METHODS OF STERILIZATION

      Sterilization is defined as a process by which an object or a material is made free from viable microorganisms by any kind of killing them or remove them by physical or chemical methods. It is necessary the place you are working with microorganisms should be free from contamination. The media and material you are using to grow specific microorganisms should be free from other microbial contamination. For this purpose, sterilization of place of work, material and media have to be done. Several physical and chemical methods are used for sterilization.

I. PHYSICAL METHODS

 Physical agents such as heat, cold, radiation and filtration are used.

1. Heat

a. Direct heat

Direct heating of instrument is an easy way of sterilization. Inoculation needle, Forceps,Scissors are commonly sterilized by direct heat from spirit lamp.

b. Dry heat on hot air oven

Glassware such as pipettes, test tubes, Petri dishes are sterilized by hot air. Hot air oven is used for this purpose. It is constructed with three walls and two chambers. The outerwall is covered with asbestos or glasswool to reduce the radiation of heat. Electric coiling runs in between intermediate and inner wall. A switch, a thermostat, two lamps and a thermometer are provided in hot air oven. The red lamp indicates the working condition and yellow or green lamp indicates the stead temperature. For sterilization the glassware’s have to be kept at 160°C for two hours in the hot air oven.

c. Steam under pressure

This is more efficient is penetrating material and preferred in sterilizing the media,clothes, rubber. Autoclave is a metallic vessel with a double wall. Inside the autoclave steam under pressure provides increased temperature. For sterilization the material should be subjected to 121°C for 15 minutes. This temperature can be achieved with steam under 15 lbs pressure per square inch.

2. Cold treatment or dessication

Cold has no germicidal effect but arrest the activity and multiplication of microorganisms

3. Radiation

By using high energy rays such as U.V. rays, X-rays, y-rays, sterilization can be achieved. UV rays are used to sterilize culture rooms, culture chambers-Laminar flow and operation theatres. They are most effective in destroying microorganisms. U.V. lamps have quartz filled with mercury. It has a wavelength of 255m. They rays of this wavelength area by nucleic acids (DNA) resulting in the damage of the cell structure and its function.

4. Filtration

There are certain materials which will be destroyed in little heat. They are called as thermolabile substances. They are heat unstable. Eg. Blood serum, vitamins and amino acids.  They are sterilized by filtration.  

The following filters are commonly used.

1. Seitz filter

2. Asbestos filter

3. Fritted glass filter

4. Bacteriological filter

5. Colloidal membrane filter

6. Porcelain filter

Of these colloidal membrane filter is most effective. It is advantageous because, it is chemically inert. The filters remove bacteria in two ways.  

1. By the mechanical sieve like action of the minute pores of the filter.

2. By the adsorption of the microbes to filter because of the difference in their electrical charges.

II. CHEMICAL METHODS

Several chemicals are known to be Germicidal in their property. Chemicals are used for destroying the microorganism and sterilize the material. Salts of metals like Mercury, Silver, and Copper are used for sterilization. In microbiology laboratories 0.1% Mercury 'chloride (l-Fgcl2) solution is used as surface sterilizing agent to sterilize leaf bits, glass tubes and work tables. Halogens like chlorine or iodine are considered as bactericidal agent. C_'hlorine is used  in drinking water and in swimming pool at l ppm (Darts per million) concentration. Alcohol, ether, potassium permanganate, organic acids, formaldehyde, phenols, potassium hydroxide are used as chemical sterilizing agents.