The aim of this study was to investigate the effect of mercury contamination on bacterial community structure and function. Bacterial communities from two sites, a mercury-contaminated site inside the harbor of Copenhagen, Denmark (CH) and a unpolluted control site, Koge Buge (KB), were compared with respect to diversity indices, of antibiotic-and heavy metal-resistance patterns, abundance and self transmissibility of plasmids in resistant isolates (endogenous isolation). Furthermore, the potential for gene transfer between indigenous bacteria was assessed by the exogenous plasmid isolation approach. It was found that resistance to all the tested compounds was higher in the mercury-polluted sediment than the control sediment. The abundance of plasmids was higher at the polluted site, where 62% of the isolates contained plasmids, whereas only 29% of the isolates from the control sediment contained plasmids. Furthermore, the frequencies of large plasmids and plasmids per isolates were found to be higher in the contaminated sediment. Exogenous plasmid isolations revealed high occurrence of Hg and tetracycline resistance, self-transmissible plasmids in CH sediment (1.8 x 10(-5) transconjugants per recipients) relative to KB sediment (3.0 x 10(-8) T/R). Shannon-Weaver diversity indices showed no difference in the diversity of the isolates from the two sites, and Hg-resistant isolates from CH were found to be as diverse as the CH isolates in total. This may be owing to high level of self-transmissible Hg resistance plasmids found in CH.