Sterilization is the elimination of all microorganisms, including spores, by means of various methods. Sterilization is a method used in the safe preparation of nutrients and other products used to protect against disease-causing microorganisms that threaten human health.

 Basic principles for effective sterilization:

1-   The object to be sterilized should be washed well.

2-   The type of sterilization used must act on the entire surface of the object to be sterilized.

3-   The objects to be sterilized must be packaged correctly.

4-   The duration of the sterilization type should be sufficient.

5-   Objects must be stored under appropriate conditions to avoid disruption of sterility.

6-   The type of the vehicle and the date of sterilization should be written on the sterilized object.

7-   The sterilized object should be used during the sterilization period.

8-   Depending on the material to be sterilized and the method of sterilization, an indicator should be placed (indicators of organic origin, which can change color with the heat and chemical factors that verify the sterility of the materials to be sterilized).

Effective application of sterilization is very important in combating infections. Protection from infections is only possible if the sterilization and disinfection rules are understood and applied.

Sterilization Methods

The choice of methods of sterilization and disinfection depends on the type of object and microorganism to be sterilized, the degree of contamination and the method used by the institution. Sterilization and disinfection are achieved by physical (heat, ray, filtration, ultrasonic sound waves) and chemical (solutions) means. The quality of the sterilization process depends on the sterilization method used, as well as on the principles of basic sterilization.

Sterilization by Physical Methods

Physical methods can be listed as hot, cold, drying, rays, electric current, sonic-ultrasonic vibrations, high pressure, osmotic changes and filtration methods.  However, the most commonly used physical method is heat. It is economical, easy to apply and reliable.

1-   Heat sterilization: The effect of heat sterilization directly on the protein structure of microorganisms. It creates coagulation in proteins. Heat sterilization takes place in two ways.

a)   Dry hot air sterilization: Dry hot air environment. Sterilization takes longer since there is no moisture in the environment. For this purpose, a Pasteur kiln is used. Dry oven (pasteur oven): generally one hour is sufficient  at 175 ° C and 3 hours for sterilization at 140 ° C. Glass and metal tools are sterilized by oils and powders (talc) that moisture cannot reach. Sterilization is possible at very high temperatures in a short time. This technique is called ultra high temperature (UHT). A liquid is heated up to 135-150 ° C and held at this temperature for 4 seconds. The effect on microorganisms is less than that of humid heat. It is considered as a disinfection method because it is thought not to kill all microorganisms. With dry air, powders, oils and glassware that do not allow permeability of moisture are disinfected. The heat-time relationship with this method is as follows: 170 ° C - 1 hour, 160 ° C - 2 hours, 120 ° C - 8 hours.

b)   Moist heat sterilization: Moist heat sterilization is carried out by the following methods. • Pressurized water vapor (autoclave): It is the most commonly used, most effective and reliable sterilization method. It kills all microorganisms, including spores. The temperature-time relationship in pressurized saturated water vapor and sterilization is as follows: 30 minutes at 121 ° C, 20 minutes at 126 ° C, and 15 minutes at 134 ° C. This method is used for sterilization of inanimate objects such as surgical instruments, liquids, cotton materials, gloves.

• Non-pressurized steam sterilization (koch boiler): The temperature of the steam of the boiling water is 100 ° C at normal atmospheric pressure (760 mm Hg). The higher the pressure, the lower the boiling point. The Koch boiler is cylindrical and autoclave-like. It works with air, gas or electricity, has a conical cap on it and a fixed manometer.  The boiler has a grill in which the article to be sterilized is placed. Water will be put in the bottom of the boiler and heated. The resulting steam fills the inside of the boiler and exits through the hole between the lid and the body of the boiler. Water is placed in a drop of water from a tap on the side to avoid water. Since there is no pressure and the temperature is not too high, it is very simple to open and close.

• Boiling: Heat-resistant microorganisms, spores and some viruses can withstand a few hours at 100 ° C. For this reason, boiling can be used only for disinfection of heat resistant metal and glassware at home. It is used for sterilization of substances not more than 100 ° C. However, there are two important issues to bear in mind: Water boils at 100 ° C at sea level 760 mm Hg. Sterilization time varies according to the type of substance to be sterilized and the killing of spores is never possible at all. 

2-   Sterilization by filtration:This is applied in sterilization of liquids. It is the process of separating the microorganisms in the liquid environment from the liquid. Sterilization of yeasts is carried out by this method. Instruments used for filtration are called filters. It is especially used for other substances that can be degraded by other sterilization methods. Chemical substances are used for disinfection rather than sterilization. This method does not cause any damage to the instruments sterilized.

3-   Ray sterilization: Ultraviolet rays, X-rays, beta and gamma rays are used for sterilization with rays. These rays act by disrupting the structure of microorganisms DNA (deoxyribonucleic acid). It is used in sterilization of special materials such as prosthesis, synthetic heart valves and surgical material in the food industry, environment and water sterilization. Preparation should be made before sterilization, so that the sterilized instrument or substance can maintain this property for a long time. For this purpose, tools such as scissors, pliers, scalpels are wrapped in tightly woven cloth covers, tins or wrapping paper.

4-   Sterilization by Chemical Methods: Chemical sterilization is most commonly done by the following methods.

a)   Chemical sterilization with gases: The gas generally used is ethylene oxide. This gas is effective on all types of microorganisms. The material removed from the sterilizer should not be used immediately. Ventilation is carried out by leaving the material at 50-60 ° C for 8–12 hours or at room temperature for 7 days.

b)   Chemical sterilization with liquids: Eliminates all forms of bacteria, fungi, tuberculosis bacilli and viruses when used properly. Usually, glutaraldehyde and formaldehyde are used as sterilization solutions. A 2% solution of glutaraldehyde is effective. Usually used for sterilization of instruments such as cystoscopes, bronchoscopes. In order to achieve sterilization, the instruments should be kept in glutaraldehyde for 10 hours.