Accumulation of toxic metals in the environment represents a public health and wildlife concern. Bacteria resistant to toxic metals constitute an attractive biomass for the development of systems to decontaminate soils, sediments, or waters. In particular, biosorption of metals within the bacterial cell wall or secreted extracellular polymeric substances (EPS) is an emerging process for the bioremediation of contaminated water. Here the isolation of bacteria from soil, effluents, and river sediments contaminated with toxic metals permitted the selection of seven bacterial isolates tolerant to mercury and associated with a mucoid phenotype indicative of the production of EPS. Inductively coupled plasma-optical emission spectroscopy and transmission electron microscopy in conjunction with X-ray energy dispersive spectrometry revealed that bacteria incubated in the presence of HgCl 2 sequestered mercury extracellularly as spherical or amorphous deposits. Killed bacterial biomass incubated in the presence of HgCl 2 also generated spherical extracellular mercury deposits, with a sequestration capacity (40 to 120 mg mercury per g [dry weight] of biomass) superior to that of live bacteria (1 to 2 mg mercury per g [dry weight] of biomass). The seven strains were shown to produce EPS, which were characterized by Fourier transform-infrared (FT-IR) spectroscopy and chemical analysis of neutral-carbohydrate, uronic acid, and protein contents. The results highlight the high potential of Hg-tolerant bacteria for applications in the bioremediation of mercury through biosorption onto the biomass surface or secreted EPS.T hroughout the twentieth century, human activities such as mining, chemical industries, and agriculture have yielded high accumulations of toxic metals in the environment. These metals, bioavailable and persistent (33), constitute a major environmental problem, adversely affecting ecosystems and public health (24). Mercury pollution is of real concern because of the high toxicity of the metal and its translocation all along the food chain: mercury is accumulated upward through the aquatic food chain and is transformed to more-toxic organic mercury forms, mainly highly neurotoxic methylmercury (24).Toxic metals are difficult to remove from the environment, since they cannot be chemically or biologically degraded and are ultimately indestructible. Physicochemical remediation of metalpolluted sites, from incineration of soils to chemical precipitation or/and ion-exchange technologies, has been widely used but remains costly and environmentally damaging. Biological approaches based on metal-resistant microorganisms have received a great deal of attention as alternative remediation processes (20,26). The biological methods used currently for mercury removal consist of Hg 2ϩ reduction to volatile metal mercury by bacterial strains harboring the mer resistance operon (1,32,35,48). Live or dead bacterial biomass has also been used for biosorption applications (52), which consist of passive immobilization of metals by th...

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