Page 137 - The Veterinary Laboratory and Field Manual 3rd Edition

106 Willy Schauwers

Many chemical agents are referred to as disinfec- wear protective gloves when handling disin-
tants, a term that is applied to substances which fectants – except for hand disinfection.
destroy microorganisms. Other terms with a • Staff must be trained.
similar meaning are germicide and bactericide.
A disinfectant that is non-injurious to human aLcoHoL
tissue is classified as an antiseptic and chemicals Absolute alcohol (100%) is not a very effective
which are used to prevent organisms growing in sterilizing agent because the power of penetration
a sterile medium are classified as preservatives. into the microorganism is poor. When diluted with
The action of a disinfectant is modified by sev- distilled water at a concentration of 70% however,
eral factors. For example, some disinfectants are it becomes effective as a skin sterilizer (disinfec-
very efficient in the absence of organic matter tant) and is often used prior to inoculations or
but are not so effective in its presence. venepunctures. It is also a useful preservative.
Chemical agents function as sterilizing agents
in the following ways: cHLorInE dErIvatIvES
Chlorine derivatives and the free chlorine pres-
• interfering with the enzymatic system of the ent are useful for disinfecting contaminated
microorganism glassware and some liquids. The disadvantages
• disruption of the cell membrane of these disinfectants are that they attack metal
• coagulation of protein and rapidly deteriorate in the presence of organic
• oxidation. material, dust and sunlight and are most effec-
tive only at a pH close to neutral. In an acid
In each case death of the microorganism occurs. environment chlorine release is accelerated.
The following points must be borne in mind Chlorine releasing disinfectants include:
when handling disinfectants.
• sodium hypochlorite (bleach): usually contains
• To assure the right concentration of the 5 or 10% available chlorine, hypochlorites
solution used, the manufacturer’s instruc- must be freshly prepared from stock solutions
tions must be observed. Measuring vessels every day
or dosage systems must be used for dosing • calcium hypochlorite granules or tablets:
purposes. If too low a dose is used, the disin- contains about 70% available chlorine, hypo-
fectant will not work properly. If too much is chlorites must be freshly prepared from stock
used, the disinfectant action will not be any solutions every day
better, so this confers no advantage and sim- • sodium dichloroisocyanurate (NADCC): con-
ply damages the environment, is expensive, tains about 60% available chlorine.
may damage materials and, not least, it is
harmful to staff. A dosage table will make it Which concentration of available chlorine to use
easier to use disinfectants (see below). for which purpose:
• The disinfectants must be used only for the
intended purpose. While that sounds logical, • a 0.1% available chlorine solution can be used
in practice this rule is not always followed. to disinfect benches
• No detergents may be added (for example, • a 0.25% available chlorine can be used for waste
all-purpose cleaners) since this could dimin- containers (for discarding used pipettes, slides)
ish the disinfectant efficacy. • a 1% available chlorine solution can be used
• For their own protection, staff must always for treatment of spills.

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