Mechanism of the oxidative stress-mediated increase in lipid accumulation by the bacterium, R. opacus PD630: Experimental analysis and genome-scale metabolic modeling.

Appropriate species of oleaginous bacteria, with their high growth rates and lipid accumulation capabilities, can be good contenders for industrial triacylglycerol (TAG) production, compared to microalgae. Further, oxidative stress (OS) can be used to significantly increase TAG yields in oleaginous microbes, but the mechanism is unexplored. In a first, this study explored the mechanism behind OS-mediated increase in TAG accumulation by the bacterium, Rhodococccus opacus PD630, through experimental analysis and metabolic modelling. Two mechanisms that could increase acetyl-CoA (TAG-precursor) levels were hypothesized based on literature information. One was OS-mediated inactivation of the aconitase (TCA cycle), and another was the inactivation of the triosephosphate isomerase (TPI; glycolysis). The results negated the involvement of aconitase in increased acetyl-CoA levels. Analysis of the metabolic model showed that inactivation of TPI, re-routed the flux through the pentose phosphate pathway (PPP), supplying both NADPH and acetyl-CoA for TAG synthesis. Additionally, inactivation of TPI increased TAG flux by 143%, whereas, inactivating both TPI and aconitase, increased it by 152%. We present experimental evidence for OS-mediated decrease in TPI activity and increase in activity of glucose-6-phosphate dehydrogenase (PPP enzyme). The findings indicate that increased flux through PPP can be explored to improve TAG accumulation on a large-scale.

Investigating the potential use of an oleaginous bacterium, Rhodococcus opacus PD630, for nano-TiO2 remediation.

The occurrence of titanium dioxide nanoparticles (nTiO2), in the effluents released from wastewater treatment plants, has raised concerns. The fate of nTiO2 and their potential impact on organisms from different ecosystems are widely investigated. For the first time, in this work, we report the responses of an oleaginous bacteria Rhodococcus opacus PD630, belonging to an ecologically important genus Rhodococcus to environmentally relevant concentrations of nTiO2, under dark and UV light conditions. We observed a dose-dependent increase in nTiO2 uptake by the bacteria that reached a maximum of 1.4 mg nTiO2 (g cell)-1 under mid-log UV exposure, corresponding to 97% uptake. The nTiO2 induced oxidative stress in bacteria that increased from 25.1 to a maximum of 100.3, 44.1, and 51.7 µmol .OH (g cell)-1 under dark, continuous, and mid-log UV, respectively. However, nTiO2 did not affect bacterial viability. Further, due to oxidative stress, the triacylglycerol (biodiesel) content from bacteria increased from 30% to a maximum of 54% CDW. Based on our findings, we propose an application of R. opacus PD 630 in nTiO2 remediation due to their high nTiO2 uptake and resistance.