Spontaneous mutation in the AgrRS two-component regulatory system of Cupriavidus metallidurans results in enhanced silver resistance via a novel mechanism involving nanoparticle formation

Background: The uncontrolled and widespread use of (nano)silver compounds has led to the increased release of these compounds into the environment, raising concerns about their negative impact on ecosystems. Concomitantly, silver resistance determinants are widely spread among environmental and clinically relevant bacteria although the underlying mechanisms are not yet fully understood. Results: In this study, we show that Cupriavidus metallidurans is able to adapt to toxic silver concentrations and explicate the genetic circuit responsible for this adaptation. None of the known silver resistant determinants present in C. metallidurans are involved in the adapted response. Instead, increased silver resistance is achieved by the concerted action of a two-component system AgrR-AgrS, previously not associated with metal resistance, and two intrinsically disordered proteins PrsQ1 and PrsQ2. Both belong to an unique group of small, uncharacterized, extracellular proteins restricted to the genera Cupriavidus and Ralstonia. This system seems to be much more efficient as it gives C. metallidurans the ability to withstand much higher silver concentrations. The latter could be facilitated by the accumulation of silver ions and the formation of silver nanoparticles. Conclusions: Detailed knowledge and exploitation of this protein family could result in novel routes for metal nanoparticle formation and metal processing relevant for biotechnical and biomedical applications. Comparison of wild-type strains with evolved strains resulting from a lab evolution experiment Spontaneous silver resistant mutants of C. metallidurans CH34, its plasmidless derivative AE104 and C. metallidurans NA4 were generated by exposure to a toxic concentration of AgNO3. To this end, CH34S1 and CH34S2 were selected on LB agar supplemented with respectively 0.5 and 2 mM AgNO3, AE104S and NA4S were generated in MM284 supplemented with respectively 3 uM and 8 uM AgNO3.