Page 139 - The Veterinary Laboratory and Field Manual 3rd Edition

108 Willy Schauwers

• Dirty reusable labware must be cleaned thor- medium is added. Both medium and tubes are
oughly before steam sterilization. Otherwise, thus sterilized with one autoclaving. If the tubes
residue will bake on during sterilization are to be filled with sterile medium, plug and
and microorganisms may not be effectively sterilize the tubes in the autoclave or dry air
destroyed if they are protected by the residue. sterilizer before adding the medium.
Furthermore, any adhering chemical residues
may damage the surfaces due to the high radIatIon
temperatures. • Ultraviolet (UV) radiation: sunlight can
• Glassware that is contaminated with blood destroy bacteria due to the action of UV light
clots, such as serology tubes, culture media, which acts as a sterilizing agent by produc-
petri dishes, and so on, must be sterilized ing peroxides which are oxidizing agents. The
before cleaning. It can best be processed in most highly germicidal rays are those hav-
the laboratory by autoclaving or by placing it ing wavelengths between 296 and 210 nm.
in a large bucket or boiler filled with water, to However, since the penetrating power of UV
which 1–2% soft soap or detergent has been light is low, its practical use is limited. UV
added, and boiled for 30 min. The glassware light is often used in safety cabinets to steril-
can then be rinsed in tap water, scrubbed ize the bench when the cabinet is not in use.
with detergent, rinsed again. • Ionizing radiation: gamma rays have more
• You may autoclave glassware or sterilize it in penetrating power than UV light and can pass
large steam ovens or a similar apparatus. If through materials. They affect organisms by
viruses or spore-bearing bacteria are present, ionizing the cell constituents. This method is
autoclaving is absolutely necessary. used in commercial laboratories and indus-
• Not all plastics are resistant to steam steril- try to sterilize disposable polystyrene Petri
ization. Polycarbonate, for example, will lose dishes, syringes, vials and so on, while they
its strength. Polycarbonate centrifuge tubes are in their packaging.
cannot be steam sterilized.
• During sterilization (autoclaving), plas- FILtratIon
tic equipment in particular should not be There are several types of filters made from diato-
mechanically stressed (for example, do not maceous earth, porcelain, asbestos, sintered glass
stack). Thus, to avoid shape deformation, and membranes, which have been used in labora-
beakers, flasks, and graduated cylinders tories. Membrane filters are gradually superseding
should be autoclaved in an upright position. other types as they are easy to handle and can
• Culture tubes which have been used previ- be discarded after use thus minimizing cleaning.
ously must be sterilized before cleaning. The They produce little loss of solution and are avail-
best method for sterilizing culture tubes is able in various levels of porosity. Diatomaceous
by autoclaving for 30 min at 121°C (15 psi). earth and porcelain filters will not be described
Media that solidifies on cooling should be since they are not used in veterinary laboratories.
poured out while the tubes are hot. After the Membrane filter units utilize membranes
tubes are emptied, brush with detergent and made from a cellulose ester which has pores
water, rinse thoroughly with tap water, rinse that range down to 5 µm. After use and steril-
with distilled water, place in a basket and dry. ization the membrane is discarded and a fresh
membrane is inserted. In veterinary laboratories
If tubes are to be filled with a medium that is pore sizes of 22 µm or 45 µm are commonly used
sterilized by autoclaving, do not plug until the for sterilization of biological fluids. Membrane

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