In order to determine whether a compound is toxic in nature, it is in most cases not enough to study how individual species react to this compound under controlled conditions in the laboratory. A study should also be made of the toxic properties of the compound for the entire community of organisms under more natural conditions. This may however create difficulties in interpreting the results, since a toxic compound may affect other factors which, in turn, affect the organisms. The compound as such may be food for the microorganisms (i.e. increase their activity) apart from being toxic (reducing the activity of the microorganisms). Several potentially toxic compounds may also occur simultaneously, and it can be difficult to know which of these is most toxic.
One way of studying toxic effects where these problems are avoided is to make use of the concept of PICT (Pollution Induced Community Tolerance). PICT is based on the fact that a toxic compound favours organisms that are tolerant to the compound in question and disfavours those whose tolerance to the compound is low when it occurs in toxic concentrations. This results in a community of the studied organisms that will be more tolerant to the toxic compound. An increased tolerance therefore implies that the compound in question must have had a toxic effect. The use of PICT produces fewer problems with other effects (e.g. nutrition effects), and PICT can also indicate which compound is most toxic in a situation where several compounds occur simultaneously.
In this project we have studied in what way PICT can be used to detect toxic effects of phenol and phenol derivatives, and for antibiotics that are used in veterinary medicine.
Our investigations were made in soil, and we have made use of the bacterial community, both because bacteria are small organisms which come into direct contact with the toxic compound, and because there are a large number of bacteria in soil, and we can therefore make measurements on millions of organisms at the same time.
We found that PICT was a good technique for the detection of toxicity in soil, with regard to both different varieties of phenol and antibiotics (Tylosin and sulfa-methoxazole, SMX). It was as sensitive as the most sensitive of the methods it was compared with. There were also fewer problems due to other ambient factors influencing the results, which would have made it difficult to draw proper conclusions.
We used a method to detect PICT, developed by us, which is based on changes in the rate of growth of bacteria. A comparison with a method for measuring PICT which is based on growth under laboratory conditions (developed in Holland) showed that our method was both faster, simpler and more sensitive in detecting effects.
The increased tolerance after exposure of the bacterial community to a toxic compound rapidly disappeared when the compound (the antibiotic Tylosin) was decomposed. This indicates a possible direct coupling between the degree of tolerance (PICT) in the bacterial community and the quantity of toxic compound. This would make it possible to use PICT to detect how effective treatment methods are.
As a secondary result, we could show that one effect of the increasing the quantity of antibiotics that impacted on bacteria was increased growth of fungi in the soil. This implies that two of the most significant decomposer groups, fungi and bacteria, compete with one another in the soil. They also appear capable of replacing one another's function to a certain extent, a fact which explains why it was difficult previously to see the effects of antibiotics in the soil.
To sum up, our investigations indicate that induced tolerance (PICT) may be a good tool for studying the effects of toxins in the soil. We have however also identified potential problems with the method, which ought to be studied further before it can be used in a more routine manner. One example is that PICT appears to vary depending on the compound being studied, and PICT may therefore be found to be more suitable for certain compounds. It is also unclear to what extent PICT can be used to show directly that a certain compound is most toxic under conditions when several potentially toxic compounds are present, since it would appear that bacteria sometimes become tolerant to closely related compounds.
Author
:
Erland Bååth
Division of Microbiological Ecology, Department of Ecology, Lund University
Literature:
Apart from a number of scientific articles, the project resulted in a doctoral thesis:
Aldén Demoling L. 2008. Pollution-induced community tolerance (PICT) of bacteria: Evaluation of phenol-and antibiotic-polluted soil. Department of Ecology, Microbial Ecology, Lund University.