Isolation and Some Properties of an Iron-oxidizing Bacterium Thiobacillus ferrooxidans Resistant to Molybdenum Ion

Abstract

Among seventy five strains of iron-oxidizing bacteria obtained from natural environments, only one strain, Thiobacillus ferrooxidans Funis 2-1, grew on Fe²⁺-medium with 1.25mM of sodium molybdate (Mo⁶⁺). In contrast, T. ferrooxidans AP19-3, the representative of Mo sensitive strains, could not grow on Fe²⁺ -medium with 1.0 mM of sodium molybdate. By comparing the levels of inhibition of iron oxidase and cytochrome c oxidase by Mo⁶⁺ or Mo⁵⁺, it was found that Mo⁵⁺ but not Mo⁶⁺ is an actual inhibitor for the iron oxidation enzyme system, especially for cytochrome c oxidase. Cytochrome c oxidase of Funis 2-1 was more resistant to Mo⁵⁺ than AP19-3. Mo⁵⁺, compared to Mo⁶⁺, strongly binds to both cells and the plasma membrane of T. ferrooxidans. Funis 2-1 cells showed a lower binding activity to Mo⁶⁺ or Mo⁵⁺ compared to AP19-3. Cytochrome c oxidase of T. ferrooxidans has been known to catalyze the oxidation of not only reduced mammalian cytochrome c but also Mo⁵⁺. Mo⁶⁺ -oxidizing activities medsured with intact cells and a purified cytochrome c oxidase from Funis 2-1 cells were higher than those of AP19-3, suggesting that Funis 2-1 cells can oxidize toxic Mo⁵⁺ more rapidly to harmless Mo⁶⁺ than AP19-3 does. Since Mo⁶⁺ is known to be chemically reduced by Fe²⁺ to give Mo⁵⁺ and Fe³⁺, the growth inhibition by sodium molybdate(Mo⁶⁺) observed in T. ferrooxidans is explained as follows: Mo⁶⁺ added to Fe⁶⁺ -medium is chemically reduced by Fe²⁺, and Mo⁵⁺ thus produced binds to the plasma membrane and inhibits iron oxidase, as a result, growth of the bacterium is stopped.