Microbial Control Methods: Sterilization and Disinfection

Methods of Sterilisation

• Microbial control methods include physical agents, chemical agents, and mechanical removal methods.

Physical Agents

• Radiation: kills microorganisms by ionizing or non-ionizing radiation
  • Ionizing radiation: X-rays, gamma rays, and cosmic rays; high penetrative power; no appreciable increase in temperature; used for sterilizing plastics, syringes, catheters, grease fabrics, and metal foils
  • Non-ionizing radiation: electromagnetic rays with longer wavelength; absorbed as heat; used for rapid mass sterilization of prepacked syringes and catheters; e.g. UV rays
• Heat: kills microorganisms by oxidation effects
  • Dry heat: uses oven; operated between 50°C to 250/300°C; holding period of 160°C for 1 hour is desirable; uses: sterilizing forceps, scissors, scalpels, swabs, and pharmaceutical products like liquid paraffin, dusting powder, fats, and grease
  • Flaming: inoculation loop or wire, the tip of forceps, and spatulas are held in a Bunsen flame till they are red hot
  • Incineration: destroys materials such as contaminated cloth, animal carcasses, and pathological materials
• Moist heat: kills microorganisms by coagulating their proteins
  • Pasteurization: process of killing pathogens in milk; does not sterilize; milk is heated at 63°C for 30 minutes (Holder method) or at 72°C for 15-20 seconds (Flash process)
  • Boiling: kills vegetative forms of bacterial pathogens; hepatitis virus can survive up to 30 minutes of boiling; endospores can survive up to 20 hours or more of boiling
  • Steam at atmospheric pressure: uses steam sterilizer (Koch/Arnold steamer); consists of a tin cabinet with a conical lid; has a perforated tray above; for routine sterilization, exposure of 90 minutes is used; for media containing sugar and gelatin, exposure of 100°C for 20 minutes for 3 successive days is used; termed as Tyndallisation or Intermittent Sterilization
  • Steam under pressure: uses autoclave; works on the principle of steam under pressure; invented by Charles Chamberland in 1879; consists of a vertical or horizontal cylinder; has an opening for keeping materials to be sterilized; has a pressure gauge and safety valve; sterilization is carried out under pressure at 121°C for 15 minutes

Chemical Agents

• Act by protein coagulation, disruption of the cell membrane, removal of sulphydryl groups, and substrate competition
• Alcohols: frequent use; no action on spores; concentration recommended 60-90% in water; uses: disinfection of clinical thermometer, disinfection of the skin – venipuncture
• Aldehydes: frequent use; formaldehyde and glutaraldehyde; formaldehyde is bactericidal, sporicidal, and has a lethal effect on viruses; aqueous solutions are used in treatment of wounds
• Gases: ethylene oxide is a colorless, highly penetrating gas with a sweet ethereal smell; effective against all types of microorganisms including viruses and spores; uses: sterilizing heart-lung machines, respirators, sutures, dental equipment, books, and clothing; also used to sterilize glass, metal, and paper surfaces, plastics, oil, some foods, and tobacco
• Formaldehyde gas: widely employed for fumigation of operation theaters and other rooms
• Beta propiolactone: used in fumigation; for sterilization, 0.2% BPL is used; has a rapid biocidal activity; very effective against viruses
• Surface active agents and metallic salts: substances that reduce the surface tension; cations are widely used in the form of quaternary ammonium compounds; markedly bactericidal, active against Gram-positive organisms; no action on spores, tubercle bacilli, and viruses
• Metallic salts: the salts of silver, copper, and mercury are used as disinfectants; act by coagulating proteins; marked bacteriostatic, weak bactericidal, and limited fungicidal activity

Mechanical Removal Methods

• Filtration: helps to remove bacteria from heat-labile liquids such as sera and solutions of sugar and antibiotics
• Filters used: candle filters, asbestos filters, sintered glass filters, and membrane filters
• Candle filters: widely used for purification of water; two types: unglazed ceramic filter (Chamberland filter) and diatomaceous earth filters (Berkefeld filter)
• Asbestos filters: disposable single-use discs; high adsorbing tendency; carcinogenic; e.g. Seitz filter
• Sintered glass filters: prepared by heat fusing powdered glass particles of graded size; cleaned easily; brittle; expensive
• Membrane filters: made of cellulose esters or other polymers; uses: water purification and analysis, sterilization and sterility testing, and preparation of solutions for parenteral use