sterilization

Sterilization and Disinfection

  • Sterilization: Process where all living microorganisms, including bacterial spores, are killed. Achieved through physical, chemical, and physiochemical means. Chemicals used as sterilizing agents are known as chemisterilants.

  • Disinfection: Elimination of most pathogenic microorganisms (excluding spores) on inanimate objects. Achieved using physical or chemical methods, with chemicals referred to as disinfectants. Not all disinfectants can kill all microorganisms.

  • Decontamination: Removal of contaminating pathogenic microorganisms using sterilization or disinfection.

  • Sanitization: Chemical or mechanical cleansing to reduce microbes on food utensils to public health-safe levels, primarily used in the food industry.

  • Asepsis: Techniques (gloves, air filters, UV rays) employed to achieve a microbe-free environment.

  • Antisepsis: Use of chemicals (antiseptics) to eliminate pathogenic microorganisms from skin or mucus membranes.

  • Bacteriostasis: Inhibition of bacterial growth without killing them.

  • Bactericidal: Chemicals that can kill or inactivate bacteria, categorized by their spectrum of activity (e.g., virucidal, fungicidal).

  • Antibiotics: Substances produced by one microbe that either inhibit or kill another microbe.

Physical Methods of Sterilization

Sunlight

  • Microbicidal activity primarily due to UV rays; effective in tropical regions.

  • Disinfects water bodies; not sporicidal, thus does not provide complete sterilization.

Heat

  • Considered the most reliable method for sterilizing heat-stable articles. Acts through oxidative effects and protein denaturation.

  • Factors affecting efficiency of heat sterilization:

    • Nature of Heat: Moist heat is more effective than dry heat.

    • Temperature and Time: Inversely proportional; increases in temperature decrease the required time.

    • Microorganism Count: Higher microbial load may require increased temperature or longer exposure.

    • Species and Strain Sensitivity: Different microorganisms respond differently to heat.

    • Material Type: Heavily contaminated materials need longer sterilization.

    • Organic Material Presence: Increases exposure time needed for sterilization.

Dry Heat Techniques

  • Red Heat: Effectiveness in sterilizing small instruments by heating until red hot.

  • Flaming: Passing articles through flame; effective against vegetative cells but not guaranteed to kill spores.

  • Incineration: Complete destruction of contaminated materials through burning.

  • Hot Air Oven: Effective for heat-stable articles; requires high temperatures for specific times.

    • Sterilization cycle considerations: time to reach temperature, maintenance at sterilizing temperature, cooling time.

    • Efficacy checked by physical (temperature charts), chemical (Browne’s tube), and biological methods (spore tests).

Moist Heat Techniques

  • Moist heat acts by coagulation and denaturation of proteins.

Below 100°C: Pasteurization

  • Holder method: 63°C for 30 minutes.

  • Flash method: 72°C for 15 seconds, rapidly cooled.

  • Vaccine bath: 60°C for 1 hour inactivates bacteria in vaccines.

  • Serum bath: 56°C for 1 hour on successive days to disinfect serum.

At 100°C

  • Boiling: Kills most vegetative bacteria but doesn’t eliminate spores. Enhanced by adding sodium bicarbonate.

  • Steaming: Involves exposing items to free steam at 100°C for specific time periods.

    • Tyndallisation: Intermittent sterilization by exposing items to steam over multiple days.

Above 100°C

  • Autoclaving: Effective sterilization using steam under pressure.

    • Advantages include better penetration and condensation of steam, requiring significant procedural precision.

    • Must ensure no trapped air; monitored through temperature control measures.

Radiation

Types of Radiation

  • Non-ionizing: UV rays (200-280 nm) that cause thymine-thymine dimers, used in surface disinfection.

  • Ionizing: Electron beams and gamma rays, penetrate better, damaging nucleic acids of microorganisms; used in food and sterilizing disposable items.

Filtration

  • Non-killing: Does not directly kill microbes but separates them from liquids.

  • Membrane filters (0.2-0.45 µm) are used for heat-labile liquids and preparation of sterility testing baths.

Chemical Methods of Disinfection

  • Disinfectants chemically destroy pathogenic microorganisms. Vary in spectrum and include chemisterilants and antiseptics.

  • Ideal properties for disinfectants include wide spectrum, rapid activity, stability, and non-toxic effects.

Classification of Disinfectants

  1. Based on consistency: Liquid (alcohols, phenols), Gaseous (formaldehyde vapor).

  2. Based on activity level: High, Intermediate, Low.

  3. Based on action mechanism: Disrupting membranes, denaturing proteins, oxidations, etc.

Example Disinfectants

  • Alcohols: Dehydrate cells and cause coagulation (e.g., ethyl, isopropyl alcohol).

  • Aldehydes: Kill all microorganisms including spores (e.g., formaldehyde, gluteraldehyde).

  • Phenols: Disrupt membranes, precipitate proteins (e.g., chlorhexidine, hexachlorophene).

  • Halogens: Oxidizing agents (e.g., chlorine, iodine).

  • Heavy Metals: Precipitate proteins; use in specific applications (e.g., silver nitrate).

  • Surface Active Agents: Disrupt microbial cell membranes (e.g., detergents).

  • Dyes: Interact with nucleic acids, effective against gram-positive bacteria (e.g., acridine dyes).

  • Hydrogen Peroxide: Releases nascent oxygen, damages proteins and DNA.

  • Ethylene Oxide: Alkylation agent; effective for heat-labile items.

  • Beta-propiolactone: Effective sporicidal agent with broad-spectrum activity.