Physical Agents Of Sterilization

Sterilization is the process of removal of all microbial life either living or in spore from certain surface. To achieve this we can use physical agents. Although the choice of sterilization process depend on the surface type and its uses after sterilization. Here we will look certain physical agents those are widely used for the sterilization process.


Because of the presence of ultraviolet rays and other cosmic rays, sunlight possesses appreciable bactericidal activity and can be used for spontaneous sterilization that occur under natural conditions. Under natural conditions sterilization power of sunlight varies according to circumstances, like direct sunlight as in tropical countryside, has an active germicidal effect due to the combine effect of ultraviolet and heat effect. Bacteria suspended in water are readily destroyed by exposure to sunlight.


Like any other life form on Earth, moisture is also important for the growth of bacteria. Four-fifth weight of bacterial cell is due to water. So, drying will kill the bacterial life. However, this is only theoretical as bacterial cells can form spores those are unaffected by drying.


Heat is the most effective method of sterilization. Materials those can be damaged by heat should be sterilized on low temperature for longer duration or by repeated cycles. Factors influencing sterilization by heat are:

• Nature of heat: dry heat or moist heat
• Temperature and time
• Number of microorganisms present
• Characteristics of the organisms, such as, species, strain, sporting capacity etc.
• Type of material from where organisms have to be eradicated.

Moist heat is considered as highest rate of sterilization because it can lead to denaturation and coagulation of protein. Steam present in moist heat can penetrate the cells and can kill spore, as steam condenses on them, increasing its water content with ultimate hydrolysis and breakdown of bacterial protein.

The time required for the sterilization is inversely proportional to the temperature of exposure and can be expressed as ‘thermal death time’, which is the minimum time required to kill a suspension of organisms in a predetermined temperature.

The sterilization time depend on the number of organisms in the suspension, presence or absence of spores, and the strain and characteristics of organism.

Dry Heat

Flaming: inoculating loop or wire is read heated before use. This is to kill any microorganism on the loop. This can be further added by dipping the loop in the disinfectant before flaming.

Incineration: this method is widely used to safely destroy material such as contaminated cloth, animal carcasses, and pathological materials.

Hot air oven: this is one of the most widely used method for sterilization by dry heat. A period of one hour at 160 C is used for the sterilization of glassware, forceps, scissors, scalpels, all-glass syringes, swabs, some pharmaceutical products such as liquid paraffin, dusting power, fats and grease.

Hot air oven is heated by the heating element within it. It must be fitted with a fan inside it to evenly distribute heat within the hot air oven. Glassware should be perfectly dry before placing in the hot air oven. Test tubes and flasks should be wrapped in paper. Cutting instruments such as those used in ophthalmic surgery, should ideally be sterilized for 2 hours a 150 C. the oven must be allowed to cool slowly for about two hours before the door is opened, since the glassware may crack due to sudden or uneven cooling.

Moist Heat

Temperature below 100 C for pasteurization of milk: the milk is heated at either 63 C for 30 minutes (the holder method) or 72 C for 15-20 seconds (the flash process) followed by cooling quickly to 13 C or lower. By these process all non sporting pathogens such as mycobacteria, brucellae and salmonellae are destroyed. Coxiella burnetii is relatively heat resistance and may survive the holder method.

Temperature at 100 C boiling: vegetative bacteria are killed almost immediately at 90 – 100 C, but sporting bacteria require prolonged periods of boiling.

Sterilization may be promoted by addition of 2% sodium bicarbonate to the water.

Steam at atmospheric pressure (100 C): an atmosphere of free steam is used to sterilize culture media which may decompose if subjected to higher temperatures.

Steam under pressure: autoclave or steam sterilizer works on the principle that water boils when its vapor pressure equals that of the surrounding atmosphere. Hence when the pressure inside a closed vessel increases, the temperature at which water boils also increases. Saturated steam has penetrative power. Sterilization by steam under pressure is carried out at temperature between 108 C and 147 C. by using appropriate temperature and time variety of materials such as dressings, instruments, laboratory ware, media and pharmaceutical products can be sterilized. There are several type of steam sterilizers are in use:

  1. Laboratory autoclave
  2. Hospital dressing sterilizers
  3. Bowl and instrument sterilizers
  4. Rapid cooling sterilizers.
    Even the domestic pressure cooker can be used as a sterilizer.
Standard Sterilization Timing and Temperature


There are two type of radiation can be used for the process of sterilization, nonionising and ionizing. Infrared and ultraviolet rays are of the nonionising low energy type, while gamma rays and high energy electrons are the high energy ionizing type.

Ultrasonic And Sonic Vibration

Ultrasonic and sonic waves can kill bacterial cells but the result have been variable. Microorganisms vary in their sensitivity to them and survivors have been found after such treatment. Therefore this method has no practical value in sterilization process.

Gaurav Singh

Editor in Chief Medical Microbiology & Recombinant DNA Technology (RDT) Labs - RDT Labs Magazine

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