BACTERIAL-RESISTANCE TO HEAVY-METALS RELATED TO EXTRACTABLE AND TOTAL METAL CONCENTRATIONS IN SOIL AND MEDIA

To assess the effects of heavy metal pollution on terrestrial systems, bacteria are frequently isolated from such environments and metal tolerance of the isolates determined by plating on media amended with high concentrations of metal salts. We have found that metal concentrations added to media in traditional assays are orders-of-magnitude higher than water soluble concentrations of Zn, Cd and Ni in soil, even highly contaminated soil. Further. many soil bacteria are intrinsically resistant to high concentrations of heavy metals, thereby precluding the need to adapt to metal contamination. Most bacteria isolated from soil were resistant to very high concentrations of heavy metals, regardless of whether or not the soils were contaminated with metals. The average tolerance to Zn of bacteria isolated from highly contaminated soil was 75 mg l-1 but the concentration of extractable Zn from the soil was only 0.47 mg kg-1 In a non-contaminated soil, the average bacterial tolerance of Zn and extractable soil concentration of Zn were 26 mg l-1 and 0.04 mg kg-1, respectively. Defining soil metal concentrations by extractable concentrations as opposed to total concentrations is a more appropriate measure of potential bacterial changes and may elucidate ecosystem changes that might otherwise go undetected.