The microbial community and sulfur oxygenase reductases of metagenomic DNA from bioreactors treating gold-bearing concentrates were studied by 16S rRNA library, real-time polymerase chain reaction (RT-PCR), conventional cultivation, and molecular cloning. Results indicated that major bacterial species were belonging to the genera Acidithiobacillus, Leptospirillum, Sulfobacillus, and Sphingomonas, accounting for 6.3, 66.7, 18.8, and 8.3%, respectively; the sole archaeal species was Ferroplasma sp. (100%). Quantitative RT-PCR revealed that the 16S rRNA gene copy numbers (per gram of concentrates) of bacteria and archaea were 4.59 × 109 and 6.68 × 105, respectively. Bacterial strains representing Acidithiobacillus, Leptospirillum, and Sulfobacillus were isolated from the bioreactors. To study sulfur oxidation in the reactors, pairs of new PCR primers were designed for the detection of sulfur oxygenase reductase (SOR) genes. Three sor-like genes, namely, sorFx, sorSA, and sorSB were identified from metagenomic DNAs of the bioreactors. The sorFx is an inactivated SOR gene and is identical to the pseudo-SOR gene of Ferroplasma acidarmanus. The sorSA and sorSB showed no significant identity to any genes in GenBank databases. The sorSB was cloned and expressed in Escherichiacoli, and SOR activity was determined. Quantitative RT-PCR determination of the gene densities of sorSA and sorSB were 1,000 times higher than archaeal 16S rRNA gene copy numbers, indicating that these genes were mostly impossible from archaea. Furthermore, with primers specific to the sorSB gene, this gene was PCR-amplified from the newly isolated Acidithiobacillus sp. strain SM-1. So far as we know, this is the first time to determine SOR activity originating from bacteria and to document SOR gene in bioleaching reactors and Acidithiobacillus species.