Heavy metal resistance to the extreme: Streptomyces strains from a former uranium mining area

Contamination of large areas by industrial or mining activities is a serious environmental problem. Selection pressure in these polluted habitats led to adaptation in microorganisms now containing special resistance mechanisms as a result to the permanent exposure to exceedingly high concentrations of heavy metals. Banks of a creek in a former uranium mining area near Ronneburg, Germany, were analyzed with regard to highly heavy metal resistant actinobacterial strains, with special attention to nickel, which is both harmful contaminant and essential for incorporation in enzymes. In this work, seven Streptomyces strains that show a high resistance to nickel are presented. Two Streptomyces mirabilis strains are able to grow on concentrations higher than 100 mmol/l nickel. Additional experiments revealed that both strains are also able to cope with 100 mmol/l zinc. Both strains are differing in plasmid equipment and production of secondary metabolites on nickel-amended complex medium. The survival at the contaminated site could be mimicked using media prepared with soil that present the composition of mobile and adsorbed heavy metals in axenic culture that are present in the environment and determine the soil microflora composition there.