Unnatural amino acid-mediated synthesis of silver nanoparticles and their antifungal activity against Candida species

Abstract

In this study, the biocompatible unnatural amino acid, 3,4-dihydroxy-l-phenylalanine (l-dopa), which is used in protein engineering, was employed in the facile synthesis of silver nanoparticles (AgNPs). The surface plasmon resonance (SPR) band of the UV–Vis spectrum at 406 nm demonstrates the possibility of formation of smaller nanoparticles; the symmetrical shape of the band demonstrates a narrow size distribution of AgNPs, the formation of AgNPs, and the face-centered cubic (fcc) crystalline structure of nanoparticles was confirmed by X-ray diffraction (XRD). Additionally, transmission electron microscopic (TEM) images revealed that these particles were spherical in shape with diameters of 2.7–12.2 nm (average = 8.7 nm). These nanoparticles exhibited antifungal activity against both planktonic and biofilm yeast cells of Candida albicans and C. dubliniensis. The minimum inhibitory concentrations (MICs) and minimum fungicidal concentrations (MFCs) were determined by microdilution assays. C. albicans were shown to be less susceptible than C. dubliniensis to AgNPs based on the MIC (ranging from 7.8 to 15.6 µg ml−1) and MFC (ranging from 31.2 to 62.5 µg ml−1). With regard to biomass quantification, AgNPs did not induce a significant reduction of the biomass of Candida species; however, treatment of biofilm with 500 µg/ml of AgNPs induced a 2.99−log10 (P < 0.001) and 3.53−log10 (P < 0.001) significant reduction in the number of culturable cells of CFUs when compared to control samples of C. albicans and C. dubliniensis, respectively. Thus, AgNPs-based antifungal agents would be an effective alternative to conventional drugs to overcome drug resistance in Candida-associated infections.