Iron and Copper Act Synergistically To Delay Anaerobic Growth of Bacteria

Abstract

ABSTRACTTransition metals are known to cause toxic effects through their interaction with oxygen, but toxicity under anoxic conditions is poorly understood. Here we investigated the effects of iron (Fe) and copper (Cu) on the anaerobic growth and gene expression of the purple phototrophic bacteriumRhodopseudomonas palustrisTIE-1. We found that Fe(II) and Cu(II) act synergistically to delay anaerobic growth at environmentally relevant metal concentrations. Cu(I) and Cu(II) had similar effects both alone and in the presence of ascorbate, a Cu(II) reductant, indicating that reduction of Cu(II) to Cu(I) by Fe(II) is not sufficient to explain the growth inhibition. Addition of Cu(II) increased the toxicity of Co(II) and Ni(II); in contrast, Ni(II) toxicity was diminished in the presence of Fe(II). The synergistic anaerobic toxicity of Fe(II) and Cu(II) was also observed forEscherichia coliMG1655,Shewanella oneidensisMR-1, andRhodobacter capsulatusSB1003. Gene expression analyses forR. palustrisidentified three regulatory genes that respond to Cu(II) and not to Fe(II): homologs ofcueRandcusR, two known proteobacterial copper homeostasis regulators, andcsoR, a copper regulator recently identified inMycobacterium tuberculosis. Two P-type ATPase efflux pumps, along with an FoF1ATP synthase, were also upregulated by Cu(II) but not by Fe(II). AnEscherichia colimutant deficient incopA,cus, andcueOshowed a smaller synergistic effect, indicating that iron might interfere with one or more of the copper homeostasis systems. Our results suggest that interactive effects of transition metals on microbial physiology may be widespread under anoxic conditions, although the molecular mechanisms remain to be more fully elucidated.