Because anaerobic mercury-resistant microorganisms have not been surveyed sufficiently and those biological features remain to be understood while anaerobic environments are common as mercury-contaminated sites such as soil and sediments, anaerobic mercury-resistant bacterial strains were isolated from Minamata Bay, Kumamoto, and characterized. We also reported identification of a mercury-resistant bacterium of the isolated strains. The genetic components for mercuryresistance of the five strictly anaerobic bacterial strains inducing the bacterium isolated from Minamata Bay were analyzed. The resistance to mercurial compounds and to other heavy metals by the five strictly anaerobic bacterial strains were also tested. Theresults showed that the anaerobic mercury-resistant bacterium isolated from Minamata Bay was identified as Clostridium butyricum Mersaru. Nucleotide sequence analysis of the mercury resistance determinant showed that C. butyricum Mersaru has merB gene (an organomercury lyase gene), identical to that of the aerobic mercury-resistant Bacillus cereus RC607. PCR-Southern hybridization showed that the two PCR products, which are amplified from chromosomal DNA of the four strictly anaerobic bacterial strains, were highly homologous to merA gene (a mercury reductase gene) and merB gene from the aerobic mercury-resistant Bacillus, respectively. The resistance spectrums to mercurial compounds and other heavy metals incicated that all Clostridia tested in this study were broad-spectrum mercury-resistant and broad-spectrum heavy-metal-resistant.
