Sterilization : Definition, Methods & Types | Short Notes

Sterilization is the killing or removal of all microorganisms, including bacterial spores which are highly resistant. Sterilization is an absolute term, i.e. the article must be sterile meaning the absence of all microorganisms.

Uses of Sterilization

1.   Sterilization for Surgical Procedures: Gloves, aprons, surgical instruments, syringes etc. are to be sterilized.

2.   Sterilization in Microbiological works like preparation of culture media, reagents and equipment's where a sterile condition is to be maintained.

METHODS OF STERILIZATION

I. Physical Methods:

1. Heat:

(a) Dry heat (inceration, red hot, flam­ing, hot air oven, infrared radiation);

(b) Moist heat (Pasteurization, boiling water, steam sterilizer, autoclave).

2. Radiation:

(a) Non-ionising radiation (Ultraviolet radiation);

(b) Ionising radiation (X-ray, gamma ray).

3. Filtration (Berkfeld, Chamber land, Seitz, sintered glass, cellulose membrane fil­ters).

II. Chemical Methods:

Chemicals are also used for sterilization. Heating provides a reliable way to rid objects of all transmissible agents, but it is not always appropriate if it will damage heat-sensitive materials such as biological materials, fiber optics, electronics, and many plastics.

I. Physical Methods:

1. Heat:

• Heat is most effective and a rapid method of sterilization. 
• The heat used kills the microbes in the substance.
• This method of sterilization can be applied only to the THERMO STABLE PRODUCTS

Types of Heat Sterilization :

a) Moist heat Sterilization
b) Dry heat Sterilization

a) Moist heat method of sterilization: Here heat is applied in the form of steam or just boiling. This  method includes techniques like

• Boiling.
• Pasteurization.
• By use of steam (Autoclave).

Boiling is preferred for metallic devices like surgical scissors, scalpels, needles, etc. Here substances are boiled to sterilize them.

Pasteurization is the process of heating the milk at a temperature of 6o degrees or 72 degrees 3 to four times. Here alternative heating and cooling kills all the microbes and molds without boiling the milk.

Using Steam (autoclaving) Here the substances are subjected to sterilization in an autoclave a steam sterilization equipment. The process is carried out at a temperature of 115 degrees for 60 min or 121 degrees for 20 min at 15psi pressure. The saturated steam is formed at boiling temperature of water, i.e.,100 degrees. This steam condenses on the material and relieves the latent heat repeatedly to convert back into the water. Further, the saturated steam under pressure penetrates all the narrow spaces leaving no microbes alive thereby making the sterilization very efficient.

It is the most common method used for drugs as it is powerful enough even to kill bacterial spores. Bacterial spores are the forms of bacteria which are inert. They form a rigid cover over the cell wall during harsh climate. This cover prevents any damage to cell and drying of the cell. By steam sterilization, these forms of bacteria are also killed as steam destroys the cell wall.

B) Dry heat methods: Here the substances are subjected to dry heat like

• Flaming
• Incineration
• Hot air oven.

Flaming is the process of exposing metallic device like the needle, scalpels, scissors to flame for few minutes. The fire burns the microbes and other dust on the instrument directly.

Incineration is done especially for inoculating loops used in microbe cultures. The metallic end of the loop is heated to red hot on the flame. This exposure kills all the germs.

Hot air oven is suitable for dry material like powders, metal devices, glassware, etc. Here thermostable materials on the racks inside the hot air oven. Then in the closed oven, hot air is circulated at particular temperature and time.

2. Radiation:

Radiation method involves exposing the packed materials to radiation for sterilization. There are two types of radiations available for sterilization i.e. non-ionic and ionic radiation.

• Non-ionic radiations are safe to the operator of sterilization, and they are like Ultra Violet radiations, they can be used even at the door entrances to prevent entry of live microbes through the air.
• Ionizing radiation sterilization. They are powerful radiation and very useful for sterilization. The operator needs to protect himself from exposure from these radiations by use of special clothing.  Ex: X-rays, γ-rays, etc.

3. Filtration

Filtration: Here the liquids are filtered through bacterial filters to remove any microbes present. This method is very effective for sterilization of heat sensitive liquids. The chances of clogging and long time duration for the process to happen are drawbacks.

Types of Filteration

A) Membrane filters: These are thin filters which are made of cellulose. They can be employed for online sterilization during injection by placing the membrane between the syringe and needle. Used for sterilization of solvents, gasses.

B) Seitz filters: These are made of asbestos or other material. They are pad like and thicker than membrane filters. They do not rupture during filtration. But the solution might get absorbed by the filter pad itself. An alternative type of filter is sintered glass filters. These are made of glass and hence do not absorb liquids during filtration. The disadvantage is  that they are very brittle and break easily.

c) Candle filters: These are made of clay like diatomous mud. This special mud has minute pores made by algae. The filters have many minute lengthy  pores. The microbes get stuck during their travel through the pore in the candle.

d) Berkefeld Filters: They are made from a fossil diatomaceous earth found in depos­its in Germany and other parts of the world and are of three grades of porosity,e.g., V (Veil) the coarsest, W (Wening) the finest, and Normal (N) intermediate. Berkefeld V is often used. A small organism (Serratia marcescens) should pass through this fil­ter.

E) Chamberland Filters: They are made of unglazed porcelain of various grades of porosity. The finer grades will pass only certain viruses of extreme minuteness (e.g., Foot and Mouth Disease virus).The most porous grade L1,clarifying filter, al­lows many microorganisms to pass. The next three L1, L2 and L3 are comparable with the Berkefeld V, N, and W candles, respectively. These filters can also be used to obtain bacterial toxin.

II. Chemical Methods:

i) ALCOHOLS :

Ethanol (80% v/v ethyl alcohol) or 2-propanol (60-70% v/v iso- propyl alcohol) solutions are used to disinfect skin and decontaminate clean surfaces.

Spectrum: Effective against fungi, vegetative bacteria, Mycobacterium species and some lipid-containing viruses.

Limitation: Not effective against spores.Concentration: Most effective at 70% in water.

Caution: Do not use near flames due to flammability. May swell rubber or harden plastics.

ii) FORMALDELYDE: 

Precautions are required when handling formaldehyde Formalin is 37% w/v formaldehyde gas in water. 

Spectrum: Active against most microorganisms. Bactericidal, sporicidal, virucidal.

Fumigation Technique : After sealing the windows and other outlets, formaldehyde gas is generated by adding 250g of KMnO4 to 500 ml formalin for every 1000 cu.ft of room volume. 

Caution: The reaction produces considerable heat, and so heat resistant vessels should be used. After starting generation of formaldehyde vapour, the doors should be sealed and left unopened for 48 hours. Formaldehyde can react with free chlorine to produce toxic gas. Remove hypochlorite solutions and hydrochloric acid from spaces to be decontaminated. Irritant vapours are released hence Neutralise with ammonia following decontamination.

iii) Peracetic acid: Peracetic acid (0.2%) is a recognized sterilant by the FDA[39] for use in sterilizing medical devices such as endoscopes.