Mercury rich geothermal springs are likely environments where mercury resistance is critical to microbial life and where microbe-mercury interactions may have evolved. Eleven facultative thermophilic and chemolithoautotrophic, thiosulfate oxidizing bacteria were isolated from thiosulfate enrichments of biofilms from mercury rich hot sulfidic springs in Mount Amiata, Italy. Some strains were highly resistant to mercury (>or=200 muM HgCl(2)) regardless of its presence or absence during primary enrichments, and three reduced ionic mercury to its elemental form. The gene encoding for the mercuric reductase enzyme (MerA), was amplified by PCR from seven strains. However, one highly resistant strain did not reduce mercury nor carried merA, suggesting an alternative resistance mechanism. All strains were members of the order Bacillales and were most closely related to previously described thermophiles belonging to the Firmicutes. Phylogenetic analyses clustered the MerA of the isolates in two supported novel nodes within the Firmicutes lineage and a comparison with the 16S rRNA gene tree suggested at least one case of horizontal gene transfer. Overall, the results show that the thermophilic thiosulfate oxidizing isolates were adapted to life in presence of mercury mostly, but not exclusively, by possessing MerA. These findings suggest that reduction of mercury by chemolithotrophic thermophilic bacteria may mobilize mercury from sulfur and iron deposits in geothermal environments.