B.Sc. 1st year Microbiology

Microbial Control Methods

1. Sterilization

• Definition: Complete destruction or removal of all forms of microbial life, including bacterial spores, fungal spores, viruses, and prions.
• Key Points:

• Achieved by physical agents (autoclaving, dry heat, filtration, radiation) or chemical agents (ethylene oxide, glutaraldehyde).
• Considered the highest level of microbial control.
• A sterilized material is absolutely free of viable microorganisms.

2. Disinfection

• Definition: Elimination of most pathogenic microorganisms (vegetative cells) from inanimate objects, but not necessarily spores.
• Key Points:

• Reduces microbial load to a safe level.
• Uses chemical agents (disinfectants) such as phenols, chlorine, alcohols.
• Applied to non-living surfaces.

3. Aseptic Technique

• Definition: Practices that prevent the entry of microorganisms into sterile environments, cultures, tissues, or materials.
• Key Points:

• Used in microbiology labs, cell culture, surgery, pharmaceutical production.
• Includes flame sterilization, hand hygiene, laminar airflow hood, sterilized instruments.

4. Sanitization

• Definition: Reduction of microbial population on surfaces or utensils to acceptable public health levels.
• Key Points:

• Common in food, dairy, and public health microbiology.
• Does not achieve sterility.
• Maintains hygiene standards in food industries, homes, hospitals.

Physical Methods of Sterilization: Dry Heat

Principles

• Oxidative Processes: High temperatures oxidize essential cell constituents (lipids, nucleic acids, enzymes).
• Protein Denaturation & Coagulation: Heat disrupts protein structure, impairing cellular functions.
• Desiccation Effect: Removes moisture, causing dehydration and inhibiting enzymatic reactions.

Overall Effect: Destruction of vegetative cells and spores (though spores are more resistant).

Methods of Dry Heat Sterilization

A. Hot Air Oven

• Most widely used for heat-stable materials.
• Operating conditions:
• 160°C for 2 hours
• 170°C for 1 hour
• 180°C for 30 minutes
• Heat transfer by conduction.
• Ensures uniform hot air circulation.

B. Flaming

• Direct exposure of instruments (loops, needles, forceps tips) to flame until red-hot.
• Destroys microbes instantly by incineration.

Applications

• Glassware (test tubes, Petri dishes, pipettes, flasks)
• Metal instruments (scalpels, forceps)
• Powders (talc, starch)
• Oils and fats (steam cannot penetrate)
• Sharps (prevent rusting/dulling)
• Glass syringes (older types)

Advantages

• Suitable for moisture-sensitive materials
• No corrosion of metal instruments
• Eco-friendly (no toxic fumes)
• Simple and cost-effective equipment

Disadvantages

• Requires long exposure time
• Less efficient than moist heat
• Not suitable for heat-labile materials (plastics, rubber)
• Poor penetration into dense materials