This document discusses different methods for sterilizing laboratory equipment. It describes sterilization using dry heat methods like flaming, hot air ovens heating to 140°C, 160°C, or 180°C. Moist heat sterilization methods are also outlined, including boiling instruments in NaOH solution, autoclaving at 121°C for 25-30 minutes, and moist heat at 100°C for vaccine preparation. Additional sterilization techniques covered are ionizing radiation using UV, X-rays or gamma rays; and chemical methods employing alcohol, hydrogen peroxide, or glutaraldehyde solution.

1. Title: Sterilization of laboratory equipment
Name: Negash Alamin
Profession: Clinical Laboratory scientist
E-mail: drnegash@gmail.com
Date: 13/11/17

2. Definition: Sterilization is a process of cleaning hardware from
microorganisms either their vegetative forms or their spores.
Methods of sterilization: heat results in the coagulation and denaturing of
protoplasm and results in the death of microorganisms.
A- Dry heat
1- Flaming: Here the equipments are passed through Bunsen flame1
without turning them red hot.
2- Red heat: Platinum loops, microbiological inoculating wires are
heated red hot in a Bunsen burner. Both Flaming and Red heat
methods do not kill spores of microorganisms.
Figure 1 Robert W. Bunsen
3- Hot air oven
In this procedure air in a closed chamber is heated in a double walled
steel chamber. Where hot air circulates in the oven by convection. The
time designated to sterilize equipments is 3 hours for 140°c, 1 hour at
160°c and 30 minute at 180°c. This method is only applicable for heat
1
Named after Robert Wilhelm Bunsen a German chemist who invented the apparatus in
about 1855.

3. resistant glassware. It is not applicable for sterilizing culture media,
liquids, rubber wares, fabrics such as lab coats and gowns.
N.B All articles including glass wares must be properly dried before
inserting them into the Hot air oven.
B- Moist heat
This method provides better sterilization property due to its high
penetrating capability due latent heat of evaporation.
Moist heat at 100°c
Blunt instruments in the surgery theatre are routinely sterilized by boiling
water with NaOH solution for 30-45 minutes; which, kills the vegetative
form in 3-5 minutes except for the spores. Sugar containing media like
sugar fermentation tube, Mac Conkey media, and DCA media are
sterilized for 30-45 minutes. The advantage of this method is that sugar
remains surrounded by a steam environment; hence, the sugar is not
destroyed or caramelized. The disadvantage is that spores are not quickly
destroyed therefore for this purpose the process must be repeated for 3
days for 20 minutes2
. This system is known as fractional steam
sterilization or tyndallisation.
Moist above 100°c
Here in this method we use an instrument called autoclave which is a
closed metallic container with an outlet valve and a safety valve fitted
with a lid cover with a space at the bottom for holding water and
generating steam. A pressure of 15 lbs. per square inch of steam is raised
to 121°c and maintained for 25-30 minutes.
This method is superior to the methods mentioned before because it has
the advantage of pressure in addition to moist heat. All vegetative and
spores are destroyed. Equipments sterilized in this method include
surgical dressing, draping, rubber gloves, bacteriological media like
peptone water, N.broth, N.agar etc.
2
Ramnik Sood: 2006: Medical laboratory technology

4. Figure 2 Autoclave apparatus (Indiamart.com picture)
Moist heat at 100°c
This method is used for the preparation of dead bacterial vaccine in
microbiology. Here the sample prepared as a vaccine candidate is freed
from vegetative organisms but its antigenicity is retained for
immunization purposes.
C- Ionization radiation
1- In this alternative method of sterilization Ultra violet rays above 400
nm of the visible spectrum act to damage or destroy bacteria DNA.
This method is used to sterilize the atmospheric environment of
surgical rooms.
2- X-rays and Gamma rays are the other methods of sterilization which
have high penetrating capability than UV rays. They alter the DNA
structure of viruses and bacteria leading to their destruction.

5. Figure 3 EM radiation with the visible spectrum shown in color ( shutter stock image)
D-Chemical methods
1- Protein denaturing agents like ethyl alcohol, Isopropyl alcohol;
oxidizing agents like hydrogen peroxide, chlorine, Iodine and
alkylating agents and chemicals like formalin, ethylene oxide, and
Glutaraldehyde solution, bacteriostatic3
and bactericidal agents like
sulphonamides and antibiotics are categorized under chemical
methods of sterilization.
For example chemo-agents like formalin gas are used in the
fumigation and sterilization of operating theatres and also incubating
apparatus. Glutaraldehyde is a colorless, oily chemical with a pungent
odor which is used in a diluted form in solutions ranging from 0.1% to
50% ratio to disinfect non-heat resistant equipment like dialysis
instruments, surgical tools, bronchoscopes, and etc.4
3
Meaning they stop the proliferation of bacteria rather than killing it.
4
CDC