Fate of a metal‐resistant inoculum in contaminated and pristine soils assessed by denaturing gradient gel electrophoresis

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Abstract
Cesium, cadmium, cobalt, and strontium are four contaminants frequently found in soils at biotoxic levels. Introduction of certain nongenetically modified bacteria has been frequently suggested as a method for the immobilization of heavy metal contaminants in soil, thereby reducing runoff and bioavailability. In this study, we have used the polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) to track the survival of five bacterial species added to soil microcosms with and without the addition of a mixture of these metals. The PCR primers targeted conserved regions of the 16S rDNA molecule present in all bacteria. The reaction products were shown to reflect the relative abundance of the bacteria both in mixtures of pure cultures and against a background of all the eubacterial species present in the soil following inoculation. Three of the species (Pseudomonas aeruginosa FRD‐1, Shewanella putrifaciens 200, and Desulfovibrio vulgaris Hildenborough) decreased rapidly following inoculation into both soils. The proportion of Alcaligenes eutrophus CH34 remained at a constant level throughout the 8‐week experiment in both soil treatments. Sphingomonas aromaticivorans B0695 showed toxic metal‐dependent survival in that its relative abundance dropped rapidly in pristine soil but remained at approximately inoculation levels throughout the experiment in contaminated microcosms.