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| Sterilization: Physical and Chemical Methods |
Sterilization:
Physical and Chemical Methods
Sterilization is the process by which all
viable microorganisms including spores are killed or eliminated.
There are
three types of sterilization procedures:
1.
Physical methods - Heat and Radiation
2.
Chemical methods.
3.
Filtration
Physical
Methods - Heat:
Heat is used to sterilize laboratory media
and glassware and instruments.
Heat kills microbes by denaturing proteins
and enzymes.
Heat resistance varies among microbes, these
differences can be expressed by D-value, Z-value and F-value.
Heat sterilization may be done by low
temperature and high temperature.
Low temperature (0-7℃) reduce the metabolic
activity of microorganisms. Sterilization by low temperature includes water
bath, vaccine bath and inspissator, lyophilization.
High temperature sterilization is divided
into two groups - Dry heat sterilization and Moist heat sterilization.
Dry heat
sterilization:
Dry heat sterilization includes incineration,
flaming, hot air oven. This
sterilization method kills microbes by
oxidation.
a) Incineration
- It is an
effective way to sterilize and dispose of contaminated papers, dressings by
burning.
b) Flaming
- It is
used to sterilize inoculating loops by heating the wire.
c) Hot air
oven- a special electric or gas oven is used, where 170℃ temperature
is maintained for 2 hours. This method is used to sterilize glass wares,
syringes, flasks and pipettes.
Moist heat
sterilization:
Moist heat sterilization includes boiling, steaming,
pasteurization, tyndallization. This
sterilization method kills microbes by
coagulation of proteins.
a) Boiling
- kills
vegetative forms of microorganisms such as bacterial species, viruses and fungi
and fungal spores within ten minutes at 100℃ temperature. But endospores resist
this temperature.
Syringes and dissecting instruments are
sterilized by boiling in water for 30 minutes.
b)
Steaming - This is a reliable sterilization method. It is may be
applied with or without pressure.
An example of steaming with pressure is
autoclaving, in which temperature is maintained above that of boiling water (
121℃).
Autoclaving is used when the material to be
sterilized is sensitive to heat or moisture, such as culture media,
instruments, dressings, intravenous equipments, solutions, transfusions.
Pasteurization - It is a method of lowering
microbial numbers and reduction of pathogens by heating.
Many thermoduric (heat -resistant) bacteria
survive pasteurization.
Milk is pasteurized at the temperature of
72℃ for 15 seconds, known as HTST (high temperature short
time) and at 140℃ for 2 seconds, known
as UHT ( ultra high temperature).
Physical
Methods - Radiation:
Radiation is the energy transmitted through
space in a variety of forms.
There are two types of radiation -
Ionizing radiation and Non-Ionizing
radiation.
Ionizing radiation - Ionizing radiation has a
wavelength less than 1nm.
Ionizing radiation forms highly reactive
hydroxyl radicals that kills microbial cells by damaging DNA.
Examples of ionizing radiation are gamma ray,
X-ray.
Non-Ionizing radiation - Non-Ionizing
radiation has a longer wavelength greater than 1 nm.
Example of non-ionizing radiation is
ultraviolet light having a wavelength of 260 nm.
Ultraviolet light kills microbes by damaging
DNA of the exposed cells and forms bonds between adjacent pyrimidine bases.
Ultraviolet light is uses to sterilize air in
a specific area, vaccines, serum and toxins.
Chemical
Methods:
a)
Alcohols - such as ethanol and isopropanol, kill bacteria and fungi
except endospores and non-enveloped viruses by protein denaturation disrupting
membranes and dissolving lipids.
70% aqueous ethanol solution is bactericidal.
Isopropanol is used as rubbing alcohol, antiseptics and disinfectants.
b)
Aldehyde - they are most effective. They form cross-links with
functional groups (-OH, -COOH, -SH) on proteins present in microbes.
40% aqueous solution of formaldehyde is used
as a disinfectant.
Glutaraldehyde, which is more effective than
formaldehyde is used to disinfect hospital instruments.
c) Phenol
- Phenol
and phenolic compounds degrade membrane of microbial cells and denature enzymes
present in cells.
For example,
detol, lysol, crysol, hexachlorophenes.
e)
Ethylene oxide - powerful disinfectant. It kills spores as well as
bacteria.
f)
Halogens - Halogens such as chlorine and iodine are used for
sterilization.
Chlorine is used to disinfect water and it is
a strong oxidizing agent.
Iodine is used for skin sterilization as
tincture iodine. Iodophors have anti-microbial activity.
"Sterilization:
Physical and Chemical Methods"
Written By
Sadia
Akhtar
Student of
Department of Microbiology
Jagannath
University.
Email-
sadiabd810@yahoo.com
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