Biosynthesis of silver nanoparticles from silver(i) reduction by the periplasmic nitrate reductase c-type cytochrome subunit NapC in a silver-resistant E. coli

Abstract

The synthesis of metal nanoparticles by using bacteria is of growing interest in nanobiotechnology as well as in the study of microbial metal metabolism. Some silver-resistant bacteria can produce considerable amounts of silver particles when exposed to silver salts at high concentration but the mechanism of biosynthesis is unknown. In this work, an Escherichia coli strain that carries chromosomally encoded silver resistance determinants has been shown to produce silver nanoparticles in the periplasmic space when it was exposed to Ag(I) salts, providing a prototypical model for studying the biosynthesis of silver nanoparticles. The synthesized silver nanoparticles are in the form of a zero-valent metallic silver lattice, and the production of which was observed to be favorable under anaerobic conditions, suggestive of the biological reduction of Ag⁺ ions. As the microbial c-type cytochromes are known to mediate respiratory reduction of metal ions, their role in the biosynthesis of silver nanoparticles was examined. A deletion mutant of the cytoplasmic membrane-anchored tetra-heme c-type cytochrome subunit of periplasmic nitrate reductase (NapC) showed markedly reduced production of silver nanoparticles. On the other hand, re-introduction of the NapC could recover the biosynthesis of the silver nanoparticles. This study has identified a molecular mechanism of biosynthesis of silver nanoparticles involving c-type cytochromes, having implications in the bioenvironmental process of mineralization and the synthetic biology of metal nano-materials.