Effect of Green-Synthesized Zinc Oxide Nanoparticles and Their Praseodymium-Doped Variants on Bacteria with Different Antioxidant Protection

Nanotechnology has emerged as a promising option in combating the worsening situation with antibiotic resistance. We studied the antimicrobial effectiveness of four types of green synthesized zinc oxide nanoparticles (ZnO-NPs), obtained via Pluronic-assisted co-precipitation by lavender and thyme essential oils and their praseodymium-doped variants. Resazurin Microtiter Assay was applied to a panel of Gram-positive and Gram-negative bacteria from Risk 1 and 2 groups and the ESKAPE group. In relation to the pro-oxidative features of the ZnO-NPs, the production of superoxide dismutase (SOD) and catalase (CAT) in the tested microorganisms was also investigated, as these enzymes are important participants in the antioxidant defense of the bacterial cell and are considered virulence factors. We hypothesized that the sensitivity of microorganisms to the action of ZnO-NPs is related to their innate levels of antioxidant enzyme activity. The results showed that all types of studied ZnO-NPs had an antibacterial effect against the entire panel of tested strains, but with different potencies. The strongest effect was found for Arthrobacter nicotianae, Oerskovia paurometabola, Bacillus subtilis, and Escherichia coli. Less inhibition was observed for bacteria from Risk group 2 maybe due to their better antioxidant protection, especially for Pseudomonas aeruginosa. Praseodymium doping contributed to enhancing the bactericidal effect. A correlation between susceptibility of bacteria to ZnO-NPs and their antioxidant enzyme activity was observed.