Rhodococcus aetherivorans BCP1 as cell factory for the production of intracellular tellurium nanorods under aerobic conditions

Abstract

Background: Tellurite (TeO₃^2-) is recognized as a toxic oxyanion to living organisms. However, mainly anaerobic or facultative-anaerobic microorganisms are able to tolerate and convert TeO₃^2- into the less toxic and available form of elemental Tellurium (Te⁰), producing Te-deposits or Te-nanostructures. The use of TeO₃^2--reducing bacteria can lead to the decontamination of polluted environments and the development of "green-synthesis" methods for the production of nanomaterials. In this study, the tolerance and the consumption of TeO₃^2- have been investigated, along with the production and characterization of Te-nanorods by Rhodococcus aetherivorans BCP1 grown under aerobic conditions.

Results: Aerobically grown BCP1 cells showed high tolerance towards TeO₃^2- with a minimal inhibitory concentration (MIC) of 2800 μg/mL (11.2 mM). TeO₃^2- consumption has been evaluated exposing the BCP1 strain to either 100 or 500 μg/mL of K₂TeO₃ (unconditioned growth) or after re-inoculation in fresh medium with new addition of K₂TeO₃ (conditioned growth). A complete consumption of TeO₃^2- at 100 μg/mL was observed under both growth conditions, although conditioned cells showed higher consumption rate. Unconditioned and conditioned BCP1 cells partially consumed TeO₃^2- at 500 μg/mL. However, a greater TeO₃^2- consumption was observed with conditioned cells. The production of intracellular, not aggregated and rod-shaped Te-nanostructures (TeNRs) was observed as a consequence of TeO₃^2- reduction. Extracted TeNRs appear to be embedded in an organic surrounding material, as suggested by the chemical-physical characterization. Moreover, we observed longer TeNRs depending on either the concentration of precursor (100 or 500 μg/mL of K₂TeO₃) or the growth conditions (unconditioned or conditioned grown cells).

Conclusions: Rhodococcus aetherivorans BCP1 is able to tolerate high concentrations of TeO₃^2- during its growth under aerobic conditions. Moreover, compared to unconditioned BCP1 cells, TeO₃^2- conditioned cells showed a higher oxyanion consumption rate (for 100 μg/mL of K₂TeO₃) or to consume greater amount of TeO₃^2- (for 500 μg/mL of K₂TeO₃). TeO₃^2- consumption by BCP1 cells led to the production of intracellular and not aggregated TeNRs embedded in an organic surrounding material. The high resistance of BCP1 to TeO₃^2- along with its ability to produce Te-nanostructures supports the application of this microorganism as a possible eco-friendly nanofactory.

Keywords: Biogenic nanostructures; Elemental tellurium; Nanorods biosynthesis; Rhodococcus aetherivorans; Tellurite; Tellurium nanorods.

Fig. 1

Kill curve of Rhodococcus aetherivorans BCP1 exposed for 24 h to increasing concentration of K₂TeO₃, with the established minimal inhibitory concentration (MIC)

Fig. 2

Rhodococcus aetherivorans BCP1 growth in LB medium, LB supplied with 100 or 500 µg/mL of K₂TeO₃ as unconditioned (a, c) or conditioned (b, d) cells, and TeO₃ ²⁻ consumption

Fig. 3

Transmission electron microscopy (TEM) micrographs of BCP1 cells grown for 120 h in the presence of 100 µg/mL (a), and 500 µg/mL (b) of K₂TeO₃. Arrows indicate the intracellular TeNRs produced by the BCP1 strain

Fig. 4

Transmission electron microscopy (TEM) micrographs of TeNRs₁₀₀ (a), and TeNRs₅₀₀ (b) extracted from the BCP1 strain grown as unconditioned cells in the presence of K₂TeO₃, and TeNRs₁₀₀ (c), and TeNRs₅₀₀ (d) recovered from those conditioned

Fig. 5

Length distribution (nm) of TeNRs₁₀₀ (a), and TeNRs₅₀₀ (b) generated by unconditioned BCP1 K₂TeO₃-grown cells, and TeNRs₁₀₀ (c), and TeNRs₅₀₀ (d) isolated from conditioned ones. Length distributions are indicated as grey filled circles, while the Gaussian fit is highlighted as a continuous black curve

Fig. 6

Scanning electron microscopy (SEM) micrographs of TeNRs₁₀₀ (a), and TeNRs₅₀₀ (b) produced by unconditioned BCP1 K₂TeO₃-grown cells, and TeNRs₁₀₀ (c), and TeNRs₅₀₀ (d) extracted from those conditioned

Fig. 7

Energy-dispersed X-ray spectroscopy (EDX) spectra of TeNRs₁₀₀ (a), and TeNRs₅₀₀ (b) unconditioned BCP1 grown cells, and TeNRs₁₀₀ (c), and TeNRs₅₀₀ (d) extracted from those conditioned ones grown in the presence of K₂TeO₃