Thiobacillus caldus and Leptospirillum ferrooxidans are widely distributedin continuous flow biooxidation tanks used to treat a variety of metal containing ores and concentrates

Using 16S rDNA-based techniques, several workers have reported that Leptosp irillum ferrooxidans and Thiobacillus thiooxidans are the bacteria most com monly encountered in biooxidation processes for the treatment of copper by heap leaching as well as the treatment of zinc-lead or arsenopyrite concent rates by continuous-flow tank leaching. We have compared the 16S rRNA genes from four strains of Thiobacillus caldus and identified two restriction en zymes which permit the 16S rDNA from T. caldus to be distinguished from clo sely related strains of T. thiooxidans. DNA has been isolated directly from continuous-flow biooxidation tanks treating copper, nickel or arsenopyrite concentrates at 40-55 degrees C. By examining the restriction enzyme patte rns of the amplified 16S rDNA, we report that even at 40 degrees C it is th e moderate thermophile T. caldus, rather than T. thiooxidans, which is the dominant sulphur-oxidising bacterium in these tanks. By examining the restr iction enzyme patterns of the 16S rDNA from fifteen strains of Leptospirill um spp isolated from different parts of the world, we have confirmed that t he leptospirilli can be divided into two major groups. Furthermore, we have found that the leptospirilli which dominate the iron-oxidising bacteria in the continuous-flow biooxidation tanks belong to the same sub-group as the L. ferrooxidans type strain (DSM2705). Restriction enzyme maps of the 16S rDNA which enable isolates of T. ferrooxidans, T. thiooxidans, T. caldus an d the subgroups of the genus Leptospirillum to be readily distinguished fro m each other are presented.