Induction of zincophore pseudopaline secretion by Cr(VI) and intracellular formation of granules from nanocrystal aggregation by Cr(III) in Pseudomonas aeruginosa

When microorganisms are challenged with toxic metals, intracellular granules are commonly observed, however, the exact nature of these granules is poorly understood. Here we show that when Pseudomonas aeruginosa CCTCC AB93066 were exposed to Cr(VI), Cr can enter the cell in the form of both Cr(VI) and Cr(III), and intracellular granules of several hundred nanometers were formed in the nucleoid region and were built up by aggregation of nanocrystals. We suggested that these nanocrystals are organic crystals. Transcriptomic profiles and liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis indicated that pseudopaline (a metallophore that can complex with Zn²⁺) production and pseudopaline-Zn²⁺ import into bacterial cells were enhanced upon Cr(VI) exposure. It was proposed that pseudopaline can scavenge Zn²⁺ which is essential for transcription alteration and DNA repair. Excessive pseudopaline might precipitate as nanospheres in the nuclear region that are further agglomerated by Cr(III) to form larger granules. During this process, Cr(III) is sequestered and immobilized. Hence we revealed pseudopaline production and zinc acquisition is crucial for alleviation of Cr(VI) toxicity and intracellular granules are composed of organic nanospheres which are aggregated by Cr(III).

Zinc-binding metallophores protect Pseudomonas aeruginosa from calprotectin-mediated metal starvation

Pseudomonas aeruginosa is known to exhibit considerable resistance to the antimicrobial activity of the metal-sequestering protein Calprotectin (CP). In this study we demonstrate that, although CP induces zinc deficiency in P. aeruginosa, a strain unable to import zinc through the two most important metal acquisition systems, namely ZnuABC and ZrmABCD, maintains significant growth capacity in the presence of high concentrations of CP. Furthermore, we have shown that nicotianamine, a molecule structurally similar to the metallophore pseudopaline, can favor the acquisition of the metal even in the presence of CP. To gain insights into the mechanisms through which metallophores can promote zinc acquisition, we analyzed the effect of nicotianamine on the activity of the metallo-β-lactamase VIM-1. Our data suggest that metallophores released by bacteria in response to zinc deficiency can extract the protein-bound metal. The ability to interfere with the binding of metals to proteins, as well as favoring the acquisition of zinc, may contribute to increasing the resistance of P. aeruginosa to the antimicrobial action of CP.