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A
Toxicology Primer for Student Inquiry:
Biological Smoke Detectors
by Charles
Drewes
FINAL
STAGES OF TOXICITY TESTING
Preliminary
experiments should provide an indication of concentration
range and duration of exposure for final stages of
testing. The following are essential considerations in
this testing.
Concentrations
in treated groups. Try to use at least 2-4 concentrations
which, based on preliminary testing, will likely cause sublethal
effects. Also, try to use at least one slightly lower concentration
that causes no effects. A minimum of 5-6 worms should be
used for each concentration, although 8-10 provide even
more statistical power. Select worms of similar size for
all groups. Use a separate container for each worm.
Controls.
In addition to groups of treated worms,
it is essential to have a control group (see Glossary).
The purpose of the control group is to verify that effects
in exposed groups are, in fact, due to the chemical itself
rather than to some other aspect of the procedure. Therefore,
the number of organisms, handling procedures, temperature,
lighting, testing methods, use of solvents to distribute
chemicals, exposure times, etc. should all be identical
to those used in treated groups. If control conditions
cause effects, then these must be subtracted from effects
in treated groups in order to obtain true measure of the
chemical’s effects.
Effects.
Results from preliminary experiments often
provide clues regarding expected types of sublethal effects
and expected timing for appearance and disappearance of
effects.
Exposure
duration and frequency of testing. One
strategy for toxicity testing is to make a single set of
short-term observations or tests of organisms after exposure
to the chemical for a fixed time period, such as
24 or 48 hours. This minimizes handling of organisms and
provides a standardized basis for comparing results between
different researchers and laboratories.
Another
testing strategy is to perform a series of repeated tests
and measurements that better describe the sequence and
time-table of symptoms and effects caused by a chemical.
This may be especially important for chemicals that rapidly
cause neurotoxicity effects that, in turn, lead to other
effects. So, if worms are not observed or tested frequently,
important effects may be missed.. There are no standard
procedures for doing this and the experimenter should exercise
his/her own judgment based on results from preliminary experiments.
Ideally,
effects should be determined while worms are still in their
original exposure container. However, this may not always
be practical or desirable, especially if filter paper is
used in the container, because it may obscure viewing or
interfere with testing. In such instances, a worm may be
very carefully removed from the test solution with a disposable
pipette so that it may be briefly examined or tested while
in another container without the chemical. Before doing
this, however, worms should be briefly and quickly rinsed
in spring water and then transferred to the new container
for viewing and/or testing.
After
testing, the worm should be replaced into the original test
solution if further exposure is desired. Use a disposable
plastic pipette for transferring worms. Special care should
be taken to avoid cross-contamination of containers or implements
that are used to handle treated worms or fluids. Repeated
observations and testing may be done at any desired interval,
but the frequency of testing should be the same in all groups,
including a control.
Reversibility
and rescue. If chemical effects on an organism are
truly “sublethal,” then organisms should survive
if exposure is promptly stopped. But survival does not
always mean full or immediate recovery from effects. Study
the persistency or reversibility of toxicity effects (recovery)
by simply placing organisms into chemical-free conditions
and continuing observations and testing. Effects may disappear
in minutes, hours, or days.
Next
Section: typical
equipment and supplies
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