Antimicrobial activity of phytofabricated silver nanoparticles using Carica papaya L. against Gram-negative bacteria

Abstract

Background and aim: Antibiotic resistance, especially in Gram-negative bacteria, is a major public health risk affecting all industries requiring the use of antibiotics, including agriculture and animal breeding. This study aimed to use papaya extracts to synthesize silver nanoparticles (AgNPs) and evaluate their antimicrobial activity against various Gram-negative bacteria.

Materials and methods: Silver nanoparticles were synthesized from the aqueous extracts of papaya seed, root, and bark, with AgNO₃ used as a reducing agent. The phytofabricated AgNPs were analyzed by ultraviolet-visible absorbance, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy, and photon cross-correlation spectroscopy (PCCS). The disc-diffusion method was used to perform antibacterial analysis, and the minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations were determined. We also investigated the antibiofilm activity of AgNPs and attempted to elucidate the potential mechanism of action on Escherichia coli ATCC 25922.

Results: Phytofabrication of AgNPs was successful with papaya root (PR-AgNPs) and papaya seed (PS-AgNPs), but not with papaya bark. Silver nanoparticles using papaya root and PS-AgNPs were both cubic and showed maximum absorbances of 2.6 and 0.3 AUs at 411.6 and 416.8 nm wavelengths and average hydrodynamic diameters X50 of 59.46 ± 7.03 and 66.57 ± 8.89 nm, respectively. The Ag in both AgNPs was confirmed by X-ray fluorescence by a distinctive peak in the spectrum at the silver Kα line of 22.105 keV. Both AgNPs exhibited broad-spectrum antimicrobial and antibiofilm activity against all Gram-negative bacteria, and PR-AgNPs were slightly better than AgNPs-PS. The MIC ranged from 16 μg/mL-128 μg/mL and 16 μg/mL-64 μg/mL, respectively, for PS-AgNPs and PR-AgNPs. The elucidation of the mechanism of action revealed interference with E. coli ATCC 25922 growth kinetics and inhibition of H⁺-ATPase proton pumps.

Conclusion: Papaya seed and root extracts were efficient reducing agents for the biogenic synthesis of AgNPs, with noteworthy antibacterial and antibiofilm activities. Future studies should be conducted to identify the phytochemicals and the mechanism involved in AgNPs synthesis.

Keywords: Carica papaya; Gram-negative; antibiotic resistance; biogenic synthesis; silver nanoparticles.

Figure-1

Coloration of the reaction mixture before and 24 h after synthesis of silver nanoparticles with papaya bark, root, and seed extracts.

Figure-2

Ultraviolet–visible spectra of phytofabricated silver nanoparticles using (a) papaya root and (b) papaya seed.

Figure-3

X-ray fluorescence spectra of papaya seed extract (black), solution of green-synthesized silver nanoparticles (AgNPs) using papaya seeds (red), papaya root extract (green), and solution of green-synthesized AgNPs using papaya roots (purple) at the characteristic energy of silver fluorescence - silver Kα line of 22.105 keV. The black and green lines coincide with the X-axis, showing that papaya seed and root extract does not contain Ag.

Figure-4

Hydrodynamic diameter of synthesized silver nanoparticles using (a) papaya root and (b) papaya seed.

Figure-5

X-ray diffraction pattern of phytofabricated silver nanoparticles using papaya root and papaya seed.

Figure-6

Fourier-transform infrared spectroscopy spectra of papaya root and papaya seed dried extracts.

Figure-7

Fourier-transform infrared spectroscopy spectra of phytofabricated silver nanoparticles with the respective extracts used (papaya root and papaya seed).

Figure-8

Principal component analysis analysis of minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and ratio MBC/MIC of phytofabricated AgNPs-PR and AgNPs-PS against selected Gram-negative bacteria. AgNPs-PR=Silver nanoparticles using papaya root, AgNPs-PS=Silver nanoparticles using papaya seed.

Figure-9

Antibiofilm activity of phytofabricated silver nanoparticles using (a) papaya root and (b) papaya seed.

Figure-10

Action of phytofabricated silver nanoparticles using (a) papaya root and (b) papaya seed on Escherichia coli ATCC 25922 growth kinetic. MIC=Minimum inhibitory concentration.

Figure-11

Action of phytofabricated silver nanoparticles using (a) papaya root and (b) papaya seed on Escherichia coli ATCC 25922 H⁺-ATPase-mediated proton pumping. MIC=Minimum inhibitory concentration.