Bioassay

A bioassay is the use of a living organism to test for the presence of a compound or to determine the amount of the compound that is present in a sample. The organism used is sensitive to the compound for which the test is conducted. Thus, the effect observed is typically the death or deteriorated health of the test organism. Depending on the test organism, soil, air, or liquid samples can be assayed.

The classic historical example of a bioassay was the use of canaries by miners in past centuries. Because canaries are more sensitive than humans to noxious gases like methane, they reacted quickly to even small amounts of the gas. This would give the miners time to escape.

Today's bioassays are more sophisticated than the canary. The ASTM (formerly known as the American Society for the Testing of Materials) has catalogued over 70 different bioassays. These are used to analyze soil, freshwater, and the sediment at the bottom of watercourses like streams and rivers, saltwater, and air.

Plants can be used as indicators of the presence of toxic compounds in the soil. In this bioassay, seeds or the mature plant is introduced into the soil of a site that is suspected of being contaminated. Failure of the seeds to germinate, or failure of the mature plant to thrive, can be evidence of contamination. If the assay is done in a controlled manner with the use of standards to provide reference points, then the geographical area of contamination can be determined.

Some species of plants can also be used to accomplish bioassays in the water. More commonly, however, the test organisms are single-celled organisms such as algae, water fleas (in particular a species called Daphnia magna, or fish (in particular the fathead minnow).

Bacteria can be used in bioassays. For example, the use of bacteria to detect and determine the amount of antibiotics or compounds that might be carcinogens in a sample has been practiced for decades. Another particularly useful bacterial bioassay involves the use of bacteria that have been designed to fluoresce (to emit light). If the bacteria are harmed by a toxic compound in the test sample, then they fail to fluoresce. The decrease in fluorescence of bacterial populations is measured in a device called a spectrophotometer. The degree of decrease can be compared to standards in order to determine the concentration of the toxic compound.

With the wide variety of bioassays available, the investigator must choose the assay with care. The test organism used needs to be appropriate for the task, and must provide a result that is readily apparent. Being able to determine the level of the toxic agent (in other words being able to quantify the agent) can be very useful.

Bioassays continue to be developed. For example, in the mid-1990s, a bioassay was introduced to detect the presence of some human hormones in the environment.