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.
Highlights
The production of metal oxide nanomaterials such as zinc oxide nanocomposites (ZnO NCs) is an emerging and currently researched subject in nanotechnology [1,2]
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 The nanocomposites were effective among three pathogens with Minimal Inhibitory Concentration (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
The molecular identification of bacterial strain was performed by MALDI-TOF-MS identification by the ultrafleXtreme mass spectrometer (Bruker Daltonics, Hamburg, Germany) using formic acid-acetonitrile extraction and BioTyper identification [26]
Summary
The production of metal oxide nanomaterials such as zinc oxide nanocomposites (ZnO NCs) is an emerging and currently researched subject in nanotechnology [1,2]. Biosynthesis of zinc oxide can be carried out with various biological materials including bacteria, fungi and plant extracts [3,4,5]. There is limited evidence of effective ZnO nanoparticle formation using plants, whereas the adoption of a microbiological approach for this purpose has still not been sufficiently described. Intracellular production involves bacteria biomass for the nanocomposite formation, while the extracellular approach, known as the post-cultured method, excludes microbial cells and uses a supernatant rich in biologically active compounds (e.g., enzymes or metabolites) [6]. As has been highlighted by literature data [7,8,9], the post-cultured method (extracellular synthesis) has its advantages, such as lower cost, simpler downstream processing (e.g., nanomaterial separation and purification processes) and possibility to reuse the bacterial cultures
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