STERILIZATION AND DISINFECTIONDr. Saad M Attash
STERILIZATION AND DISINFECTION Sterilization: is a process whereby all living organisms, including spores are destroyed. Disinfection: a process that destroys only the vegetative form of organisms, and leaves intact any spores the may be present. The term disinfectant can be used to describe any chemical substance with either a bactericidal or a bacteriostatic action.Methods of Sterilization and Disinfection: I . Physical Methods 1. heat: a. dry heat - hot air oven - infra red radiation b. moist heat - pasteurization - boiling - tyndallization c. steaming under pressure (autoclave) 2. light (ultraviolet light) 3. ionizing radiation 4. filtration I I . Chemical Methods 1. inorganic (halogens): a. iodine: - tincture iodine - povidone iodine b. chlorine 2. organic: a. alcohol: - ethanol 70% - isopropyl alcohol 50-70% b. aldehyde - formaline
I . Physical Methods 1. Heat: a. Dry heat - Hot air oven: for sterilization of instruments. Its an inefficient sterilizing agent because it’s a poor conductor of heat and it penetrate feebly. Its therefore necessary that the oven be fitted with fans to circulate the air through the chamber around the assembled load. A temperature of 160 c0 will sterilize the content after 1 hour. Such high temperature damages fabrics, rubber, so its used for: syringes, oily fluid, powders, metallic objects. - Infra red radiation: heating to 200 c0, for sterilizing surgical instruments.
b. Moist heat - Pasteurization: milk is pasteurized by being held in water at 63 c0 for 30 minutes. Instruments that cannot withstand great heat , can be disinfected by hot water, e.g: endoscopes and cystoscope is held in water at 75 c0 for 10 minutes. All vegetative organisms will be killed but spores will survive, however they are of no importance in urinary tract infection. - Boiling: the advantage of boiling over pasteurization is that it destroy the virus of hepatitis, But will not destroy spores. In boiling we use 100 c0 for 30 minutes. Syringes and other instruments are sterilized by this method. - tyndallization: it’s the steaming of a solution for 20 minutes for 3 successive days. After the first steaming only spores are left behind, these germinate in the medium, and are destroyed by the next steaming, any that survive should be killed after the third steaming. This method fails when applied to drugs, suture material, instruments.
c. Steaming under pressure (autoclave)it’s the most important method of sterilization. We use 121 c0 for 10 minutes, or 126 c0 for 5 minutes, or 134 c0 for 3 minutes. The principle is that water boils when its vapor pressure is equal to that of the surrounding atmosphere. So the boiling point of water can be increased by raising the atmospheric pressure.1. moist heat kills by denaturing and coagulating enzymes and proteins, whereas dry heat acts by destructive oxidation of cell constituents. so moist heat kills rapidly and at lower temperature. 2. steam condense, so large amount of heat emitted. 3. steam penetrate better than hot air, due to that its density is half that of air.
Failure in sterilization may be due to: 1. improper packing of the load: the packing must be loose so that steam can circulate around them. 2. defect in autoclave: the defect is due to the presence of air mixed with the steam. 3. defect in the steam: over saturated and superheated steam. 4. faulty operation of the autoclave: the responsibility for efficient sterilization must remain as always with those who use the finished product, namely the surgeons. Test of efficiency for heat sterilization: 1. thermocouple 2. Browne’s tube: contain chemical indicator in an ampoule which change from red to green at specific temperature. 3. autoclave tape: its a tape printed with sensitive ink which change its colour at specific temperature. 4. spores of non pathogenic organisms: this method is of little value for routine use. Because the growth is often slow, so that when a positive culture is obtained the load has probably been used.
2. Light (ultraviolet light): It can not penetrate, so used to disinfect surfaces. 3. ionizing radiation: both γ rays and β rays, its useful method because good penetration, so can be used for closed packs. used for: plastic, rubber, fabrics. 4. filtration: used by microbiologist to sterilize fluids containing sera and amino acids which is denaturated by heating, also used in sterilizing air in operating theatre.
• I I . Chemical Methods A satisfactory agent should have: 1. active against wide range of organisms and spores. 2. have rapid action. 3. should not be vitiated by organic matter like blood, pus, stool. 4. should not be toxic, irritant, not inducing hypersensitivity. 1. Inorganic (halogens): the only useful one is halogens, they have a rapid action against both vegetative organisms and spores. So they are true sterilizer, but the disadvantage that they are annulled by foreign organic matter. a. Iodine - tincture iodine: the most important iodine, contains 2.5% iodine and 2.5% potassium iodide and 90%ethanol . it’s the best skin disinfectant, but its irritating and cause allergic dermatitis. - povidone iodine: (betadine) less irritating and stain less than tincture, its inferior to tincture.b. chlorine powerful disinfectant, used to sterilize water.
2. Organic (alcohol): a. Alcohol - ethanol 70%: effective against vegetative form. - isopropyl alcohol 50-70%: useful as skin disinfectant b. aldehyde - formaline : its a true sterilizer, it penetrate organic matter slowly. The disadvantage it has irritating odour. c. Phenol not a true sterilizer Advantage: continued activity in the presence of organic matter. So used for disinfection of feces. Disadvantage: toxic and expensive. - dettol (choroxylenol): less irritant and toxic than other, but also less efficient. Inactivated by organic matter. - hibitane (chlorhexidine): powerful against vegetative organisms, especially gram +ve, but not the spores of low toxicity. Soluble in water and alcohol. - hexachlorophane: insoluble in water and alcohol, Its toxic and not used widely.