
Disadvantages
• Requires high temperature
• Longer exposure time
2. Moist Heat Sterilization
Moist heat kills microbes by coagulation of proteins. It is more effective than dry heat.
a. Below 100°C
Example: Pasteurization
• Used for milk, beverages.
• Temperatures: 63°C for 30 min (Holder method) or 72°C for 15 sec (HTST method).
b. At 100°C
Example: Boiling
• 100°C for 10–20 minutes.
• Used for household utensils.
Does not kill spores → not reliable for medical use.
c. Above 100°C (Autoclaving)
• Autoclave uses steam under pressure:
• 121°C at 15 psi for 15–20 min
Suitable for:
• Surgical instruments
• Culture media
• Dressings
• Rubber materials
Why autoclave is highly effective?
Steam increases protein denaturation and penetrability.
2. Filtration Sterilization (Mechanical Method)
Used for heat-sensitive materials like vaccines, sera, antibiotic solutions, and culture media for microorganisms.
Types of Filters
Filter Type Pore Size Use
Membrane Filters 0.22 µm Sterilize liquids (syringes & lab filters)
HEPA Filters 0.3 µm Purify air in ICUs, operation theaters, laminar flow hoods
Principle
Microorganisms are trapped mechanically while fluid passes.
Example
Sterilization of injectable drug solutions.
3. Chemical Methods of Sterilization
Used for heat-sensitive equipment such as catheters, endoscopes, and plastic instruments.
A. Liquid Chemical Sterilants
Chemical Concentration Use
Alcohol (Ethyl/Isopropyl) 70% Skin disinfection
Phenol & Cresol 1-5% Surface disinfection
Chlorine Compounds 0.5-1% Water & surface disinfection
Hydrogen Peroxide 6-25% Equipment sterilization
Glutaraldehyde 2% Endoscopes, respiratory equipment
B. Gaseous Sterilization
Used for delicate instruments that are heat and moisture sensitive.
Gas Used Application
Ethylene Oxide (EO) Sterilizing syringes, plastic instruments, pacemakers
Formaldehyde Gas Room fumigation, biosafety cabinets
• Advantages
• Penetrates deep into materials.
Disadvantages
• Toxic & requires aeration period.
4. Radiation Sterilization
A. Non-Ionizing Radiation
Ultraviolet (UV) Light (260 nm)
• Used for surface sterilization in operation rooms and biosafety cabinets.
• Poor penetration → limited use.
B. Ionizing Radiation
Gamma Rays (Cobalt-60 Source)
• Used for plastic syringes, IV sets, disposables, food sterilization.
• High penetration and effective.
Comparison of Sterilization Methods
Method Suitable for Example Drawback
Dry Heat Glassware, powders, metals Hot Air Oven Long heating time
Moist Heat Surgical tools, culture media Autoclave Not suitable for oils
Filtration Heat-sensitive solutions Membrane filters Cannot remove viruses
Chemicals Plastic instruments Glutaraldehyde Toxicity
Radiation Disposables Gamma rays Expensive setup
Applications of Sterilization
• Hospitals & Clinics – surgical tools, catheters
• Pharmaceuticals – injectable products, culture media
• Food & Beverage – milk pasteurization, canned food
• Laboratories – inoculation loops, media, glassware
Conclusion
Sterilization is a vital process to eliminate all forms of microbial life, ensuring safety in medical procedures, laboratory work, pharmaceutical preparations, and the food industry. The choice of sterilization method depends on the nature of the material, type of microorganisms, and availability of equipment.
FAQs
1. What is the most reliable method of sterilization?
Autoclaving (steam under pressure) is considered the most reliable due to high efficiency and rapid action.
2. Which method is used for sterilizing disposable syringes?
Gamma radiation or Ethylene oxide gas.
3. Why is dry heat less effective than moist heat?
Dry heat requires higher temperature and longer time as it penetrates less efficiently than steam.
4. What is the pore size of a sterilizing membrane filter?
0.22 micrometers.
5. Can boiling water sterilize surgical instruments?
Boiling only kills most pathogens but does not kill spores, so it is not suitable for sterilization.
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Writer: Vandita Singh, Lucknow (GS India Nursing Group)