Sterilization: A Comprehensive Description Introduction Sterilization is one of the most essential processes in microbiology, medicine, and laboratory sciences. It is defined as the complete elimination or destruction of all forms of microbial life, including bacteria, viruses, fungi, spores, and prions, from any object, surface, or medium. Unlike disinfection, which only reduces the microbial load, sterilization ensures total microbial eradication, making materials absolutely safe for use in medical, pharmaceutical, and research purposes. Sterilization has been practiced for centuries, with early methods involving heat and sunlight. With the advancement of medical science and technology, new and precise methods have emerged, including steam sterilization, chemical sterilants, radiation, and filtration. Understanding sterilization is crucial for healthcare professionals, microbiologists, laboratory technologists, and industries such as food processing, pharmaceuticals, and biotechnology. --- Historical Background of Sterilization Ancient practices: Boiling water and exposure to fire were the earliest forms of sterilization. Louis Pasteur (1822–1895): Developed the concept of microbial life destruction through heat, leading to “Pasteurization.” Joseph Lister (1827–1912): Introduced antiseptic surgery, emphasizing sterilization in medical practices. Modern era: Autoclaves, ethylene oxide sterilizers, and radiation techniques were introduced to achieve effective sterilization for large-scale use in hospitals and industries. --- Principles of Sterilization The main principle of sterilization is to destroy all microorganisms, including their most resistant forms such as bacterial endospores. To achieve sterilization, different agents are used, including: 1. Physical agents – heat, filtration, and radiation. 2. Chemical agents – gases, liquids, and disinfectants with sterilizing properties. 3. Physicochemical agents – combinations of heat and chemicals. The choice of sterilization method depends on: Nature of the material (heat-sensitive or heat-stable). Type of microorganisms present. Intended use of the material (medical instruments, lab media, food, etc.). Cost and practicality of the method. --- Methods of Sterilization Sterilization methods can be broadly classified into physical, chemical, and radiation methods. --- 1. Heat Sterilization Heat is the most widely used method because it is reliable, inexpensive, and easily available. a) Moist Heat Sterilization Moist heat is more effective than dry heat because water conducts heat better, penetrates faster, and denatures microbial proteins. Types of moist heat sterilization: 1. Boiling (100°C for 10–30 minutes): Kills most vegetative bacteria, fungi, and some viruses. Does not kill spores (thus, not a true sterilization method). Used in emergencies and household practices. 2. Tyndallization (Intermittent sterilization): Exposing material to steam at 10

1. Sterilization

2. What is sterilization
 The complete destruction of all forms
of microbial life from the substance
which is being sterilized.
 Sterilant:
It is an agent( may be a physical or
chemical) that destroys or eliminate all
forms of microbial life from the
sterilization substance.

3. Methods of Sterilization;
1. Physical method
2. Chemical method
1. Physical Method:
 Sunlight
 Drying
 Dry heat – flaming, incineration and hot air
 Moist heat- pasteurization, boiling and
steam under pressure.
 Filtration
 Radiation

4. Sunlight:
Sunlight contains U.V.
Rays and other heat Rays that
has bactericidal activity under
natural conditions.
Drying :
Drying in air has deleterious effects on
many bacteria.

5. Dry heat :
Flaming – sterilizing substances are shown
into the flame ,(E.g. Inoculating loops, are
sterilized by flaming , mouths of culture tubes,
glass slides, were also sterilized by this
method.)

6. Incineration- Materials were directly
burnt by fire, E.g. Hospital wastages,
dressings, pathological material were
directly burnt into fire.

7. Hot air – sterilization by applying hot air to the
substances, and this is the most widely used method of
sterilization. It is by means of Hot air oven. A holding
period of 160 C for 1 hour is used.
⁰
This method is used to sterilize glass wares, foreceps,
scissors, all glass syringes etc.

8. Moist heat
Pasteurization – heating the substance at the temperature of
63⁰C for 30 minutes or 72⁰C for 15 to20 seconds (flash
process). Mostly milk is sterilized by this method.
Boiling - boiling the substance at 100⁰C for 10 to 30
minutes can effectively sterilize the materials.
Steam under pressure-
applying moist heat with pressure
especially at 121 C at 15lbs
⁰
pressure (Autoclaving) by an
instrument called Autoclave. This
is most effective method of
sterilization. E.g. Microbiological
media, glass wares can be
sterilized by this method.

9. Filtration:
Heat sensitive substances especially solutions can be
sterilized by filtering the substance by Membrane filter in
which the pore size is less than 0.22µm. so that the
microorganism cannot penetrate the filter.

10. Radiation:
U.V. rays, X-rays, gamma rays were also
used in sterilization. U.V rays is used for
sterilization of hospital wards, operation theatres,
intensive care units, inoculation rooms etc.

11. Chemical Methods
1. Alcohols
2. Aldehydes
3. Halogens
4. Gases

12.  Alcohols
Alcohols such as ethyl alcohol and isopropyl
alcohol were used at concentration of 60 -70% in
water is used as skin antiseptic.
 Aldehydes
Formaldehyde and Glutaraldehyde were very
active against bacteria and viruses. Formaldehydes
were used in the preservation of anatomical
specimen.

13.  Halogens
Iodine mixed with alcohol were used as
skin disinfectant. It is very active against
bacteria
Similarly chlorine is also used as
disinfectant, in water supplies, food and
dairy industries for many years. It is very
active against bacteria.

14. Gases
Ethylene oxide gas – it is used to sterilize heat
sensitive materials, surgical equipments,
sutures, cannulas, syringes, gloves etc.
Formaldehyde gas- it is used to fumigate
laboratory, operations theatres and other
rooms.