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What is Sterilization | 2 Types Of Sterilization |

What is Sterilization


It is a process by which an article, surface or medium is made free of all micro-organisms either in the vegetative or spore form.
The process of killing or removing bacteria and all other forms of living micro-organisms and there spares from preparation.

Its Aim:
Is to provide a product that is safe and eliminates the possibility of introducing.

Importance Of Sterilization:

   1.Prevents the growth of diseases 

If left unchecked any medical device, bacteria comes onto it or not disinfected properly, it is highly likely that bacteria will grow.

   2.Prevents the spread of Diseases

If surgical equipment is not properly sterilized, patients treated are exposed to a disease the previous patient had.

 Method Of Sterilization 

What is Sterilization  2 Types Of Sterilization

   A. Physical method 

          2. Heat
               a)Dry Heat
                    b)Moist Heat
    B. Chemical Method 
           It is of 2 types:


  • Alcohol
  • Aldehyde
  • Phenol
  • Halogen
  • Heavy metals
  • Surface active agents
  • Dyes
  • Formaldehyde
  • Ethylene oxide
  • Plasma      





Physical method of sterilization



        Dry Heat



     Moist Heat




Gamma ray- Chamber

Sunlight has an active germicidal effect due to its content of ultraviolet rays. It is a natural method of sterilization in cases of water in tanks, rivers and lakes.


Heat is the most reliable and commonly employed method of sterilization. It should be the method of choice unless contraindicated. 


     a) Dry heat kills the organisms by denaturation of bacterial protein, oxidative damage by the toxic effect of elevated levels of electrolytes. The possibility of DNA damage incriminated as one of the mechanisms of inactivation of  microbes.
    b) Moist heat kills the microorganisms by denaturation and coagulation of proteins. 

Factors Influencing

             1. Nature of heat: Dry and Moist heat  
             2. Temperature and duration: The time required for sterilization is inversely                               proportional  to temperature to which organisms are exposed.

             3. Type of material: A high content of organic substances tend to protect the                               vegetative form and spores against the lethal action of heat. 

             4. Characteristic of microorganisms and spores present: Bacterial spores are killed by moist heat at 121C for 15 min. Most vegetative bacteria, fungi, and viruses are killed in 30 min at 65C by moist heat.

Dry Heat Sterilization 

It employs higher temperature at the range of 160-180C and require exposure time up to 2hrs, depending upon the temperature employed.

The benefit of dry heat include good penetrability and non corrosive nature which make it applicable for sterilizing glass wares and metal surgical instruments. It is also used for sterilizing non-aqueous thermo stable liquids and thermo stable powders.

Dry heat destroy bacterial endotoxins (or pyrogens) which are difficult to eliminate by other means and this property make it applicable for sterilizing glass bottles which are to be filled aseptically.

The following procedures are used for sterilization by dry heat.

        a)Red Heat 
        d)Hot air oven 

Red Heat
Inoculating wire or loops, tips of forceps and needles, are held in the flame of a Bunsen burner till they become red hot.

Glass slides, scalpels, mouth of culture tubes are passed through Bunsen flame without allowing them to become red hot.

By this method infective material is reduced to ashes, by burning. Instrument needs incinerator for this purpose. Soiled dressing, animal carcasses, bedding and pathological materials are dealt with this method.

Hot Air Oven

It is the most widely used method of sterilization by dry heat. The oven is electrically heated and it is fitted with a fan to ensure to ensure adequate and even distribution of hot air in the chamber. It is also fitted with a thermostat that maintain the chamber air at a chosen temperature.

Temperature and Time

160C = 2 hrs 
170C= 1 hrs
180C=30 min

It is used for sterilization of 
a)Glass wares like glass syringes, Petri dish, flask, pipettes, and test tubes.
b)Surgical instruments like scalpels, forceps, scissors etc.

c)Chemical such as liquid paraffin, fats, sulfonamides powders etc.


a)It should not be overloaded.

b)The material should be arranged in a manner which allow free circulation of air.

c)Material to be sterilized should be perfectly dry.

d)Test tube, flasks etc should be fitted with cotton plug.

e)Petri dishes and pipettes should be wrapped in paper.

f) Rubber materials(except silicane material) or any inflammable material should not be kept inside the oven.

g)The oven must be allow to cool for 2 hrs  before opening the doors, since the glass wares may crack by sudden cooling.

Sterilization Control

a)Thermocouples may also be used to record temperature

b)Brownies tube with green spot is available. After proper sterilization green color is produced(after 2hrs at 160C)

Moist Heat Sterilization

Moist heat sterilization involves the use of steam in the range of 121C-134C. Steam under pressure is used to generate high temperature for sterilization. Saturated steam act as as an effective sterilizing agent. 

Moist heat can be categorised into 3 groups:
  1. At a temperature below 100C
  2. At a temperature  at 100C
  3. At a temperature above 100C

At a temperature below 100C

1. Pasteurisation of milk-
Two types of method:
  • Holder method(63C for 30 sec)
  • Flash method(72C for 20 sec followed by cooling quickly to 13C or lower)
All non-sporing pathogens such as mycobacteria, brucellae, salmonellae are killed except coxiella burnetii which being relatively heat resistant may survive in holder method.

Some serum or egg media such as loweinstein-Jensen's and loeffler's medium, are rendered sterile by heating at 80-85C  temperature for half an hour daily on three consecutive days.This process of sterilization is called inspissation .The instrument used is called inspissator.

3.Vaccine Bath
Bacterial vaccine are sterilized in special vaccine bath at 60C for 1 hour. Serum or body fluids can be sterilized by heating for one hour at 56C in a water bath or several successive days.

4.Low Temperature Steam Formaldehyde(LTSF)-
Items which cannot stand by temperature of 100C may be sterilized by a method known as low temperature steam-formaldehyde sterilization.

At a Temperature 100C


Boiling for 10 to 30 min may kill most of the vegetative forms but many spores withstand boiling for a considerable time. When better methods are not available, boiling may be used for glass syringes and rubber stoppers. It is not recommended for the sterilization of instruments used for surgical procedures.


Steam at 100C for 20min on three successive days is used. This is known as tyndallisation or intermittent  sterilization. It is used for sterilization of egg , serum or sugar containing media which are damaged at higher temperature of autoclave. The instrument commonly used is koch's or Arnold's steam sterilizer.

3.Steam Sterilizer at 100C for 90 min-

Koch's or Arnold's steam sterilizer is usually used for media which are decomposed at higher temperature of autoclave. The article are kept on a perforated tray which steam can pass. They are exposed to steam at higher temperature for 90min.

At a Temperature above 100C(under pressure)

Water boils when its vapour pressure equals that of the surrounding atmosphere. When the atmospheric pressure raised then the boiling temperature is also raised. At normal pressure water boils at 100C but when  pressure inside a closed vessel increases, the temperature at which water boils also increases. The principle has been applied in autoclave and pressure cooker.


What is Sterilization | 2 Types Of Sterilization |
Autoclave machine

  • Steam above 100C or saturated steam has a better killing power than dry heat.
  • Bacteria are more susceptible to moist heat as bacterial protein coagulates rapidly.
  • Saturated steam can penetrate porus material easily.
  • The condensed water produces moist conditions for killing the microbes present.
  • Autoclave is modified pressure cooker or boiler.
  • It consist of a vertical or horizontal cylinder of gun metal or stainless steel in a supporting iron case.
  • The lid is fastened by screw clamps or rendered air tight by an absestos washer.
  • The lid bears a discharge tap for air and steam, a pressure gauge and a safety valve.
  • The cylinder is filled with sufficient water and the material to be sterilized is placed on the tray.
  • The lid is closed with a discharge tap open.
  • The heater is put on, safety valve is adjusted to the required pressure.
  • After boiling of water, steam and air mixture is allowed to escape till all the air has been displaced.
  • When the air bubble stop coming in the pail it indicates all the air has been displaced.
  • The discharge tap is now closed.
  • The steam pressure raised inside and when it reaches at a desired  set level, a safety valve opens and excess steam escapes.
  •  Holding period counted.
  • After the holding period is stopped and autoclave allow to cool till the pressure gauge indicated that the pressure inside the vessel is equal to atmospheric pressure.
  • The discharge tap is opened slowly and air is allow to enter the autoclave.
  • The lid is now opened and sterilized material is removed.
Sterilization Control

Temperature -121C
Holding time -15 minutes
Pressure - 15 lb per square inch

  • To sterilize culture media, rubber material,  gowns, dressing, gloves etc.
  • It is particularly useful for material which cannot withstand the higher temperature of hot air oven.
  • For all glasses syringes , hot air oven is a better sterilizing material.
  • The air must be allowed to escape from the chamber as temperature of air-steam mixture is lower than that of pure steam.
  • Materials should be arranged in such a manner as to ensure free circulation of steam inside the chamber.
Sterilization control
  1. Thermocouple - It is to record the temperature directly by a potentiometer.
  2. Bacterial Spores-Spores of bacillus thermophilus are used as the test organisms has an optimum growth temperature at 55C-65C and its spores are killed in 12 minutes at 121C.
  3. Chemical Indicators-Browne's tube containing red solution which turns green when exposed to temperature of 121C for 15 minutes in autoclave.
  4. Autoclave tapes.


Low Temperature Sterilization By Ozone.

Ozone sterilizer uses Oxygen, water, electricity to produce ozone within the sterilizer and provide sterilization without producing toxic chemicals.

4. Filtration

This method of sterilization is useful for substances which get damaged by heat process e.g sera, sugars , antibiotic, solutions etc.

Uses of Filtration
  • To sterilize sera, sugar, antibiotic solutions.
  • separation of toxins and bacteriophages from bacteria.
  • To obtain bacteria free filtrates of clinical samples for virus isolation
  • Sterilization of hydatid fluid
  • Filter discs retain the organisms which can then be cultured e.g testing of water samples for cholera vibrios or typhoid bacilli.
  • Purification of water 
Types of filter
  • Candle filter 
  • Asbestos disc filters 
  • Sintered glass filters
  • Membrane filters
  • Air filters
  • Syringe Filters
Candle filters

These have been used widely for purification of water for industrial and drinking purposes. These filters are made in the form of hollow candles.

Candle filters are of 2 types
  1. Diatomaceous earth filters
  2. Unglazed porcelain Filters
Asbestos Disc Filters 
These are made up of magnesium silicate. The filter disc is supported in a metal mount.

Sintered glass filters
These are prepared by fusing finely powdered glass particles. These are available in different  pore size.

Membrane Filters
Membrane filters are made up of cellulose filters. These are routinely used in water analysis.

Air Filters 
These filters are used to deliver clean bacteria-free air to cubicle or a room. High Efficiency Particulate Air (HEPA) are used in air filtration in laminar air flow system.

Syringe Filters 
Syringe fitted with membrane of different diameter are available.

5. Radiation

 Two types of radiation :

1.Ionising radiation
  • It includes gamma rays,  x-rays, cosmic rays. 
  • They have very high penetrating power
  • They are highly lethal to all cell including bacteria
  • They can damage DNA by various mechanisms 
2.Non-ionising radiation
  • These includes infrared or Ultraviolet radiation.
  • Infrared is used for rapid mass sterilization of syringes and catheters.
  • Ultraviolet radiation with wavelength of 240 to 280 nm has marked bactericidal activity.
  •  It acts as a denaturation of bacterial protein and interference with DNA replication.
  • UV radiation is used for disinfecting closed areas such as bacteriological laboratory, inoculating hoods , laminar flow, and operation theaters.
  • Most vegetative bacteria are susceptible to UV radiation but spores are highly resistant. 

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