In vitro biosynthesis of Ag, Au and Te-containing nanostructures by Exiguobacterium cell-free extracts

Abstract

Background: The bacterial genus Exiguobacterium includes several species that inhabit environments with a wide range of temperature, salinity, and pH. This is why the microorganisms from this genus are known generically as polyextremophiles. Several environmental isolates have been explored and characterized for enzyme production as well as for bioremediation purposes. In this line, toxic metal(loid) reduction by these microorganisms represents an approach to decontaminate soluble metal ions via their transformation into less toxic, insoluble derivatives. Microbial-mediated metal(loid) reduction frequently results in the synthesis of nanoscale structures-nanostructures (NS) -. Thus, microorganisms could be used as an ecofriendly way to get NS.

Results: We analyzed the tolerance of Exiguobacterium acetylicum MF03, E. aurantiacum MF06, and E. profundum MF08 to Silver (I), gold (III), and tellurium (IV) compounds. Specifically, we explored the ability of cell-free extracts from these bacteria to reduce these toxicants and synthesize NS in vitro, both in the presence or absence of oxygen. All isolates exhibited higher tolerance to these toxicants in anaerobiosis. While in the absence of oxygen they showed high tellurite- and silver-reducing activity at pH 9.0, whereas AuCl₄^- which was reduced at pH 7.0 in both conditions. Given these results, cell-free extracts were used to synthesize NS containing silver, gold or tellurium, characterizing their size, morphology and chemical composition. Silver and tellurium NS exhibited smaller size under anaerobiosis and their morphology was circular (silver NS), starred (tellurium NS) or amorphous (gold NS).

Conclusions: This nanostructure-synthesizing ability makes these isolates interesting candidates to get NS with biotechnological potential.

Keywords: Aerobiosis; Anaerobiosis; Exiguobacterium; Metal(loid); Nanostructure; Reduction.

Fig. 1

Metal(loid) reduction by Exiguobacterium strains under aerobic and anaerobic conditions. Reduction assays of Ag (a), Au (b) and Te (c) were carried out as described in Methods. Blue, red and green bars represent pH 7.0, 8.0 or 9.0, respectively. (+), aerobic test, (−) anaerobic test. Bars represent the average of 3 independent trials. **, p < 0.01; *, p < 0.05; nd, not determined; ns, not significant

Fig. 2

Assessing metal(loid)-reducing activity of crude extracts from E. acetylicum MF03, E. aurantiacum MF06 and E. profundum MF08. Reduction of Ag (a), Au (b) and Te (c) was carried out as described in Methods. Black and white bars represent no treatment or toxicant exposure, respectively. +, aerobic tests; −, anaerobic tests. Bars represent the average of 3 independent trials. **, p < 0.01; ns, not significant

Fig. 3

Characterization of silver nanostructures. Electronic micrographs (left) and EDS analysis (right) of in vitro generated AgNS, under anaerobic (a) and anaerobic conditions (b) by crude extracts of E. acetylicum MF03. c and d, AgNS generated aerobically and anaerobically, respectively, by E. profundum MF08

Fig. 4

Characterization of gold nanostructures. Electron micrographs (left) and EDS analysis (right) of in vitro generated Au-NS, under aerobic (a) and anaerobic (b) conditions by crude extracts of E. acetylicum MF03; AuNS generated under aerobic (c) and anaerobic (d) conditions by E. aurantiacum MF06

Fig. 5

Characterization of tellurium nanostructures. Electronic micrographs (left) and EDS analysis (right) of NS from Te generated in vitro. TeNS generated aerobically (a) and anaerobically (b) by crude extracts of E. acetylicum MF03 and TeNS generated aerobically (c) and anaerobically (d) by E. profundum MF08