Biofabrication of gold nanoparticles by Shewanella species

Background: Shewanella oneidensis MR-1 (MR-1) and Shewanella xiamenensis BC01 (SXM) are facultative anaerobic bacteria that exhibit outstanding performance in the dissimilatory reduction of metal ions. Shewanella species have been reported to produce metal nanoparticles, but the mechanism and optimization are still not extensively studied and clearly understood. Herein, the effects of pH, biomass, gold ion concentration, and photoinduction are evaluated to optimize gold nanoparticle (Au@NP) production by Shewanella. Results: The highest amount of Au@NPs produced by SXM and MR-1 were 108 and 62 ppm, respectively, at pH 5 when 2.4 g/L biomass was immersed in 300 ppm gold ions and 50 mM lactate under a light intensity of 100 µmol/m²/s. By scanning election microscopy and zeta potential analysis, the proposed mechanism of Au@NP formation was that Shewanella used lactate as electron donors for the Mtr pathway, stimulated by photosensitive proteins resulting in the nucleation of NPs on the cell membrane. Besides, the resting cells retained the ability for biofabrication of nanoparticles for nearly 25 days. Conclusions: The optimal conditions evaluated for Au@NPs production by Shewanella were biomass, pH, ions concentration, and photoinduction. To the best of our knowledge, this is the first attempt to explore a two-step mechanism for Au@NPs formation in Shewanella. First, the HAuCl₄ solution reacted with sodium lactate to form metallic gold ions. Second, the metallic gold ions were adsorbed onto the outer membrane of cell, and the formation of Au@NPs at the surface was triggered. Shewanella-based Au@NPs production could be a potential ecofriendly solution for the recovery of Au ions from secondary resources like industrial waste.[Figure not available: see fulltext.].