Abstract
Microbes are suitable candidates to recover and decontaminate different environments from soluble metal ions, either via reduction or precipitation to generate insoluble, non-toxic derivatives. In general, microorganisms reduce toxic metal ions generating nanostructures (NS), which display great applicability in biotechnological processes. Since the molecular bases of bacterial reduction are still unknown, the search for new -environmentally safe and less expensive- methods to synthesize NS have made biological systems attractive candidates. Here, 47 microorganisms isolated from a number of environmental samples were analyzed for their tolerance or sensitivity to 19 metal(loid)s. Ten of them were highly tolerant to some of them and were assessed for their ability to reduce these toxicants in vitro. All isolates were analyzed by 16S rRNA gene sequencing, fatty acids composition, biochemical tests and electron microscopy. Results showed that they belong to the Enterobacter, Staphylococcus, Acinetobacter, and Exiguobacterium genera. Most strains displayed metal(loid)-reducing activity using either NADH or NADPH as cofactor. While Acinetobacter schindleri showed the highest tellurite () and tetrachloro aurate () reducing activity, Staphylococcus sciuri and Exiguobacterium acetylicum exhibited selenite () and silver (Ag+) reducing activity, respectively. Based on these results, we used these bacteria to synthetize, in vivo and in vitro Te, Se, Au, and Ag-containing nanostructures. On the other hand, we also used purified E. cloacae glutathione reductase to synthesize in vitro Te-, Ag-, and Se-containing NS, whose morphology, size, composition, and chemical composition were evaluated. Finally, we assessed the putative anti-bacterial activity exhibited by the in vitro synthesized NS: Te-containing NS were more effective than Au-NS in inhibiting Escherichia coli and Listeria monocytogenes growth. Aerobically synthesized TeNS using MF09 crude extracts showed MICs of 45- and 66- μg/ml for E. coli and L. monocytogenes, respectively. Similar MIC values (40 and 82 μg/ml, respectively) were observed for TeNS generated using crude extracts from gorA-overexpressing E. coli. In turn, AuNS MICs for E. coli and L. monocytogenes were 64- and 68- μg/ml, respectively.
Highlights
More than half of the elements in the Periodic Table are represented by metals and metalloids
Four different culture media were individually inoculated with each sample and incubated at 25 or 37◦C for 24 h: LB (Sambrook and Russell, 2001); R2A (0.5 g/L yeast extract, 0.5 g/L peptone, 0.5 g/L casamino acids, 0.5 g/L glucose, 0.5 g/L soluble starch, 0.3 g/L sodium pyruvate, 0.3 g/L K2HPO4, and 0.5 g/L MgSO4 adjusted to pH 7.2); Acidiphilium (2 g/L (NH4)2SO4, 0.1 g/L KCl, 0.5 g/L K2HPO4, 0.5 g/L MgSO4, 0.3 g/L yeast extract and 1 g/L D-glucose adjusted to pH 3.0); and Aciduric thermophilic Bacillus strains (ATB) (0.2 g/L (NH4)2SO4, 0.5 g/L MgSO4, 0.25 g/L CaCl2, 3 g/L KH2PO4, 1 g/L yeast extract, 1 g/L tryptone, 1 g/L glucose and 1 g/L starch adjusted to pH 4.3)
A total of 47 bacterial strains isolated from environmental samples were assessed for metal(loid) resistance
Summary
More than half of the elements in the Periodic Table are represented by metals and metalloids. Protein carbonylation (Contreras and Vásquez, 2010), [4Fe-4S] cluster dismantling of iron-sulfur proteins (Calderón et al, 2009), membrane lipid peroxidation (Lemire et al, 2013; Pradenas et al, 2013), blocking of cysteine and methionine residues (Stolz and Oremland, 1999), increased cell permeability, and the interference of metabolic pathways have been observed (Carotti et al, 2000) Given their great biotechnological potential, we focused our interest in characterizing NS containing silver, gold or the metalloids tellurium and selenium. Gold is found as metallic gold (Au0) as well as in six other oxidation states, 1+ [Au (I)] and 3+ [Au (III)] being the most commonly found in nature (Nies and Grass, 2009)
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