Synergistic antibacterial and antibiofilm activity of silver nanoparticles biosynthesized by lignin-degrading fungus

Background The fabrication of silver nanoparticles (Ag-NPs) through green chemistry is an emerging area in the field of medical nanotechnology. Ag-NPs were fabricated by enzymatic reduction of AgNO 3 using two lignin-degrading fungus Aspergillus flavus (AfAg-NPs) and Emericella nidulans (EnAg-NPs). The prepared Ag-NPs were characterized by different spectroscopic techniques. Antibacterial activity of prepared Ag-NPs was demonstrated against selected Gram negative ( Escherichia coli and Pseudomonas aeruginosa) and Gram positive ( Staphylococcus aureus) bacteria in the term of minimum bactericidal concentration (MBC) and susceptibility constant (Z). The synergistic antibacterial activity of Ag-NPs with four conventional antibiotics was also determined by the fractional inhibitory concentration index (FICI) using the checkerboard microdilution method. The antibiofilm potential of Ag-NPs was also tested. Results The plasmon surface resonance of biosynthesized Ag-NPs shows its characteristic peaks at UV and visible region (~450 and 280 nm). Fourier transform infrared spectrometer (FTIR) analysis confirms the nature of the capping agents as protein (enzyme) and indicates the role of protein (enzyme) in reduction of silver ions. The average particle size and charge of synthesized Ag-NPs was ~100 nm and ~−20 mV, respectively. X-ray diffraction (XRD) and TEM analysis confirmed the purity, shape, and size (quasi-spherical, hexagonal, and triangular) of Ag-NPs. Energy-dispersive X-ray spectroscopy (EDX) data validate the biological synthesis of Ag-NPs. Low MBC and high susceptibility constant indicate the high antimicrobial strength of biosynthesized Ag-NPs. The antibacterial analysis demonstrates the synergistic antimicrobial activity of Ag-NPs with antibiotics. This study also shows that biosynthesized Ag-NPs have ability to inhibit the biofilm formation by 80–90 %. Conclusion The Aspergillus flavus and Emericella nidulans -mediated biosynthesized Ag-NPs have significant antimicrobial activity and demonstrate synergistic effect in combination with antibiotics. It suggests that nanoparticles can be effectively used in combination with antibiotics to improve the efficacy of antibiotics against pathogenic microbes. The substantial antibiofilm efficiency of biosynthesized Ag-NPs would also be helpful against sensitive and multidrug-resistant strains.