Escherichia coli may harbor genetic mercury resistance markers which makes this bacterial species a promising alternative for bioremediation processes. The objective of this study was to investigate phenotypic and genetic characteristics related to diversity and mercury resistance among 178 Escherichia coli strains isolated from residential, industrial, agricultural, and hospital wastewaters and recreational waters at Rio de Janeiro city. Genetic and conventional methods were carried out in order to determine mercury resistance. Random amplification of polymorphic DNA (RAPD-PCR) and denaturing gradient gel electrophoresis (DGGE) were used to investigate genetic variability. RAPD data revealed a high degree of polymorphism among E. coli mercury resistant strains and showed reproducibility and good discriminative results. DGGE typing detected diversity within the merA gene fragment. Our findings represent an improvement in epidemiological studies of ？？E. coli and support the evidence of nonclonal nature of mercury resistant E. coli strains circulating in rural and urban aquatic systems in Rio de Janeiro city. 1. Introduction Chemical contamination of aquatic systems consists of a relevant pollution pattern causing drastic impacts on human, animal, and ecosystem health [1]. Among the various chemical contaminants, mercury plays an important role and once released in aquatic systems, mercury can resist to natural degradation processes and persist for a long time in these environments without losing its toxicity [2]. The concern about environmental contamination by this metal is due to its high toxicity, especially to the nervous system, and its bioaccumulation and biomagnification, providing persistence and wide distribution in global aquatic environment. Even regions with no mercury discharging may be affected [2–7]. Mercury toxicity to humans and other organisms is related to the chemical form to which the organisms were exposed, the route and time of exposure, dose, nutritional status, individual susceptibility, and genetic predisposition [3, 4, 6, 8]. Symptoms and contamination sources are rather different in exposure to elemental mercury, inorganic or organic mercury compounds [3, 4]. Human contamination by this metal may occur by different pathways such as vapors inhalation, contaminated food and/or water consumption, and to a lesser extent through skin contact [3, 6]. Mercury exposure triggers a series of effects including neurological, renal, cardiovascular, respiratory, gastrintestinal, hepatic, genotoxic, immunological, dermal, reproductive, and
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