BACTERIAL-RESISTANCE TO HEAVY-METALS RELATED TO EXTRACTABLE AND TOTALMETAL 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 tol erance of the isolates determined by plating on media amended with hig h concentrations of metal salts. We have found that metal concentratio ns added to media in traditional assays are orders-of-magnitude higher than water soluble concentrations of Zn, Cd and Ni in soil, even high ly contaminated soil. Further. many soil bacteria are intrinsically re sistant to high concentrations of heavy metals, thereby precluding the need to adapt to metal contamination. Most bacteria isolated from soi l were resistant to very high concentrations of heavy metals, regardle ss of whether or not the soils were contaminated with metals. The aver age tolerance to Zn of bacteria isolated from highly contaminated soil was 75 mg l-1 but the concentration of extractable Zn from the soil w as 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 elucid ate ecosystem changes that might otherwise go undetected.