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Sterilization
Lab 3
Abeer Saati
Sterilization
Sterilization
is a term referring to any process that eliminates (removes)
or kills all forms of microbial life, including transmissible
agents (such as fungi, bacteria, viruses, spore forms) present
on a surface, contained in a fluid, in medication, or in a
compound such as biological culture media.
CLASSIFICATION OF METHODS
Sterilization and disinfection are done by:
(A). Physical methods
1. Heat
2. Radiation
(B). Chemical methods
(C). Mechanical methods
(A) Physical methods
1.Sterilization by heat:Heat is most effective and a rapid method of sterilization and
disinfection. Excessive heat acts by coagulation of cell proteins. Less heat
interferes metabolic reactions. Sterilization occurs by heating above
100°C which ensure lolling of bacterial spores. Sterilization by hot air in
hot air oven and sterilization by autoclaving are the two most common
method used in the laboratory.
Types of heat :
A. Sterilization by moist heat
B. Sterilization by dry heat
(A) Physical methods
A. Sterilization by Moist Heat
Moist heat acts by coagulation of protein, breakage of DNA strands, and loss
of functional of cell membrane.
Sterilization above 100°C: (Autoclaving)
Autoclaving is one of the most common methods of sterilization. Principle: In
this method sterilization is done by steam under pressure. Steaming at
temperature higher than 100°C is used in autoclaving. The temperature of
boiling depends on the surrounding atmospheric pressure. A higher
temperature of steaming is obtained by employing a higher pressure. When the
autoclave is closed and made air-tight, and water starts boiling, the inside
pressures increases and now the water boils above 100°C. At 121°C
temperatures is obtained. This is kept for 15 minutes for sterilization to kill
spores. It works like a pressure cooker.
(A) Physical methods
B. Sterilization by Dry Heat
Dry heat at 160°C (holding temperature for one hour is required to kill the
most resistant spores). The articles remain dry. It is unsuitable for clothing
which may be spoiled.
--Red Heat. Wire loops used in microbiology laboratory are sterilized by
heating to 'red' in bunsen burner or spirit lamp flame. Temperature is above
100°C. It leads to sterilization.
-- Sterilization by Hot Air
Hot Air Oven (Sterilizer). It Is one of the most common method used for
sterilization. Glass wares, swab sticks, all-glass syringes, powder and oily
substances are sterilized in hot air oven. For sterilization, a temperature of 160°C
is maintained for one hour. Spores are killed at this temperature. It leads to
sterilization.
(A) Physical methods
2.Radiation sterilization:Methods of sterilization using radiation such as X-rays, gamma rays, or
UV. The major target for these radiation is microbial DNA.

Gamma rays are very penetrating and are commonly used for
sterilization of medical equipment, such as syringes, needles.

X-rays are a form of ionizing energy allowing to irradiate large
packages for sterilization of medical equipment.

Ultraviolet light (UV) is useful only for sterilization of surfaces and
some transparent objects.
(B). Chemical methods
Chemicals that destroy pathogenic bacteria from surfaces.
Some chemical have very narrow spectrum of activity and
some have very wide. Those chemicals that can be safely
applied over skin and mucus membranes are called
antiseptics
(B). Chemical methods
Classification of disinfectants:
1. Based on consistency
a. Liquid (E.g., Alcohols, Phenols)
b. Gaseous (Formaldehyde vapor, Ethylene oxide)
2. Based on spectrum of activity
a. High level
b. Intermediate level
c. Low level
3. Based on mechanism of action
a. Action on membrane (E.g., Alcohol, detergent)
b. Denaturation of cellular proteins (E.g., Alcohol, Phenol)
c. Damage to nucleic acids (Ethylene Oxide, Formaldehyde)
(C). Mechanical methods
Filtration does not kill microbes, it separates them out. Membrane filters with
pore sizes between 0.2-0.45 μm are commonly used to remove particles from
solutions that can't be autoclaved. It is used to remove microbes from heat labile
liquids such as serum, antibiotic solutions, sugar solutions, urea solution.
Different types of filters are:
•Earthenware filters
•Asbestos filters
•Sintered glass filters
•Membrane filters
Thank you