Zinc Oxide Nanocomposites-Extracellular Synthesis, Physicochemical Characterization and Antibacterial Potential

Abstract

This research presents, for the first time, the potential of the Lactobacillus paracasei LC20 isolated from sweet whey as a novel, effective and accessible source for post-cultured ZnO nanocomposites synthesis. The obtained nanocomposites were subjected to comprehensive characterization by a broad spectrum of instrumental techniques. Results of spectroscopic and microscopic analysis confirmed the hexagonal crystalline structure of ZnO in the nanometer size. The dispersion stability of the obtained nanocomposites was determined based on the zeta potential (ZP) measurements-the average ZP value was found to be -29.15 ± 1.05 mV in the 7-9 pH range. The ZnO nanocomposites (NCs) demonstrated thermal stability up to 130 °C based on the results of thermogravimetric TGA/DTG) analysis. The organic deposit on the nanoparticle surface was recorded by spectroscopic analysis in the infrared range (FT-IR). Results of the spectrometric study exhibited nanostructure-assisted laser desorption/ionization effects and also pointed out the presence of organic deposits and, what is more, allowed us to identify the specific amino acids and peptides present on the ZnO NCs surfaces. In this context, mass spectrometry (MS) data confirmed the nano-ZnO formation mechanism. Moreover, fluorescence data showed an increase in fluorescence signal in the presence of nanocomposites designed for potential use as, e.g., biosensors. Despite ZnO NCs' luminescent properties, they can also act as promising antiseptic agents against clinically relevant pathogens. Therefore, a pilot study on the antibacterial activity of biologically synthesized ZnO NCs was carried out against four strains (Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae and Pseudomonas aeruginosa) by using MIC (minimal inhibitory concentration). Additionally, the colony forming units (CFU) assay was performed and quantified for all bacterial cells as the percentage of viable cells in comparison to a control sample (untreated culture) The nanocomposites were effective among three pathogens with MIC values in the range of 86.25-172.5 μg/mL and showed potential as a new type of, e.g., medical path or ointment formulation.

Keywords: Lactobacillus paracasei; antimicrobial activity; extracellular synthesis; mechanism of formation; nanocomposites; organic deposit; zinc oxide.

Figure 1

SEM micrograph (A), TEM micrograph (B), EDX spectra (C) and selected area (electron) diffraction (SAED) (D) of biologically synthesized ZnO NCs.

Figure 2

XRD pattern of biologically synthesized (A), reference ZnO NCs (B) and control (C).

Figure 3

FT-IR spectra for biologically synthesized ZnO NCs in the υ = 400–4000  cm⁻¹ range; υ (cm⁻¹): 1: 1600–1700, 2: 1400, 3: 1100–1200, 4: 800–900, 5: 700–800, 6: 500–600.

Figure 4

The molecular fingerprint of biologically synthesized ZnO NCs (A); the MS/MS spectra for specific signals (B–G).

Figure 5

The proposed mechanism of ZnO NC formation.

Figure 6

Fluorescence spectra of water (A), zinc nitrate (B), biologically synthesized ZnO NCs (C).

Figure 7

Zeta potential in the 2–10 pH range (A) and thermogravimetric TG/DTG curves (B) for biologically synthesized ZnO NCs.

Figure 8

Quantification of bacterial cell viability (%) at different concentrations of ZnO NCs and ampicillin.