Handbook of Solvents - George Wypych - ChemTech - Ventech!

15.2.2 A simple test to determine toxicity using bacteria 1097
is used to determine if chemicals will harm sewage treatment plants. It is rarely used to determine
the toxicity of compounds. The test provides values comparable to those observed
using sewage sludge as the source of bacteria (Sun et al., 1993; Toussaint et al., 1995;
Elnabarawy et al., 1988).
Toxitrak TM has been used to determine toxicity in water samples. There is a report
(Gupta and Karuppiah, 1995) in which they tested water samples from a river in Maryland
using this method, Microtox TM , Ceriodaphnia dubia (a variation of the standard Daphnia
test), and this test. With four samples, they got comparable results. The Toxitrak test is not
used extensively. Apparently the test follows the reduction of dye resazurin by cells of Bacillus
cereus (Liu, 1989).
Dozens of tests using bacteria as indicator organisms in toxicity studies have been proposed
(Bitton and Dutka, 1986). Very few of these have been characterized extensively.
Bacterial tests are inexpensive, inoffensive to animal rights groups, the cells are easy to
grow and to prepare. And it is thought there is enough universality at the biochemical level,
that a compound that is toxic for a bacterium will be toxic for a higher organism.
Ideally an alternative test for toxic chemicals would be inexpensive. It would not require
any specialized equipment, only equipment normally found in a laboratory should be
required. The test should not require that the personnel be specially trained in the techniques.
It should involve only the procedures that any laboratory worker would know. It
should be the sort of test that any laboratory with a tangential interest in toxicology could
carry out. Water quality laboratories, industrial safety laboratories, agricultural research
laboratories, should be able to carry out the procedure. Small samples should be tested.
Tests with fish often involve quite large volumes of toxic chemicals and this can limit their
utility. This has been discussed in detail by Blaise (1991).
The sensitivity of a test can be a problem. Every test for toxic chemicals has some
chemicals that the test is very sensitive to. And every test has some toxic chemicals that it is
not sensitive to. Often several tests will be run and the median value for toxicity is judged to
be representative for the toxicity of that chemical. It can be argued that if a test system is extremely
sensitive to a chemical, than it must be concluded that this chemical could be toxic
for humans at this very low level. If it damages one life form, it could very well damage another.
Tests with live animals, LD50 tests, are usually not as sensitive as tests with Daphnia,
with animal cells, with bacterial tests. Tests with live animals usually involve injecting the
toxin into the stomach and it is not always certain what effect the acidic conditions of the
stomach may have on the toxin. And humans rarely come in contact with toxins in this fashion.
It can be argued that this sort of animal test is not indicative of the toxicity of the chemical
(Ruelius, 1987). On the other hand, chemical analysis is often much more sensitive than
any toxicity test. There are reports of chemicals in the range of parts per trillion yet few tests
for toxic chemicals can detect less than parts per million or occasionally, parts per billion.
Yet we see public policy formulated on the basis of these chemical tests.
15.2.2.3 An alternative
A simple, inexpensive and rapid test to determine the toxicity of chemicals has been developed.
(Botsford, 1997, 1998, 1999). The assay uses the bacterium Rhizobium meliloti as the
indicator organism. This bacterium reduces the thiazole tetrazolium dye, MTT
(3-[4,5-dimethylthiazole-2-y]-2,5-biphenyl tetrazolium bromide) readily. The dye is almost
completely reduced after a 20 minute incubation at 30 o C. The dye turns dark blue
when reduced and this can be followed readily with a simple spectrophotometer. Reduction
of the dye is inhibited by toxic compounds. More than 200 compounds have been tested and
the results obtained with this test are comparable to those obtained with other tests
(Botsford, 2000a).