A Molecular Mechanism for Bacterial Susceptibility to Zinc

Abstract

Transition row metal ions are both essential and toxic to microorganisms. Zinc in excess has significant toxicity to bacteria, and host release of Zn(II) at mucosal surfaces is an important innate defence mechanism. However, the molecular mechanisms by which Zn(II) affords protection have not been defined. We show that in Streptococcus pneumoniae extracellular Zn(II) inhibits the acquisition of the essential metal Mn(II) by competing for binding to the solute binding protein PsaA. We show that, although Mn(II) is the high-affinity substrate for PsaA, Zn(II) can still bind, albeit with a difference in affinity of nearly two orders of magnitude. Despite the difference in metal ion affinities, high-resolution structures of PsaA in complex with Mn(II) or Zn(II) showed almost no difference. However, Zn(II)-PsaA is significantly more thermally stable than Mn(II)-PsaA, suggesting that Zn(II) binding may be irreversible. In vitro growth analyses show that extracellular Zn(II) is able to inhibit Mn(II) intracellular accumulation with little effect on intracellular Zn(II). The phenotype of S. pneumoniae grown at high Zn(II):Mn(II) ratios, i.e. induced Mn(II) starvation, closely mimicked a ΔpsaA mutant, which is unable to accumulate Mn(II). S. pneumoniae infection in vivo elicits massive elevation of the Zn(II):Mn(II) ratio and, in vitro, these Zn(II):Mn(II) ratios inhibited growth due to Mn(II) starvation, resulting in heightened sensitivity to oxidative stress and polymorphonuclear leucocyte killing. These results demonstrate that microbial susceptibility to Zn(II) toxicity is mediated by extracellular cation competition and that this can be harnessed by the innate immune response.

Characterizing the conformational dynamics of metal-free PsaA using molecular dynamics simulations and electron paramagnetic resonance spectroscopy

“…Recombinant PsaA was cloned from S. pneumoniae D39 genomic DNA, gene SPD_1463, into pCAMcLIC01-PsaA as previously described [19]. The recombinant sequence of PsaA 8 …”

“…It has a two-lobed organization comprising of N-and C-terminal (β/α) 4 -domains linked by a rigid helix, and bisected by a cleft in which the Mn 2+ ion binds [18,19] (Figure 1a). …”

Enterococcus faecalis zinc-responsive proteins mediate bacterial defence against zinc overload, lysozyme and oxidative stress

“…Such a connection between zinc resistance and higher tolerance to oxidative stress and lysozyme has, to the best of our knowledge, never been described previously. A zinc burst, described to occur inside macrophages (Botella et al, 2011), could indirectly affect the mechanisms of oxidative stress response, as it is known that zinc is able to compete with manganese (Kloosterman et al, 2008;McDevitt et al, 2011), a crucial metal in oxidative stress response, in protein binding. McDevitt et al (2011) found that extracellular zinc inhibits manganese acquisition in Streptococcus pneumoniae by competing for binding to the solute-binding protein PsaA.…”

“…A zinc burst, described to occur inside macrophages (Botella et al, 2011), could indirectly affect the mechanisms of oxidative stress response, as it is known that zinc is able to compete with manganese (Kloosterman et al, 2008;McDevitt et al, 2011), a crucial metal in oxidative stress response, in protein binding. McDevitt et al (2011) found that extracellular zinc inhibits manganese acquisition in Streptococcus pneumoniae by competing for binding to the solute-binding protein PsaA. Similarly, we have previously reported that the operon ef0575-ef0577, which encodes a PsaA-like solute-binding protein (EF0577), is upregulated by zinc and downregulated by manganese (Abrantes et al, 2011), revealing an E. faecalis manganese transporter that is modulated by zinc concentrations.…”

Manipulation of the Mononuclear Phagocyte System by Mycobacterium tuberculosis

“…The exact mechanism(s) of copper toxicity in M. tuberculosis remain to be identified. The mechanism(s) of zinc ion toxicity may also include inactivation of iron -sulfur clusters (Xu and Imlay 2012) and inhibition of manganese uptake through transport competition in the bacterial perisplam (McDevitt et al 2011). In addition, it was shown recently that P-ATPase-mediated copper export is required for copper supply to periplasmic Cu,Zn-SOD and resistance to oxidative stress in Salmonella enterica (Osman et al 2013).…”