Effects of Culturing Factors and Reaction Conditions on the Synthesis of Gold Nanoparticles by Four Enterobacterial Species

Introduction Synthesis of gold nanoparticles (AuNPs) using enterobacteria confers many advantages, but the efficiency depends on the bacterial strains and culture conditions. Therefore, the selection of the most effective strains and the optimization of the culturing factors and reaction conditions are required. Methods Gold resistance and AuNP synthesis were evaluated across 105 enterobacterial strains. The AuNPs synthesized by the four selected strains were characterized using scanning electron microscopy (SEM). AuNP synthesis was improved by optimizing several parameters, including culture media, inducer concentrations, aeration, cultivation times, reaction temperatures, and incubation times. Results Fifty-six strains synthesized AuNPs at concentrations ranging from 7.56 ± 0.28 to 77.92 ± 3.92 μg/mL. The AuNPs synthesized by the four selected strains, including Citrobacter freundii ENTSF 1-3, Enterobacter cloacae ENTSF 8-1, Hafnia alvei ENTSF 15-1, and Morganella morganii SFTCBS1, appeared spherical to slightly polyhedral, with average sizes ranging from 22 to 28 nm. Lennox Luria-Bertani (LB) medium and static conditions were favourable culturing parameters for AuNP synthesis by all four strains. The most favourable reaction conditions varied among the four strains as follows: C. freundii ENTSF 1-3 (at 37°C for 72 h and 120 h); E. cloacae ENTSF 8-1 (at 55°C for 72 h and 120 h); H. alvei ENTSF 15-1 (at 37°C for 72 h and 120 h, as well as at 55°C for 24 h and 48 h); and M. morganii SFTCBS1 (at 55°C for 120 h). Discussion AuNP synthesis by the four selected strains offers several advantages, including a simple growth medium, a short cultivation time, a rapid reaction, convenient product harvesting, stable AuNPs, and no requirement for extra equipment. Conclusion This study determined the most suitable cultivation and reaction conditions to enhance the yield of AuNPs.