Active biosynthesis of gold nanoparticles mediated by obligate methylotrophic bacteria Methylophilus sp

Obligate methylotrophic bacteria are traditional biotechnological producers due to their ability to grow rapidly in extremely simple growth media. The aim of this study was to isolate a highly efficient methylotrophic producer from natural sources specifically for the "green" synthesis of metal nanoparticles. It was demonstrated that a pure bacterial culture of the genus Methylophilus is the most technologically advanced for the active biosynthesis of gold nanoparticles (AuNPs). The optimal conditions for AuNPs biosynthesis were pH 5.5, 25-28 °C, 0.45 mM HAuCl₄ and cells in the active growth phase. Under these conditions, bacteria of the genus Methylophilus synthesized small extracellular gold nanoparticles with a gold core measuring 4-14 nm in 6 h with a yield of up to 15 ± 3%. Continued cultivation under the same conditions for 15 h resulted in a maximum nanoparticle yield of up to 37 ± 6% with a gold core measuring 9-24 nm. After 15 h of active synthesis phase, the formation of reduced gold occurred intracellularly. Extracellular AuNPs exhibited high antioxidant activity and were characterized using FT-IR spectroscopy, SEM, TEM, XRD, and DLS. The nanoparticles shell, at least twice the size of the core, consists primarily of proteins such as porins and flagellin. The increase in the concentration of AuNPs covered with a characteristic shell outside the cells, but in the active phase of their growth, indicates a specific mechanism of their synthesis.