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FEMS Microbiology Letters 204 (2001) 49^53

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Adaptation of the bacterial community to mercury contamination
Anne Kirstine Muller, Lasse Dam Rasmussen, SÖren Johannes SÖrensen * «
Department of General Microbiology, University of Copenhagen, SÖlvgade 83 H, DK-1307 Copenhagen K, Denmark Received 25 April 2001 ; received in revised form 31 July 2001; accepted 1 August2001 First published online 14 September 2001

Abstract
The utilisation of 31 sole carbon sources by bacterial communities of soil in the presence of increasing concentrations of Hg(II) was measured by a colour development assay. The assay was performed on Biolog microtitre plates (Ecoplates) in the presence of Hg(II) and compared to Hg(II)-free Ecoplates. Furthermore, community tolerance toHg(II) was measured by colour development in microtitre plates supplemented with LB broth and by enumeration of colony-forming units on LB agar plates. Both microtitre plates supplemented with LB and LB agar plates contained increasing concentrations of Hg(II). The difference in substrate utilisation profile, as shown by growth on 31 different carbon substrates in the Ecoplates, suggested anadaptation of the soil community that correlated with the metal exposure level in the soil. Similarly, growth on microtitre plates supplemented with LB and plate-spreading data showed an increased community tolerance with increasing levels of mercury in the soil. Both the multi-function microtitre plate assay (Ecoplate) and the LB broth microtitre plate assay are suitable for evaluating the adaptationof the bacterial community in soil to a heavy metal pollutant. ß 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Microbiological Societies.
Keywords : Mercury; Community tolerance ; Ecoplate

1. Introduction Many parameters are used to indicate the e¡ects of heavy metals on the soil microbial community. E¡ects can be measured by the size of the microbial biomassand/or by the abundance of speci¢c organisms and di¡erent microbial-mediated processes [1,2]. However, such measurements do not always indicate the direct e¡ects of heavy metals on the microbial community as they fail to account for the impact of other factors such as soil pH and organic matter content. A property speci¢cally connected with the heavy metal concentration in the soil is the toleranceof the microbial community to the compound of interest. As the heavy metal exerts a selection pressure upon the micro-organisms, a measurable increase in community tolerance will result. Analysis of plate-spreading results have shown the proportion of heavy metal-resistant bacteria in soil to increase with increasing concentration of the metal [3^5]. A resistance index determined by plate counthas been proposed

* Corresponding author. Tel.: +45 35 32 20 53; Fax: +45 35 32 20 40. E-mail address : sjs@mermaid.molbio.ku.dk (S.J. SÖrensen).

as a relevant parameter when evaluating the e¡ect of heavy metals [6]. Measurement of [3 H]thymidine incorporation into bacteria in the presence of increasing levels of heavy metal has also been used to show increased tolerance of the soil bacterialcommunity with increasing concentration of metal in the soil [7]. The increased tolerance of the community may also result in changes in the functional performance. Monitoring functional changes in microbial communities by the sole carbon source utilisation pro¢le has previously been used to study the e¡ect of Hg(II) in soil [8,9] and community tolerance to Zn(II) [10]. The aim of this researchwas to develop a rapid multifunction test for evaluating the tolerance of soil bacterial communities to a speci¢c pollutant. Soil was treated with four di¡erent concentrations of mercury. After 1 week the adaptation of the bacterial communities was measured by the growth kinetics in microtitre plates containing di¡erent mercury concentrations. Growth on microtitre plates where each well contains...