Synergistic broad-spectrum antibacterial activity of Hypoxis hemerocallidea-derived silver nanoparticles and streptomycin against respiratory pathobionts

Abstract

Respiratory tract infections arise due to the introduction of microbes into the airway, disrupting the normal, healthy, complex interdependent microbiome. The selective disruption of this community can be either beneficial or dangerous. Nanoparticles are a potential tool for modifying this population. Coated silver nanoparticles (AgNPs) were synthesized using ethanolic extracts of Hypoxis hemerocallidea (EEHH), a Southern African plant used extensively in traditional medicine and the source of many bioactive secondary metabolites. The room temperature reaction between silver nitrate and EEHH forms largely spherical AgNPs with an average diameter of 6-20 nm. These nanoparticles show similar levels of antibacterial activity as the broad-spectrum antibiotic streptomycin against Bacillus cereus, Streptococcus pneumonia, Escherichia coli, Pseudomonas aeuroginosa, and Moraxella catarrhalis. However, the AgNPs synergistically increase the antibacterial activity of streptomycin when they are applied in combination (30-52%). AgNPs are reiterated to be promising dual-function antibiotics, synergistically enhancing activity while also acting as delivery agents for small molecules.

Figure 1

Structures of major known secondary metabolites in Hypoxis hemerocallidea.

Figure 2

Nanoparticle formation: (A) UV spectrum of the AgNPs formed, (B) surface charge: FTIR spectra of the extract and AgNPs.

Figure 3

Morphology: (A) TEM, (B) Particle size distribution (C) TEM-SAED, (D) EDX.

Figure 4

XRD pattern of green synthesized AgNPs using HH.

Figure 5

Zeta potential and size distribution of the AgNPs. Analyses were conducted in triplicate for three freshly prepared samples, and each run is plotted in the different colours.

Figure 6

The antibacterial activity of (i) EEHH, (ii) AgNPs, (iii) AgNPs and Streptomycin (10 μg/mL for all experiments), (iv) zone of inhibition on agar plate.