Active resistance to the mercuric ion is widely distributed in environmental microbes and results from the action of mercuric reductase. Five mercury resistant bacteria: Escherichia coli Z1, E. coli Z3, Pseudomonas putida Z2, Serratia marcescens Z4 and Xanthomonas sp. Z5 were isolated and identified from sludge sample. The presence of mercury resistance determinants was screened by polymerase chain reaction (PCR) using merA-specific primers. Based on the analysis of merA amplicons, high

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... ty was recorded between the merA region of the strains P. putida Z2 and Xanthomonas sp. Z5 with those of Tn5053; while the merA of E. coli Z1 was analogous to those of Tn21. In case of the bacterial strains E. coli Z3 and S. marcescens Z4 a great matching was obtained between their merA and those of Tn5036. The effect of mercury stress upon the structure of mercury reducing biofilm at the species level and the type of mercury resistance determinants was studied in a continuous bioreactor. Community analysis suggested that the bacterial strain E. coli Z3 containing Tn5036-like determinant is well adapted strain that tolerated elevated levels of mercury whereas the other strains showed a less fitness under these extreme conditions.