STERILIZATION
Sterilization
is the killing or the total elimination of all forms of microbial life
including spores, spores are highly resistant to destruction, the term
sterilization therefore is an absolute term and the article must be free from
all forms of microbial life.
Disinfection
on the other hand is the killing of many organisms with an aim to reduce the
number of contaminating organisms to a level that cannot cause infection,
during disinfection, some organisms and bacteria spores may survive but
pathogenic should be destroyed.
Disinfectants
are the chemicals that are used for disinfection, but they are used only on inanimate
objects. Antiseptic on the other hand are mild form of disinfectant which are
used externally on living tissues to kill micro-organisms for example on the
surface of the skin or on mucus membrane.
Sterilization
is used for sterilizing instrument that are used for surgical procedures for
example, gloves, aprons, syringes, scalpels, etc. sterilization is also used in
the microbiology laboratory for microbiological works in the microbiology
laboratory such as media preparation, reagent, sterilizing equipment and
situation in which a sterile condition is to be maintained.
METHODS
FOR STERILLIZATION INCLUDES THE FOLLOWING:
1. The
use of physical agent
2. The
use of chemical agent
1.
The
use of physical agent: it includes; i. Heat ii. Radiation
iii. And filtration.
i.
Heat:
Heat
is the most effective and rapid method for sterilization, heat acts by
coagulating cell protein and interfering with the metabolic reactions in the
microbial cell. The two most common method of heat sterilization include;
a. Sterilization
by Moist heat
b. Sterilization
by Dry heat
A.
Sterilization
by Moist heat: Moist heat act by denaturing and
coagulating the cell protein breaking the DNA strands and resulting in a
general loss of functional integrity of the cell membrane.
Sterilization can take place at either
100oC or above 100oC. The following methods are used at
100oC
i.
Boiling:
Boiling
at 100oC is done in a water bathe for 30 minutes, instruments such
as rubber goods, syringes, surgical instrument that may be damaged at
temperatures above 100oC are usually sterilized using this method,
by this method, all bacteria and certain spores are destroyed, this method
leads to a destruction of all pathogens.
ii.
Steaming:
steaming is more effective than dry heat at the same temperature; this is
because steam has more penetrating power due to heat of condensation. Steaming
can be done in a steam sterilizer which works at 100oC under a
normal atmospheric pressure without any extra pressure, at temperature higher
than 100oC, some culture Medias become damaged, and are therefore
sterilized using this method. The steam sterilizer is a metallic vessel which
has two perforated diaphragm, water is boiled using electricity or gas, and the
steam passes through the perforations. At the roof of the steam sterilizer is a
small opening where the steam can escape. Sterilization using steam can be done
using two different ways;
i.
Single exposure to the steam
continuously for one and the half hours, all pathogens are destroyed, using
this method.
ii.
Fractional sterilization: this method is
also known as tynderlization invented by a scientist known as john tyndal. It
is used for materials and culture media that are heat labile materials and
culture medium, the material is steamed sterilized for 20 minutes, followed by
incubation at 37oC overnight, this procedure is repeated for another
2 consecutive days, the spores are allowed to germinate to vegetative cells
during the period of incubation and are destroyed during steam sterilization on
the second and the third days.
Sterilization
above 100oC
Autoclaving is one of the most common methods of
sterilization. In autoclaving, sterilization is achieved by the use of steam
under pressure. Temperatures used are higher than 100oC and the
temperature of boiling used depends on the surrounding atmospheric pressure.
When pressure is increased, the temperature of boiling used depends on the
surrounding atmospheric pressure. When pressure is increased, the temperature
also increases. The autoclave is closed and made air tight. Water starts to
boil and the inside pressure increases. The water boils above 100oC.
At 15lbs per square inch of pressure, a temperature of 121oC is
obtained. When kept for 15 minutes, spores are killed.
PARTS
OF AN AUTOCLAVE
An autoclave is a metallic cylindrical vessel. On
the lid are;
i.
Gauge for indicating the pressure.
ii.
A safety valve which is set to open at
any desired pressure.
iii.
A stop clock to release the pressure.
Inside
the autoclave is a perforated diaphragm. Water is placed below the diaphragm
and heated from below by electricity, gas or stove. To operate an autoclave,
the following steps are to be followed;
a. Place
materials into the autoclave.
b. Close
the lid, leave the stop clock open.
c. Set
the safety valve at the desired pressure.
d. Turn
on the heat. Air is forced out until steam comes out through the tap.
e. Close
the top to allow the pressure inside the autoclave to build up until it reaches
the set level. At the set level, the safety valve below open and allow excess
steam to escape.
f. Keep
this for 15 minute holding time.
g. Stop
the heating.
h. Cool
the Autoclave below 100oC.
i. Open
the stop clock slowly to allow air to enter the autoclave.
To check for the efficiency of an autoclave, the
spores of Bacillus steazothermophilus are
used. Spores can withstand 121oC heat up to 12 minutes. Strips of
paper containing the spores are placed along with materials to be sterilized
after autoclaving, the strips are cultured at 50oC - 60oC
and checked to see if spores will germinate. If there is no growth, then the
autoclave is functioning properly.
PASTEURIZATION
The art of heating at temperature below 100oC
aimed at killing pathogenic organism. Pasteurization was developed by a
scientist “Louis Pasteur”. It was used for food that are especially heat
sensitive when sterilization temperature may alter the nutritive valve of the
food or may result in a change of colour, or flavor of the food. Pasteurization
does not sterilize the food as many living organism including spores are not
destroyed. There are two (2) methods used in pasteurization namely “HTST” (High
Temperature Short Time). In this, heating is done at 72oC for 15
seconds or “LTST” (Long Temperature Short Time) In this, heating is done
between 63oC - 66oC for 30 minutes.
B. DRY
HEAT STERILIZATION: Dry heat was the first method of sterilization. The process
is a larger process than most heat sterilization. The destruction of
micro-organisms through the use of dry heat is a gradual phenomenon. With longer
exposure to lethal temperatures, the number of micro-organisms that are
destroyed increases. A forced ventilation of hot air can be used to increase
the rate at which heat is transferred to the organism, and reduce the
temperature and amount of time needed to achieve sterility. At higher
temperatures, shorter exposure times are required to kill organisms. The mechanism
by which dry heat achieves microbial destruction are;
i.
Protein denaturalization
ii.
Oxidative damage
iii.
Toxic effect of an elevated electrolyte
in the absence of water.
Temperatures used are 160oC held for at
least 1 hour to kill moist resistant spores. The articles remain dry. Dry heat
has an advantage that it can be used on powders and other heal stable items
that are adversely affected by a steam.
I.
HOT
AIR STERILIZATION: the hot air oven is one of the
most common methods used for sterilization in the microbiology laboratory. Glass
wares, swab sticks, powder and oily substances are sterilized in the hot air oven.
A temperature of 160oC is maintained for 1 – 2 hours and at this
temperature and holding time, spores are destroyed. This leads to
sterilization.
The hot air oven is an apparatus with double
metallic walls and a door. There is an air space between these walls. The apparatus
is heated by electricity or gas at the bottom. On heating, the air at the
bottom becomes hot and passes between the two walls from below upward and then
passes in the inner chamber through the holes on the top of the apparatus. A thermostat
is fitted to maintain a constant temperature at 160oC.
II.
FLAMING:
Another
method of dry heat sterilization is flaming. Flaming is done loops and straight
wires. Leaving the loop in the flame of a Bunsen burner or alcohol lamp until
it turns red hot. Ensures that any infectious agent gets inactivated. This method
is commonly used for small metals or glow objects but it is not suitable for
large objects. During initial flaming, infectious materials has a tendency to
be sprayed from the wire surface, therefore it is killed, thereby contaminating
nearby surfaces and objects. To prevent this, special heaters have been
developed which surrounds the inoculating loop with a heated case, ensuring
that such case materials does not further contaminate the area. Sometimes,
carbon residues are left on the heated object, of the object heat is not
enough. To prevent this, the object can be dipped into ethanol, before briefly
touching the Bunsen burner flame. The ethanol ignites and turns off leaving fewer
residues.
III.
INCINERATION:
This
is a waste treatment process that involves the combustion of organic substances
contained in waste materials. This method also burns any organism off to ash.
It is used to sterilize
medical and bio-hazardous waste before it is discarded with non-hazardous waste.
“There
are mini furnaces created for the purpose of incineration”.