Methods of sterilization, advantages, disadvantages, uses and monitoring

Here is a discussion of different methods of sterilization , advantages and disadvantages and monitoring of each method .
The main methods of sterilization are :

 Heat sterilization 

- Heat is the most practical, efficient and cheap method of sterilization and disinfection of objects that can withstand high temperature , that's why Heat is the most frequently used mean of sterilization .

- There are two types of sterilization : Moist (steam) and Dry heat .

A. Steam sterilization 

- In this method , the sterilizing agent is saturated water steam under high pressure , which is obtained by heating water under controlled conditions in a closed vessel ( Autuclave ) in order that boiling point of water rises above 100'C .
- Saturated steam carries the same temperature of boiling water .
- It is essential to make steam saturated and free of any residual air because air acts as an insulator, reduces temperature and hinders penetration .

- Sterilization requires exposure to moist heat at 121'C for 20-30 minutes Or at 134'C for 3-6 minutes .

- Steam sterilization is the most common and preferred method for sterilizing items that can withstand high temperature and moisture such as :
culture media , surgical instruments and dressings .

Advantages of Steam sterilization :

1. A good ability of hot saturated steam to penetrate into loaded porous items .
2. Liberation of latent heat of vaporization after cooling due to condensation of the steam .
3. No toxicity .
4. Low cost .

Monitoring of steam sterilizers (Autoclaves)

a. Physical measurement of tracks of the time, temperature and pressure .

b. Chemical indicators : by chemically impregnated paper strips which change their color at certain temperature e.g 121 or 134'C .

c. Biological indicators : which are paper strips containing the spores of Bacillus Sterothermophillus , placed in the depth of the load to be sterilized .After finishing the sterilization cycle , Spore strips are incubated in a fluid medium and checked for viability .

B. Dry heat sterilization

1. Incineration : It is applicable for dead animal bodies and infectious hospital waste such as used surgical dressings, needles,.....etc.

2. Red heat :
Inoculating wires, loops and points of forceps are sterilized by holding them in the flame until they are red .

3. Hot air sterilizer :
 In this method , the sterilizing agent is hot dry air which is applied at :
- 170'C for 60 minutes  Or 
- 160'C for 120 minutes .

Advantages of Hot air sterilizer

1. Can be used for sterilizing powders, waterless oils and glassware .
2. No corroding effect .
3. Low cost .

Disadvantages of Hot air sterilizer

1. Slow and uneven penetration of heat into the materials .
2. Needs prolonged exposure .
3. Damage to the rubber items and some fabrics .

Monitoring of Hot air sterilizer 

Bacillus Subtilis spores are used as a biological indicator .

Sterilization using Ethylene Oxide (EO) gas 

- Exposure time in EO usually varies from 3 to 6 hours .
- It is ideal to sterilize instruments that cannot be subjected to steam or hot air without damage .
- It has some flaws like high cost and toxicity in addition to long exposure duration .

Bacillus Subtilis spores are used as a biological indicator .

Sterilization by Irradiation 

Two types of ionizing radiation are commonly used for sterilization : 
1. Gamma rays : which are emitted by radioactive elements such as Cobalt 60 .
2. High energy electons (B-rays) : produced by electron accelerators .
- Ionizing radiation has a high penetrating power and is, therefore, a practical mean for prepacked heat-sensitive items such as bone grafts, surgical sutures, disposable plastic syringes , glove and IV sets .

Monitoring : Bacillus Pumilus 

Sterilization by Filtration 

- Fluids can be rendered free of bacteria by passage through filters with a pore size less than 0.45 micron ,However, most viruses and certain bacteria as mycoplasma can pass through filters with a pore size as small as 0.22 micron .

- This method is used for sterilizing fluids that cannot withstand heat such as Antibiotic solutions, blood products, hormones, vitamins.....etc.
Read Also :
Difference between Sterilization, Disinfection, Cleaning and Antiseptics

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