Related Arsenic Resistance Transposons Occur in Leptospirilli Isolated from Different Continents

Abstract

Acidithiobacillus caldus and Leptospirillum ferriphilum dominate the microbial consortium used in continuous-flow, stirred tank processes used to treat gold-bearing arsenopyrite concentrates in South Africa. These microorganisms were adapted to high concentrations of arsenic over several years and both types of microorganisms were found to contain two sets of arsenic resistance genes. One set was present on all isolates of a species irrespective of whether they were highly arsenic resistant or not, while a second, transposon-located set was present in only those strains that had been adapted to high concentrations of arsenic. We isolated a Leptospirillum ferrooxidans from the microbial consortium present in arsenopyrite treatment tanks from Tamboraque (near Lima, Peru) that had not been inoculated with an arsenic-adapted consortium from South Africa. This allowed us to determine whether these microorganisms had acquired similar arsenic resistance mechanisms as contained on the transposons in the highly arsenic resistant South African cultures. Several isolates of both Leptospirillum ferriphilum and L. ferrooxidans from Europe as well as a "Leptospirillum ferrodiazotrophum" were also screened to detect whether they contained similar arsenic resistance transposons even though they had not been selected for enhanced arsenic resistance. Transposons containing arsenic resistance genes that were identical or closely related to those from South Africa were found in both L. ferrooxidans and L. ferriphilum isolates from South America and Europe. The widespread occurrence of arsenic resistance transposons suggests that it should be possible to select for highly arsenic resistant biomining microorganisms from many different sources and therefore, unnecessary to acquire pre-adapted arsenic resistant consortia.