Physical Methods of Sterilization

Physical Sterilization - The Germ Gauntlet

⭐ Autoclave: 121°C, 15 psi for 15-20 min is crucial for sporicidal action.

Dry Heat Sterilization - Desert Scorchers

• Mechanism: Kills by protein denaturation, oxidative damage, and toxic effects of elevated electrolyte levels.
• Methods:
  • Hot Air Oven:
    • Temperatures/Times: 160°C for 2 hours; 170°C for 1 hour; 180°C for 30 minutes.
    • Uses: Glassware (syringes, Petri dishes), surgical instruments (scalpels, forceps if not heat-sensitive), powders (talc, sulphonamides), oils, fats, waxes.
    • Biological indicator: Spores of Bacillus atrophaeus.

    ⭐ Hot Air Oven: Standard holding time is 160°C for 2 hours. Used for glassware, oils, powders.

  • Incineration: Burning to ashes. For contaminated dressings, animal carcasses, pathological material. Temp: 870-980°C.
  • Flaming: Passing items through a Bunsen burner flame. For inoculating loops, needles, tips of forceps.
  • Infrared Radiation: For rapid mass sterilization of pre-packed items like syringes and catheters.
• 📌 Mnemonic: "Dry Heat Oven Sterilizes Glassware, Powders, Oils" (Dry Heat Oven, Syringes/Scalpels, Glassware, Powders, Oils).

Moist Heat (Non-Autoclave) - Steamy Subduers

• Utilizes temperatures at or below 100°C; denatures microbial proteins.
• Pasteurization: Kills non-sporing pathogens (e.g., Mycobacterium, Salmonella, Brucella).
  • Holder method: 63°C for 30 min.
  • Flash (HTST): 72°C for 15-20 sec.
  • Used for milk, vaccines, serum. Spores survive.
• Inspissation: Thickening by evaporation.
  • Heating at 80-85°C for 30 min on 3 successive days.
  • For media with egg/serum (Löwenstein-Jensen, Loeffler’s serum slope).
• Boiling:
  • At 100°C for 10-30 min. Kills most vegetative bacteria & viruses.
  • Not sporicidal reliably; some spores resist.
• Tyndallization (Fractional Sterilization):
  • Steam at 100°C for 20-30 min on 3 successive days.
  • For heat-labile media (sugars, gelatin).

⭐ Inspissation is the method of choice for sterilizing heat-sensitive egg-based media like Löwenstein-Jensen (LJ) medium.

Autoclaving - Pressure Point Power

• Principle: Moist heat (steam) under ↑ pressure (↑P → ↑Boiling Point of $H_2O$).
• Mechanism: Saturated steam denatures & coagulates proteins.
• Key Parameters:
  • 121°C at 15 psi (~1.05 kg/cm²) for 15-20 min.
  • 134°C at 30 psi (~2.1 kg/cm²) for 3-5 min (flash; prions).
• Monitoring:
  • Chemical: Autoclave tape, Bowie-Dick test (air removal).
  • Biological: Geobacillus stearothermophilus spores. 📌 Gee, Steam Treats Bugs!
• Uses: Surgical instruments, media, dressings. Not for oils, powders.

⭐ Geobacillus stearothermophilus spores are the most resistant biological indicators for validating autoclave efficacy.

Radiation & Filtration - Ray & Sieve Shield

• Radiation Sterilization (Cold Sterilization)
  • Non-ionizing:
    • UV (260 nm): DNA damage (pyrimidine dimers). Surfaces, air (OT, LAF), water. Low penetration.
    • Infrared (IR): Heats; mass sterilization (syringes).
  • Ionizing (Gamma rays, X-rays, Electron beams):
    • Mechanism: DNA damage (free radicals). High penetration.
    • Dose: 2.5 Mrad (25 kGy).
    • Uses: Disposables (gloves, syringes), drugs, food.

    ⭐ Gamma radiation source: Cobalt-60 ($^{60}$Co).

• Filtration Sterilization (Mechanical Sieving)
  • Heat-labile liquids (sera, antibiotics) & air (HEPA).
  • Membrane filters: Pore size 0.22 µm (bacteria), 0.01 µm (viruses).
  • HEPA filters: Remove >99.97% of particles >0.3 µm (air). ​

High‑Yield Points - ⚡ Biggest Takeaways

• Autoclave (121°C, 15 psi, 15 min) is most reliable for surgicals & media.
• Hot Air Oven (160°C, 2 hours) sterilizes glassware, oils, and powders.
• Incineration ensures complete destruction of contaminated waste.
• Filtration sterilizes heat-labile liquids (e.g., sera) & air (HEPA filters).
• Ionizing radiation (Gamma, X-rays) sterilizes pre-packaged disposables.
• UV radiation (non-ionizing) is for surface and air disinfection.
• Tyndallization: intermittent steaming for heat-sensitive, spore-containing media.