Disinfection and Sterilization
Disinfection is the process of freeing of an article from most of the micro-organisms. Disinfectant or germicide is a substance which destroys harmful microbes (usually not the spores) on the surface of an article, e.g., phenol, chlorine. etc. Sterilization is the process of freeing of an article from all living microorganisms including bacterial spores. Antiseptics are chemicals used to destroy or inhibit the growth and multiplication of micro-organisms on the surface of the skin.. e.g.. dettol. savior’, ethyl alcohol (ethanol) etc. Clearly the difference between disinfectants and antiseptics is of degree only. A disinfectant in low concentration may act as antiseptic. Deodorant is a substance which suppresses offensive odours, e.g.. bleaching powder, room air-fresheners, etc. Detergent is a surface cleaning agent which removes micro-organisms along with dirt; it acts by lowering surface tension, e.g., soap. Fumigation is a process by which killing of animal forms. specially the arthropods and rodents is accomplished by the use of gaseous agents, such as formaldehyde gas, sulphurdioxide gas, hydocyanic acid gas.
Disinfecting/sterilizing agents can be classified as follows:
- Natural : (a) Air; (b) Sunlight
- Physical : (a) Heat : High temperature:
- Dry heat,
- Moist heat.
Low temperature (56=60’C):
- Water bath.
- Vaccine bath.
- Agents damaging cell membrane:
- Cataionic, anionic, nonionic, amphoteric agents: Soap. Bile salt.
- Phenolic compounds:
(i) Phenol. (ii) Cresol, (iii) Lysol.
- Alcohol : Ethyl alcohol.
- Agents modifying functional group of protein:
- Oxidizing agents: (1) (ii)
Chlorine; (iii) Iodine, (iv) Chiorhexidine.
- Alkalytin‘ g agent: Formaldehyde.
- Agents denaturing protein:
(i) Benzoic acid, (ii) Propionic acid.
- Gaseous: (i) Formaldehyde gas, (ii) SO2 gas, (iii)
Hydrocyanic acid gas, (iv) Cresol fumigation.
Natural Disinfectants of Disinfetcion
Fresh air and sunlight kills microbes. Drying kills microbe and attenuates it. The sunlight, due to its ultra-violet rays, exerts germicidal effect on organism, e.g., diphtheria bacilli and tuberculosis bacilli are killed within 1 hour on exposure to direct sunlight.
Physical Disinfectants of Disinfetcion
It includes heat in various forms as follows: Dry heat. (i) Burning of articles of small value, e.g., rags, pillos and mattresses, etc.; this is the best method of disinfection. Burning is done in a furnace. The excreta from cholera and enteric fever patients are burnt by mixing them with sawdust and kerosene oil. In plague hit areas, thatched quarters are destroyed by burning. Sputum and other discharges are best destroyed by burning; burning should not be done in open place. Naked flame is used for sterilizing platinum loops, needles, rotary lancets, etc. (ii) Hot dry air: Hot air oven is electrically heated with a thermostat and a fan and blower; the temperature and holding time: 160°C for 1 hour. It is useful only for disinfecting leather goods, books, India rubber, etc. Because of the high temperature needed for disinfecting.
Serbian Barrel. This is used for steam disinfection. Here current steam is used, and the clothing is disinfected within an hour. It consists of a barrel which is perforated at the bottom and on the top has a lid fitted with hooks. Clothes are hanged on the hooks. Steam, generated below. enters the barrel through the perforated bottom. The barrel is placed on a metal tank containing water.
Autoclave. Here, steam is used under regulated pressure (at 121°C temperature under 15 Ibs/sq. inch pressure for 15 minutes). It is used for the sterilization of culture media and destruction of old cultures. It is also used for the sterilization of surgical equipment and dressing.
Radiation. (I) Ultraviolet radiation (240-280 nm wv) can be used for sterilizing air in hospital ward and operation theatre. UV is equally effective for gram-positive and gram-negative organisms. (ii) Ionizing radiation (2.5 mF WV-X, (3 and y rays): Plastic syringes, catheter. anaesthetic instrument and transfusion set can be sterilized.
Chemical Disinfectants of Disinfection
Chemical disinfectants chiefly act by oxidizing, reducing, coagulating and by direct action on micro-oranisms. They are used to disinfect faeces. urine, sputum, floors and walls of room, wooden/iron articles, surgical instruments, and also to disinfect houses and buildings. Chemical disinfectants are : (1) Oxidizing agents, e.g., KMNO 4; (2) Reducing agents, e.g., SO2; (3) Coagulants, e.g., Perchloride of mercury: (4) Direct action on bacteria, e.g., Carbolic acid, Formalin, etc. The commonly used chemical disinfectants are mentioned hereunder
Perchloride ofmercury or Corrosive sublimate (HgCl2). It is a powerful disinfectant. It kills microorganisms in 1 in 1000, dilution; in 1 in 500
dilution it will destroy spore–c- after one_ hour’exposue. This “‘disinfectant is poisonous and
colourless. So, to avoid accidents, aniline blue has to be added to the disinfectant to impart colour. It corrodes metals, hence metallic vessels are not to be used.
Phenol (Carbolic acid). Pure phenol is not an effective disinfectant. The phenol (crude pheonl) that is commonly used as disinfectant is a mixture of phonol and cresol. It is effective against gram- positive and gram-negative bacteria; its action is slow in acid-fast bacillus and spore-bearing organism. Floors, walls, ceillings can be disinfected by mopping 3.5% to 5% solution of phenol. It has no bad effect on fabrics. It is cheap but poisonous.
Phenol derivatives. These are cresol, lysol. izol, cellin (lysol, izol and cellin are cresol emulsions). These are less poisonous, but 3 to 10 times as powerful as phenol. Cresol is a general disinfactant. For disinfection of stool and urine cresol is used in 5% to 10% strength. In laboratories, a 2% solution of lysol is often used to receive contaminated tubes, pipettes, etc.
KMI1104 (Potassium permanganate). It is a good disinfectant for water purification; used in 5% solution. During cholera epidemic, wells and other sources of water can be disinfected with this chemical. 5% KMNO4 has germicidal effects; it oxidizes the organic matter which is food for bacteria and bacteria are starved to death. It is expensive and soon becomes inert by organic matters.
Bleaching powder. It is a hygroscopic powder, whitish is colour. It deteriorates on exposure due to absorptiOn of moisture and CO4 and with liberation of chlorine. It is prepared by pasing Cl gas through slaked lime. Ca(OH)2+C12 = Ca0C12+H20. It is kept in airtight containers. An ideal bleaching powder should contain 33% of chlorine (as a rough field check, the solution should have a strong chlorine odour and a ‘slippery’ feel that does not ‘burn’ the fingers). Bleaching powder spoils clothes by bleaching. For practical use, both powder and sloution can be used. As a powder it is used to disinfect faeces, damp corners of privies, latrines, drains, etc. It acts both as a germicide and as a deodorant. 5% solution can be used to disinfect stools, and the contact period is 1-2 hours. Solution (1 in 30) is used to disinfect rooms; 1% solution is germicidal and 10% solution will kill spores. Bleaching powder is widely used for chlorination of water. Colour-fast clothing ca n be disinfected by soaking in 2% chlorine solution for 38 minutes.
Chlorine tablets (Halazone tablets). One tab.t containg 4 mg of halazone is enough to disinfect i litre of water in 1/2 to 1 hour.
Lime. Used as fresh –wick lime or 10% to 20% aqueous suspension is a god disinfectant and is the cheapest. Stool and urine ban be disinfected in 10-20% acqueous suspension in 2 hours time. As lime wash, it is used for disinfecting walls. As a deodorant, lime is sprinkled in stables and in public urinals and latrines.
Formalin. Commercial formalin is a 40% (w/v) solution of formaldehyde in water. It does not spoil fabrics and metals. A 2% solution (20 ml in one litre of water) can be used for disinfecting walls and furniture.
Iodine. In 1-2 percent alcoholic solution, iodine is one of the most effective skin antiseptics, but it stains skin and prodtices sensitivity reactions in some individuals. It is cheap, readily available and quick in action.
Iodophors. These are complexes of iodine and ‘solubilizers’, e.g., povidone-iodine (betadin), and are claimed to have the activity as iodine. but are not irritants, and do not stain skin.
Chlorhexidine. This is a very useful skin antiseptic. (i) Hibitane (0.5 percent aqueous or alcoholic solution) is an effective hand. lotion. Creams or lotions containing 1 percent chlorhexidine are used for disinfection of hand, (ii) Savlon (an aqueous solution containing 1.5% V/V chiorhexidine gluconate and 15% W/V cetrimide) is an antiseptic for general use combining antiDisinfectionbacterial activity and cleansing properties. It is always diluted before use.
Chloroxylenol (Dettol). It is a relatively nontoxic antiseptic and can be used in high concentration. Dettol is non-irritating, but readily inactivated by organic matter and hard water. It is active against streptococci, but not effective against some gram-negative bacteria. Dettol (5%) is suitable for disinfection of instruments and plastic equipment in 15 minutes.
Quaternary Ammonium compound.Centrimide (which is present in Savlon) and berzalkoniurn chloride are examples of quarternany ammonium antiseptics. These can be used to disinfect plastic appliances like Lippes loop, exposure time being 20 minutes.