Definition of the term sterilization
Sterilization
is a process of rendering an object free of any form of living organisms.
Physical Methods of Sterilization
• Dry heat
sterilization
• Moist heat
sterilization
•
Sterilization by filtration
•
Sterilizing by irradiation
Dry Heat Sterilization
• Is
achieved by applying dry heat, organisms are killed by:
o Protein
denaturation
o Oxidative
damage
o Toxic
effect of elevated levels of electrolytes.
Types of Dry Heat Sterilization:
Red Heat:
Used to sterilize metallic objects by holding
them in flame till they are red hot
Used for materials such as inoculating wires,
needles and forceps.
Flaming
The article is passed over a flame without
allowing it to become red hot
Used for materials such as mouth of culture
tubes, cotton wool plugs and glass slides.
Incineration (Burning to Ash):
This is an excellent method for rapidly
destroying materials
Used for materials such as soiled dressings,
animal carcasses and pathological materials.
Hot Air
Oven
Sterilization by hot air oven requires temperature of 160°C for one hour or 180° C for 30 minutes. Materials sterilized by this method are liquid paraffin, dusting powder and glass wares.
Moist Heat Sterilization
• The lethal
effect of moist heat is by denaturation and coagulation of proteins.
Moist Heat
Sterilization Methods;
• Heating
Below 100C
o
Pasteurization
A method used to sterilize milk.
Temperature employed is either 63oC for 30
minutes (Holder method) or 72oC for 15 to 20 seconds (flash method).
Organisms like mycobacterium, Salmonella and
Brucella are killed
o Vaccine
bath
Used for killing non-sporing bacteria, which
may be present in vaccines.
ƒIn a vaccine bath, the vaccine is
treated with moist heat for one hour at 60o C.
• Boiling at
100 oC
o
Tyndallization
This is a process by which the medium to be
sterilized is heated at100 oC for three consecutive days for 30 minutes.
This method is used for sterilization of egg
or serum containing media and some pharmaceutical preparations that cannot withstand
higher temperatures.
o Steam at
Atmospheric Pressure (100oC)
Free steam is used to sterilize culture media and some pharmaceutical preparations which may decompose if subjected to higher temperature.
o
Autoclaving/Steam under Pressure (Temperature above 100oC)
Autoclaving is the most commonly used method
for sterilizing materials in the laboratory.
Requires the use of autoclave machines.
Materials to
be sterilized should be placed in a bucket.
Articles to
be reused should be washed after sterilization (e.g. culture media plates containing
isolates).
Autoclaving is used to sterilize cultures,
rubber goods, syringes, scissors, forceps, gowns and dressings.
In this apparatus, material for sterilization
is exposed to 121 oC for 15 to 20 minutes
at 15lb pressure per square inch.
Saturated steam condenses on the surface of
the micro-organisms and transfers its latent heat of vaporization. The organism
is rapidly destroyed by protein coagulation.
Sterilization by Filtration
• This is a
method of sterilization useful for antibiotics solutions, sera and carbohydrate
solution by using:
o Glass
sintered filters
o Zeits
filters
o Membrane
filters
Chemical Sterilization and Materials
Sterilized
Chemicals as a Sterilizer: Mode of
Action
• Chemical
substances act as sterilizing agents by:
o
Coagulation of bacterial protoplasm (e.g. heavy metals)
o Disruption
of cell membrane by chemical substances
They may alter physical and chemical
properties of cell membranes, thus resulting in killing or inhibiting the
bacterial cell.
o Oxidation
or burning out the bacterial protoplasm (e.g. halogens).
o Affecting
bacterial enzymes or coenzyme systems, thus causing interference of bacterial
metabolism.
Common Chemicals Used in Sterilization
• Alcohol
(Ethanol)
o Absolute
alcohol is not a very effective sterilization agent.
o When
diluted to 70% it is effective as a skin sterilizer.
• Chlorine
(hypochlorite solution or Jik)
o It is the
disinfectant of choice in microbiological laboratories with broad spectrum activity
against a wide range of micro-organisms including spores and viruses.
• Glycerol
o 50%
glycerol solution will kill contaminating organisms
o It is used
for the preservation of certain viruses
• Phenols
and cresols (Lysol)
o 5%
phenol/cresol is used mainly for discarded cultures, infected pipettes and
other infected materials.
Gaseous
Sterilization and Materials Sterilized.
• Ethylene
Oxide
o This is a
highly penetrative, corrosive and microbicidal gas which is used in the industry
for the sterilization of single-use, heat sensitive medical devices such as prosthetic
heart valves and plastic catheters.
o Ethylene
oxide sterilization is usually carried out at temperatures below 60oC.
o Care must
be taken because of the hazards, which include toxicity to personnel, flammability
and explosion risk, as well as the effect of residual ethylene oxide in the
sterilized product.
•
Formaldehyde gas
o Is used to
disinfect laboratory cabinets, baby incubators and anaesthetic machines.
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