Sterilisation and Disinfection



Microorganisms are the causative agents of infection. They include bacteria, viruses, fungi, and parasites. Infection preventions often rely on placing barriers between the host and microorganisms. Protective barriers are physical, mechanical or chemical processes, which help to prevent the spread of infectious microorganisms from client to client, staff to client or vice versa due to lack of infection prevention practices or from contaminated instruments or equipments.




It is the combination of efforts made to prevent entry of microorganisms into any area of body where they are likely to cause infection. The goal of asepsis is to reduce or eliminate the number of microorganisms on both animate surfaces and inanimate objects to a safe level.



It is the prevention of infection by killing or inhibiting the growth of microorganisms on skin and other body tissues.



It is the process that makes inanimate objects safer to be handled by staff especially cleaning personnel before cleaning.



It is the process that physically removes all visible blood, body fluids or any other foreign material such as dust or soil from skin or inanimate objects.



It is the process that eliminates most, but not all disease causing microorganisms from inanimate objects.



It is the process that eliminates all microorganisms including bacterial endospores from inanimate objects.


Processes: (How to make instruments/items safer to contact)


Level of contact

with body

Appropriate infection

prevention process.

Intact skin Decontamination Instruments/gloves prior to cleaning.

Exam. tables or other surfaces contaminated by body fluids.

Intact mucous membrane

or broken skin

Decontamination & cleaning


Surgical instruments
Blood vessels or tissues

beneath the skin

Decontamination & cleaning


Surgical instruments.


Decontamination & Cleaning:

They are effective infection prevention measures that can minimize the risk of infection to health care workers and are also useful to break infection transmission cycle.



Decontamination is the first step in handling used surgical instruments, reusable gloves and other items. The objective of decontamination is to protect individual handling surgical instruments and other items, which have been in contact with blood or body fluids from serious diseases. Immediately after use these items should be placed in 0.5 % chlorine solution or other locally available disinfectants for 10 minutes. Person should wear gloves while handling used instruments and equipments. After decontamination surgical instruments should be immediately rinsed with water to help prevent corrosion and remove visible organic material before being cleaned.

Larger surfaces or tables can be decontaminated with wiping with suitable disinfectant such as 0.5 % chlorine solution at least daily. After decontamination items need to be further processed by cleaning and finally by sterilization or HLD.



Cleaning is a crucial step in providing safe, infection –free equipments and instruments. A thorough cleaning with detergent and other water physically removes organic material such as blood and secretions. Use of detergent is important for effective cleaning, since water alone will not remove protein, oil and grease. Use of liquid is preferable.

Cleaning is important because,

  • It is the most effective way to reduce the number of microorganisms or used instruments and equipments.
  • Both sterilization and disinfection procedures are not effective without prior cleaning.



  • Physical Methods:
    1. Heat:

Because of its reliable effects, ease of use, and economy, heat is the most common method used for the elimination of microorganisms. Heat can be used in several ways.

Moist heat or heat under steam pressure is the agent used in autoclaves. Putting steam under 15 psi (Pound per square inch) achieves a temperature of 1210 C. At this temperature, all microorganisms and their endospores are destroyed in approximately 15 minutes of exposure for media preparation and for hospital waste disposal autoclaving should be done for 45-60 minutes. Moist heat is the sterilization method of choice for heat-stable objects.

Dry heat may also be used as sterilizing agent, although it requires much longer exposure times and higher temperatures than moist heat. This method may be used for heat-stable substances that are not penetrated by moist heat, such as oils. Dry heat is commonly used to sterilize glassware.

Boiling and Pasteurization are both methods that achieve disinfection but not sterilization; neither method eliminates spores. Boiling (1000C) kills most microorganisms in 10 minutes. Pasteurization, used mostly in food industry, eliminates food-borne pathogens as well as organisms responsible for food spoilage.

Vaccines of non-sporing bacteria are heat inactivated in special vaccine baths at 60 0C for one hour. Serum or body fluids containing coagulable proteins can be sterilized by heating for one hour at 56 0C in a water bath on several successive days.

An atmosphere of free steam is used to sterilize culture media, which may decompose if subjected to higher temperatures.


Control of sterilization:

  • Biological control (Bacterial spores):
    • Bacillus stearothermophilus – Autoclave
    • Bacillus subtilis – ETO (Ethylene oxide) sterilizer
    • Bacillus pumilis – Radiation
    • Clostridium tetani (nontoxigenic strain) – Hot air oven


  • Chemical indicators:

Such as pellets of sulphur in test tube or Browne’s sterilizer control tubes may be used. At 1200 C sulphur pellets gets melted with change in shape and red solution of Browne’s tube turns red to green, when adequate temperature is maintained for adequate time.

  • Thermocouple:

It is placed in a test article and the attached wire is brought outside the chamber door to connect with a potentiometer to record the temperature directly.


  1. Filtration:

Filtration methods may be used with both liquid and air. Filtration of liquids is accomplished through the use of thin membrane filters composed of plastic polymers or cellulose esters. The liquid is pulled through small, mechanically introduced pores with a vacuum. Organisms larger than the size of the pores are retained. Filters of various sizes are available. The most common application of filtration is in the sterilization of heat-sensitive solutions, such as vaccines and antibiotic solutions. Filtration of air is accomplished with the use of high-efficiency particulate air (HEPA) filters. These filters are used in laboratory hoods and in rooms of immunocompromised patients.

  1. Radiation:

            Radiation may be used in two forms, ionizing and nonionizing. Ionizing radiation in the form of gamma rays or electron beams, is of short wavelength and high energy. This method of sterilization is used by the medical industry for the sterilization of disposable supplies, such as syringes, catheters, and gloves. Nonionizing radiation in the form of ultraviolet rays is of long wavelength and low energy. Because of its poor penetrability, usefulness of nonionizing radiation is limited; it is commonly used to disinfect surfaces.


  • Chemical methods:

Just as physical methods are used mainly to achieve sterilization, chemical agents are used mainly as disinfectants. Some chemical agents, however, may be used to sterilize. These are known as chemo sterilizers.



ETO sterilization is used for products that cannot withstand heat.  It is specially used for sterilizing plastic and rubber article, catheter, respirator, heart-lung machine, sutures, dental equipments and clothing. ETO gas infiltrates packages as well as products themselves to kill micro organisms that are left during production or packaging processes. This gas, mixed with air at a ratio of at least 3% ETO gas, forms an explosive mixture. Pure ETO gas boiling point is 10.73 ºC at atmospheric pressure. It is mixed with Nitrogen or CO2 to avoid its explosive property. It has a potential toxicity to human being, including mutagenicity and carcinogenicity.


Chemical agents commonly used as disinfectant and antiseptics:


Type Agents Action Application and precaution
Alcohol (50-70%) Ethanol, isopropanol, benzyl alcohol Denature proteins; make lipids soluble Skin antiseptics
Aldehydes Formaldehyde (8%), gluteraldehyde (2%) React with NH2, -SH, and –COOH groups Disinfectants; kill endospores, Toxic to humans
Halogens Tincture of iodine


Inactivates proteins Skin disinfectants
  Chlorine and chlorine compounds Reacts with water to form hypochlorous acid (HCIO); oxidizing agents Use to disinfect drinking water

Surface disinfectant

Heavy metals Silver nitrate Precipitate proteins Eye drop
  Mercuric chloride React with –SH groups

Lyses cell membrane

Disinfectant: toxic at high concentration
Detergents Quaternary ammonium compounds Disrupt cell membranes Skin antiseptics; disinfectants
Phenolics Phenol, carbolic acid, Lysol, hexachlorophene Denature proteins; disrupt cell membranes Disinfectant at high concentrations; used in soaps at low concentrations
Gases Ethylene oxide Alkylating agent Sterilization of heat-sensitive object.