Sterilization and Disinfection

Sterilization is the killing or removal of all microorganisms, including bacterial spores. It is an absolute germ free state. Disinfection is the killing of many but not all, microorganisms. Specially bacterial spores are not killed. Disinfectant is a chemical agent that is used for disinfection.

Methods of sterilization: I. Heat II. Ionizing radiations III. Filtration IV. Chemicals I. Heat: It can be again a. Dry heat b. Moist heat c. Pasteurization

a. Dry Heat: Sterilization by dry heat requires temperatures in the range of 180°C for 2 hours. Used primarily for glassware. Used less frequently than autoclaving. Dry heat can be applied by i. Red heat: The article to be sterilized is directly held in the flame and heated till it becomes red hot. ii. Hot air oven: Articles are sterilized in hot air oven at temp range of 1600C-1800C.

1600C is sterilization temperature. Uses: Glassware, sealed containers. etc are sterilized by this method.

b. Moist heat: It can be applied by Boiling Steaming 1. Boiling: Simple boiling is used to sterilize some glassware.

Boiling done for 20-30 minutes for three consecutive days is called Tyndalization. It is used for materials which can not withstand prolonged boiling. 2. Steaming Most common method to be used. It is normally applied above 1000C under high pressures. The instrument used for it is called autoclave.

Principle of autoclaving: In autoclave steam or moist heat is produced at a temp of 1150C. Water boils at 1000C. At this temp vapour pressure equals the atmospheric pressure i.e. 760 mm Hg. When water is heated in a closed vessel the vapour pressure rises with a subsequent rise in boiling point. The boiling point rises to 1150C.

Thus steam produced at this temp is super heated. This steam when condenses on the articles thermal energy is released from it and along with moisture this heat denatures the microbial proteins. Autoclave is used to sterilize most of the instruments and culture media.

Different temperatures for autoclave at different pressures. 1150C at 10 psi 1210C at 15 psi 1320C at 27 psi

Stovetop autoclaves—the simplest of autoclaves

Dental autoclave

It is used primarily for milk. c. Pasteurization: It is used primarily for milk. It consists of heating the milk to 62°C for 30 minutes followed by rapid cooling OR heating it at 720C for 20 seconds. This is sufficient to kill the vegetative forms of the milk-borne pathogens, e.g. Mycobacterium bovis, Salmonella, Streptococcus etc In industry UHT is the method of choice, different from pasteurization. The temp applied is 140oc for 4 sec and then rapidly cooled down.

II. Ionizing radiations: The three types of radiation used to kill microorganisms 1.Ultraviolet (UV) light and 2. X-rays. 3. Gamma rays Used in industry to sterilize disposable syringes, gloves, catheters etc. III. Filtration: Filtration is the preferred method of sterilizing certain solutions, e.g. IV fluids. Antibiotic solutions. Cellulose acetate membrane filter most common. Pore size 0.02-1 µm. Removes bacteria.

High-efficiency particulate air (HEPA) filters remove almost all microorganisms larger than about 0.3 micron in diameter. They are used to remove micro organisms from air. Used specially in TB labs.

Filter sterilization with a disposable, presterilized plastic unit.

IV. Chemicals: They can be a. Gases like Ethylene dioxide used in plastic industry. Highly toxic inflammable ,potentially explosive gas . b. Liquids like Glutraldehyde used to disinfect endoscopes.

Methods of Disinfection: Disinfectants are strong chemicals used for disinfecting non living articles. They are toxic and corrosives for living tissues. Antiseptics: Disinfectants which are less irritant and can be applied to living tissues are called antiseptics e.g. methylated spirit, Alcohol, Iodine, Chloroxylenol (dettol) etc.

Lister was the first to use phenol (carbolic acid) to control surgical Types of Disinfectants. 1.Phenol and Phenolic compounds. Phenol (carbolic acid) and phenolics are good disinfectants. Phenolics are derivatives of Phenol. Cresols is derived from coal tar form an important group of phenolics. 2.Bisphenols: Bisphenols are derivatives of phenol. Lister was the first to use phenol (carbolic acid) to control surgical infections in the operating room.

Hexachlorophene and Triclosan are important bisphenols. 3. Biguanides: The best known biguanide is chlorhexidine used for microbial control on skin and mucous membranes. Hibiscrub (chlorhexidine + cetrimide ) Hibisol (chlorhexidine + glycerine)

4. Halogen compounds. The halogens, particularly iodine and chlorine, are effective antimicrobial agents. The most common commercial preparation of Iodine is povidone-iodine (Pyodine). A liquid form of compressed chlorine gas used extensively for disinfecting municipal drinking water, water in swimming pools, and sewage.

3. Aldehydes: Formaldehyde as gas used to sterilize operation theatres and other spaces. Glutaraldehyde a chemical relative of formaldehyde less irritating and more effective than formaldehyde. Glutaraldehyde used to disinfect hospital instruments, including endoscopes and respiratory therapy equipment

4. Alcohol: Alcohols effectively kill bacteria and fungi but not spores and non enveloped viruses. Most commonly used alcohols are ethanol and Isopropanol. The recommended conc. of ethanol is 70% Isopropanol is slightly superior to ethanol as an antiseptic and disinfectant However, alcohols are unsatisfactory antiseptics when applied to wounds.