Molecular genetics of Thiobacillus ferrooxidans and other mesophilic, acidophilic, chemolithotrophic, iron- or sulfur-oxidizing bacteria
The bacteria primarily responsible for decomposing metal sulfide ores and concentrates at temperatures of 40 °C or below have been identified as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans (or related Leptospirillum spp.) T. thiooxidans and recently, T. caldus. These obligately acidophilic, autotrophic, usually aerobic, iron- or sulfur-oxidizing chemolithotrophic bacteria occupy an ecological niche that is largely inorganic and very different from that populated by the more commonly studied non-acidophilic heterotrophic bacteria. It has been of particular interest to discover how these `biomining' bacteria are phylogenetically related to the rest of the microbial world. Based on 16S rRNA sequence data, the thiobacilli have been placed in the Proteobacteria division close to the junction between the β and γ sub-divisions. In contrast, the leptospirilli have been positioned within a relatively recently recognized division called the Nitrospira group. T. ferrooxidans is the only biomining bacterium whose molecular biology has been studied in some detail. Of the approximately 50 genes cloned or sequenced and published, by far the majority that can be tested are expressed and produce proteins which are functional in Escherichia coli (a member of the γ sub-division of Proteobacteria). These observations together with phylogenetic comparisons of most T. ferrooxidans protein sequences have confirmed the unexpectedly close relationship between T. ferrooxidans and E. coli. A special challenge has been the isolation of the various components of the iron-oxidation system and as a result of a global effort, this is almost complete. Several plasmids, transposons and insertion sequences have been isolated from T. ferrooxidans. These genetic elements are interesting because they may contain non-essential genes which are thought to improve the fitness of the bacterium and are frequently mobile. They have provided some fascinating insights into genetic exchanges that have occurred between T. ferrooxidans and other bacteria. There are clear indications that some of the other `biomining' bacteria are even more important than T. ferrooxidans in many commercial biomining processes. The molecular biology of these bacteria is almost unstudied.