Rapid biological synthesis of silver nanoparticles and their enhanced antibacterial effects against Escherichia fergusonii and Streptococcus mutans

Emergence of antibiotic resistance has become an increasingly important public health issue. Although several new antibiotics have been developed in the last few decades, none of them show improved activity against multidrug-resistant bacteria. Silver nanoparticles (AgNPs) have long been known for their broad-spectrum antibacterial effects. The development of a rapid, dependable, simple, cost-effective, biocompatible, and environmentally friendly method to synthesize nanoparticles is an essential aspect of current biomedical research. This paper describes the extracellular biochemical synthesis of AgNPs using supernatants from Bacillus cereus cultures and characterization of the synthesized AgNPs, using several analytical techniques. The nanoparticles showed a maximum absorbance at 420 nm in ultraviolet–visible spectra. Particle size analysis by dynamic light scattering and transmission electron microscopy revealed the formation of homogeneous and well-dispersed nanoparticles with an average size of 10 nm. We investigated the dose-dependent antibacterial activity of AgNPs against Escherichia fergusonii and Streptococcus mutans. In addition, the efficiency of AgNPs with various broad-spectrum antibiotics against these test strains was evaluated. The results show that the combination of antibiotics with AgNPs has significant antimicrobial effects. The greatest enhancement was observed with gentamycin and vancomycin against E. fergusonii and S. mutans, respectively. This work supports that AgNPs can be used to enhance the activity of existing antibiotics against Gram-negative and Gram-positive bacteria. Keywords: Bacillus cereus, Escherichia fergusonii, Streptococcus mutans, Silver nanoparticles, Antibiotics, Sub-lethal concentratio