Additive effects of metal excess and superoxide, a highly toxic mixture in bacteria

Abstract

Heavy metal contamination is a serious environmental problem. Understanding the toxicity mechanisms may allow to lower concentration of metals in the metal-based antimicrobial treatments of crops, and reduce metal content in soil and groundwater. Here, we investigate the interplay between metal efflux systems and the superoxide dismutase (SOD) in the purple bacterium Rubrivivax gelatinosus and other bacteria through analysis of the impact of metal accumulation. Exposure of the Cd 2+-efflux mutant DcadA to Cd 2+ caused an increase in the amount and activity of the cytosolic Fe-Sod SodB, thereby suggesting a role of SodB in the protection against Cd 2+. In support of this conclusion, inactivation of sodB gene in the DcadA cells alleviated detoxification of superoxide and enhanced Cd 2+ toxicity. Similar findings were described in the Cu +-efflux mutant with Cu +. Induction of the Mn-Sod or Fe-Sod in response to metals in other bacteria, including Escherichia coli, Pseudomonas aeruginosa, Pseudomonas putida, Vibrio cholera and Bacillus subtilis, was also shown. Both excess Cd 2+ or Cu + and superoxide can damage [4Fe-4S] clusters. The additive effect of metal and superoxide on the [4Fe-4S] could therefore explain the hypersensitive phenotype in mutants lacking SOD and the efflux ATPase. These findings underscore that ROS defence system becomes decisive for bacterial survival under metal excess.

Increasing the copper sensitivity of microorganisms by restricting iron supply, a strategy for bio‐management practices

“…Similar experiments were conducted using the ZntA mutant of E. coli to check the effect of Cd 2+ on SOD activity. Likewise, excess Cd 2+ in the medium resulted in an induced activity of the Mn‐Sod and decreased activity of the Fe‐Sod only in the ZntA mutant ( accompanying paper (Steunou et al , 2020b)). Altogether, these results showed that the accumulation of copper affected the expression and activity of superoxide dismutases presumably because of iron homoeostasis dysregulation linked to copper stress.…”

“…The periplasmic copper protein CopI (Durand et al , 2015) is noteworthy induced under copper stress in R. gelatinosus and V. cholerae in response to Cu + . Cadmium stress in R. gelatinosus and V. cholerae also resulted in the induction of SodB in the CadA ATPase‐deficient mutants ( accompanying paper (Steunou et al , 2020b)).…”

“…The induction of SODs is not fortuitous but functional. Indeed, mutants in which both the efflux system CopA or CadA and the superoxide dismutase SodB are missing are also extremely sensitive to Cu + and to Cd 2+ stress ( accompanying paper (Steunou et al , 2020b)).…”

Cadmium and Copper Cross-Tolerance. Cu+ Alleviates Cd2 + Toxicity, and Both Cations Target Heme and Chlorophyll Biosynthesis Pathway in Rubrivivax gelatinosus

“…subtilis and other bacteria (Kihlken et al, 2008). As for copI, the copZ deletion in B. subtilis resulted in an increased sensitivity to both Cu + and Cd 2+ and expression of the copZA operon was induced in response to elevated concentrations of both metals (Moore et al, 2005;Steunou et al, 2020a). Similarly, in Pseudomonas (P.) aeruginosa Cu + induces the activation of the czcCBA operon involved in the efflux of Zn 2+ , Cd 2+ , and Co 2+ (Caille et al, 2007).…”

“…A band at about 22 kDa apparent molecular weight was also noticeably induced in the cadA mutant. This band corresponds to the cytosolic superoxide dismutase SodB (Steunou et al, 2020a). Similarly, in the cadR − mutant, CopI is also more induced compared to the wild-type FIGURE 7 | Expression level of HemN-H 6 in R. gelatinosus WT and HemN-H 6 strain in malate medium (-) or challenged with 1000 µM CdCl 2 (Cd 2+ ) or 500 µM CuSO 4 (Cu + ).…”