Page 138 - The Veterinary Laboratory and Field Manual 3rd Edition

The selection, use, maintenance and quality control of laboratory equipment and supplies 107

GLycEroL objects to be sterilized in the same way as for
Glycerol in a 50% solution will kill contami- the autoclave method. Cotton-wool plugs, if
nating microorganisms. It is used for the used, should not be too thick, otherwise the
preservation of certain viruses and bacteria that hot air cannot penetrate.
are not affected by the glycerol. • Moist heat: A common method of wet steriliza-
tion is achieved by using an autoclave (steam
PHEnoLIcS sterilizer) at a temperature of 121°C (15 psi)
• Lysol is a powerful phenolic antiseptic used for 20 min or at 110°C (10 psi) for 40 min. A
mainly for disinfection of discarded culture temperature of 100°C (boiling) will kill non-
plates, contaminated pipettes and other sporing organisms within 10 min. Most spores
potentially infectious material. Solutions of will be killed in 30 min at 100°C but some
1 to 9% are generally used. spores can resist boiling for several hours. The
• Hycolin: a phenol derivative used at a 1% solu- addition of 2% sodium carbonate increases the
tion for all forms of laboratory disinfection. bactericidal effect of boiling water and spores
that resist boiling water for 10 h have been
Always read the manufacturer’s instructions
before using a chemical disinfectant. Used killed in 30 min this way. This method is suit-
incorrectly, all disinfectants can be poten- able for sterilizing contaminated instruments
tially hazardous to laboratory staff. following post-mortems so that they may be
used again quickly. It is unsuitable if instru-
ments are to be stored in a sterile condition.
Sterilization • Biological fluids may be sterilized by heating
Laboratory ware and instruments may need to them in a water bath at 56°C for periods of
be sterilized but the procedure is also important 1 h daily if necessary. The principle is called
for the preparation of culture media and some Tyndallization but at this low temperature
reagents. Sterilization can be achieved using a more than three exposures may be necessary.
variety of methods and it is a case of choosing This method of sterilization cannot be used
one that is appropriate for the situation. when the fluid potentially contains resistant
spores. It is a useful method to sterilize media
PHySIcaL MEtHodS oF StErILIzatIon containing protein or sugars which may be
• Dry heat: The application of dry heat is a simple ‘broken down’ at higher temperatures and
method of sterilization providing the material when filtration is not feasible.
to be sterilized is not adversely affected by the
heat. An example is nichrome wire loops used StEaM StErILIzatIon
in bacteriology. These can be heated and ster- Steam sterilization (autoclaving) is defined as
ilized in a Bunsen flame in excess of 300°C. the destruction or irreversible inactivation of all
For glassware/metal articles that must remain reproducible microorganisms under exposure to
dry, a hot air oven is used at a temperature of saturated steam at 121°C (15 psi).
160°C. This temperature must be maintained The following points must be observed.
for 1 h (2 h if the objects are heavy or bulky),
then the glassware is allowed to cool slowly • Efficient steam sterilization is assured only if
before removing it from the oven (when the the steam is saturated and has unrestricted
temperature is around 40°C). Note that rapid access to all contaminated areas.
cooling will cause contraction and unsterile • To prevent pressure build-up, containers or
air being sucked into the vessel. Prepare the vessels must always be open.

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