Interpreting Results; Disposal Of Test Cultures - Hach DR2400 Manual

Interpreting Results

Disposal of Test Cultures

Summary of Method
Toxicity_None_Other_Eng_Ody.fm
The % Inhibition results obtained are only a relative measurement. They do not
represent a true quantitative measurement of toxic concentration. The %
Inhibition does not necessarily increase in direct proportion to the concentration
of toxins. To determine the minimum inhibition concentration of a toxin, it is
possible to make tenfold dilutions of the sample and determine the % Inhibition
for the dilutions until the sample is diluted sufficiently so that no inhibition is
observed. This is the No Observed Effect Concentration (NOEC).
Due to the many variables involved in the test, the limits of detection are on the
order of 10% Inhibition. This would correlate to the Lowest Observable Effect
Concentration (LOEC). If a sample shows less than 10% Inhibition, repeat the
test. After several repetitions, look at the series of data to determine the
likelihood of toxicity. Results below 10% are not reliable, but can be used to
surmise some presence of toxicity if they are consistent. See examples below:
Data Points: % Inhibition
7%, 9%, 5%, 8%, 5%
7%, -4%, -5%, 5%, 1%
-7%, -9%, -5%, -8%, -5%
Some toxins will increase respiration and will give a negative % Inhibition on
this and all other respiration-based toxicity tests. After repeated testing, samples
that always give a % Inhibition that is more negative than -10% should be
considered toxic.
Dispose of active bacterial cultures grown during incubation by using one of
these methods:
• Autoclave used test containers at 121 °C (250 °F) for 15 minutes at 15 pounds
of pressure. Once the containers are sterile, pour the contents down the drain
with running water. The reaction tubes may be washed and re-used.
• Sterilize test containers by using a 1:10 dilution of commercial laundry
bleach. Pour the test container contents and test containers into the bleach
solution. Allow 10–15 minutes of contact time with the bleach solution. Then
pour the liquid down the drain and wash the reaction tubes for reuse.
This method is based on the reduction of resazurin, a redox-active dye, by
bacterial respiration. When it is reduced, resazurin changes color from blue to
pink. Toxic substances can inhibit the rate of resazurin reduction. A chemical
accelerant has been added to shorten the reaction time.
Toxicity
Conclusion
May be slightly toxic
Most likely not toxic
May be slightly toxic
Toxicity
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