Impact of the Mg-citrate transporter CitM on heavy metal toxicity in Bacillus subtilis

Bacillus subtilis possesses a secondary transporter CitM that is specific for the complex of citrate and Mg but is also capable of transporting citrate in complex with the heavy metal ions Zn, Ni and Co. We report on the impact of CitM activity on the toxicity of the heavy metals Zn, Ni and Co + in B. subtilis. In a CitM deletion mutant or under conditions when CitM is not expressed, the toxic effects of the metals were reduced by the presence of citrate in the medium. In contrast, the presence of citrate dramatically enhanced toxicity when the Mgcitrate transporter was present in the membrane. It is demonstrated that the complex of Ni and citrate is transported into the cell and that the uptake is responsible for the enhanced toxicity. At toxic concentrations of the metal ions, the cultures adapted by developing resistance against these ions. Resistant cells isolated by exposure to one of the metal ions remained resistant after growth in the absence of toxic metal ions, and were cross-resistant against the other two toxic ions. Resistant strains were shown to contain point mutations in the citM gene, which resulted in premature termination of translation.