PMB 222: Principles of Sterilization and Disinfection

General Principles of Physical and Chemical Sterilization

  • Physical Sterilization: Removal or destruction of all microbial life.
    • Moist Heat (Autoclaving): Steam under pressure at 121∘C121^\circ\text{C} for 15−20 minutes15-20\,\text{minutes}.
    • Dry Heat: Hot air ovens at 160∘C160^\circ\text{C} for 2 hours2\,\text{hours}.
    • Radiation: Ionizing (Gamma rays, X-rays) or Non-ionizing (UV light).
    • Filtration: Membrane filters for heat-sensitive solutions.
  • Chemical Sterilization: Uses agents like Ethylene oxide gas, formaldehyde gas, or glutaraldehyde to destroy all life including spores.

Chemical Disinfection and Microbiology of Air

  • Disinfection: Process of eliminating most pathogenic microorganisms (excluding spores) on inanimate objects.
  • Microbiology of Air: Air carries bacteria (Staphylococcus, Streptococcus), fungi (Aspergillus), and viruses. Sampling methods include settle plates, impingement in liquid, and filtration.

Properties of an Ideal Chemical Disinfectant

  • Broad-spectrum: Effective against Gram-positive/negative bacteria, viruses, fungi, and spores.
  • Rapid Action: Minimizes microbial kill time for efficiency.
  • Non-toxicity: Low risk of skin irritation or respiratory issues for humans and animals.
  • Surface Compatibility: Should not corrode metals, plastics, or fabrics.
  • Stability: Long shelf life; resistant to light, heat, or air degradation.
  • Residual Effect: Provides ongoing protection after application.
  • Economic/Practical: Cost-effective, odorless or pleasant smelling, and biodegradable.

Factors Affecting the Activity of Chemical Disinfectants

  • Concentration: Generally, higher concentrations increase killing power, though some agents plateau.
  • Contact Time: Adequate duration is essential for penetration and microbial kill.
  • Temperature and pH: Higher temperatures generally increase activity. Efficacy varies with pH (e.g., hypochlorite is better at low pH; glutaraldehyde at high pH).
  • Organic Matter: Blood, pus, or feces can inactivate agents or shield microbes.
  • Microbial Load: Higher numbers of organisms require longer exposure.
  • Surface Properties: Porous surfaces (wood) absorb agents, whereas smooth surfaces (steel) are easier to treat.

Major Groups of Chemical Disinfectants

  • Alcohols: Ethanol and isopropanol (50−70%50-70\%); bactericidal but ineffective against spores.
  • Aldehydes: Formaldehyde (3−8% aqueous solution3-8\%\,\text{aqueous solution}) and Glutaraldehyde; sporicidal and high-level.
  • Halogens: Chlorine (0.10.1 to 0.25 ppm0.25\,\text{ppm} for water) and Iodine (Skin antiseptics).
  • Phenolics: Phenol (first used by Joseph Lister), Cresol, and Chloroxylenol (Dettol).
  • Biguanides: Chlorhexidine (Savlon); potent, non-irritating topical antiseptic.
  • Quaternary Ammonium Compounds (QACs): Benzalkonium chloride; effective against enveloped viruses.
  • Oxidizing Agents: Hydrogen peroxide (3%3\% for wounds) and Potassium permanganate (1:50001:5000).
  • Dyes: Acriflavine, Gentian violet (antifungal), and Brilliant green.

Evaluation of Potency and Activity

  • Extinction Time Method: Measures the time needed for complete killing.
  • Phenol Coefficient Method: Compares effectiveness against Salmonella typhi or Staphylococcus aureus.
    • Phenol coefficient=dilution of test compounddilution of phenol\text{Phenol coefficient} = \frac{\text{dilution of test compound}}{\text{dilution of phenol}}
  • Bacteriostatic/Fungistatic Tests:
    • Agar Diffusion Test: Also known as the Kirby-Bauer test; uses the zone of inhibition measurement.
    • Broth Dilution Method: Uses serial dilutions in liquid media to assess growth via turbidity.
    • Minimum Inhibitory Concentration (MIC): The lowest concentration that prevents visible growth.