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

Background: Tellurite (TeO ₃ ^2- ) is recognized as a toxic oxyanion to living organisms. However, mainly anaerobic or facultative-anaerobic microorganisms are able to tolerate and convert TeO ₃ ^2- into the less toxic and available form of elemental Tellurium (Te ⁰ ), producing Te-deposits or Te-nanostructures. The use of TeO ₃ ^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 TeO ₃ ^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 TeO ₃ ^2- with a minimal inhibitory concentration (MIC) of 2800 μg/mL (11.2 mM). TeO ₃ ^2- consumption has been evaluated exposing the BCP1 strain to either 100 or 500 μg/mL of K ₂ TeO ₃ (unconditioned growth) or after re-inoculation in fresh medium with new addition of K ₂ TeO ₃ (conditioned growth). A complete consumption of TeO ₃ ^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 TeO ₃ ^2- at 500 μg/mL. However, a greater TeO ₃ ^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 TeO ₃ ^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 K ₂ TeO ₃ ) or the growth conditions (unconditioned or conditioned grown cells).
Conclusions: Rhodococcus aetherivorans BCP1 is able to tolerate high concentrations of TeO ₃ ^2- during its growth under aerobic conditions. Moreover, compared to unconditioned BCP1 cells, TeO ₃ ^2- conditioned cells showed a higher oxyanion consumption rate (for 100 μg/mL of K ₂ TeO ₃ ) or to consume greater amount of TeO ₃ ^2- (for 500 μg/mL of K ₂ TeO ₃ ). TeO ₃ ^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 TeO ₃ ^2- along with its ability to produce Te-nanostructures supports the application of this microorganism as a possible eco-friendly nanofactory.