Iron and Copper Act Synergistically To Delay Anaerobic Growth of Bacteria

Abstract

bTransition 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 bacterium Rhodopseudomonas palustris TIE-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 for Escherichia coli MG1655, Shewanella oneidensis MR-1, and Rhodobacter capsulatus SB1003. Gene expression analyses for R. palustris identified three regulatory genes that respond to Cu(II) and not to Fe(II): homologs of cueR and cusR, two known proteobacterial copper homeostasis regulators, and csoR, a copper regulator recently identified in Mycobacterium tuberculosis. Two P-type ATPase efflux pumps, along with an F o F 1 ATP synthase, were also upregulated by Cu(II) but not by Fe(II). An Escherichia coli mutant deficient in copA, cus, and cueO showed 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.

Potential Role of Nitrite for Abiotic Fe(II) Oxidation and Cell Encrustation during Nitrate Reduction by Denitrifying Bacteria

“…Fe 2ϩ is known to be toxic due to the formation of oxygen radicals in the Fenton reaction under oxic conditions (64). There are only a few studies concerning the toxicity of Fe 2ϩ under anoxic conditions (65,66). Some nitrate-reducing Fe(II)-oxidizing strains are not able to grow on dissolved Fe(II) but instead require chelated Fe(II), for example, Paracoccus ferrooxidans (41) and Pseudogulbenkiania strain MAI-1 (67), and it was suggested by those authors that this was due to Fe 2ϩ toxicity.…”

Nonredundant Roles for Cytochrome c 2 and Two High-Potential Iron-Sulfur Proteins in the Photoferrotroph Rhodopseudomonas palustris TIE-1

“…We therefore looked for insight into its function by investigating its transcriptional response to metal stress. In a previously performed microarray (25), Rpal_4085 was upregulated by an anoxic Fe (II) shock, while PioC was not. We therefore hypothesized that it might be involved in mitigating metal toxicity.…”

Anaerobic Copper Toxicity and Iron-Sulfur Cluster Biogenesis in Escherichia coli

“…In industrial and mining waste streams, micromolar to millimolar concentrations of copper have also been found (4). Copper at high concentrations in water has become a threat to all living organisms (5). As a transition metal, copper was believed to produce reactive oxygen species via the Fenton reaction under aerobic conditions (6,7).…”