Combined efficacy of biologically synthesized silver nanoparticles and different antibiotics against multidrug-resistant bacteria

Abstract

Biological synthesis of nanoparticles is a growing innovative approach that is relatively cheaper and more environmentally friendly than current physicochemical processes. Among various microorganisms, fungi have been found to be comparatively more efficient in the synthesis of nanomaterials. In this research work, extracellular mycosynthesis of silver nanoparticles (AgNPs) was probed by reacting the precursor salt of silver nitrate (AgNO3) with culture filtrate of Aspergillus flavus. Initially, the mycosynthesis was regularly monitored by ultraviolet-visible spectroscopy, which showed AgNP peaks of around 400-470 nm. X-ray diffraction spectra revealed peaks of different intensities with respect to angle of diffractions (2θ) corresponding to varying configurations of AgNPs. Transmission electron micrographs further confirmed the formation of AgNPs in size ranging from 5-30 nm. Combined and individual antibacterial activities of the five conventional antibiotics and AgNPs were investigated against eight different multidrug-resistant bacterial species using the Kirby-Bauer disk-diffusion method. The decreasing order of antibacterial activity (zone of inhibition in mm) of antibiotics, AgNPs, and their conjugates against bacterial group (average) was; ciprofloxacin + AgNPs (23) . imipenem + AgNPs (21) > gentamycin + AgNPs (19) > vancomycin + AgNPs (16) > AgNPs (15) . imipenem (14) > trimethoprim + AgNPs (14) > ciprofloxacin (13) > gentamycin (11) > vancomycin (4) > trimethoprim (0). Overall, the synergistic effect of antibiotics and nanoparticles resulted in a 0.2-7.0 (average, 2.8) fold-area increase in antibacterial activity, which clearly revealed that nanoparticles can be effectively used in combination with antibiotics in order to improve their efficacy against various pathogenic microbes.

Keywords: Aspergillus flavus; antibacterial agents; mycosynthesis; silver nanoparticles.

Figure 1

Extracellular synthesis of AgNPs at the beginning of the reaction and after 96 hours (completion of reaction). Notes: (A) Culture filtrate: Aspergillus flavus with AgNO₃ solution (0.1 M). (B) Positive control: filtrate without silver salt. (C) Negative control: sterile deionized water with only AgNO₃ solution. Abbreviation: AgNPs, silver nanoparticles.

Figure 2

Ultraviolet-visible spectra of aqueous medium containing cell free culture filtrate and silver salt (0.1 M) and of controls over time.

Figure 3

X-ray diffraction pattern of nanoparticle film on copper surface obtained from culture filtrate of Aspergillus flavus.

Figure 4

Transmission electron micrograph of AgNPs produced by culture filtrate of Aspergillus flavus. Abbreviation: AgNPs, silver nanoparticles.

Figure 5

Zones of inhibition of standard antibiotics and AgNPs against multidrug-resistant microorganisms. Notes: (A) Bacillus; (B) M. luteus; (C) S. aureus; (D) E. faecalis; (E) E. coli; (F) P. aeruginosa; (G) A. baumanii; (H) K. pneumoniae. Abbreviation: AgNPs, silver nanoparticles.