Abstract
Silver nanoparticles offer a possible means of fighting antibacterial resistance. Most of their antibacterial properties are attributed to their silver ions. In the present work, we study the actions of positively charged silver nanoparticles against both methicillin-sensitive Staphylococcus aureus and methicillin-resistant Staphylococcus aureus. We use aberration-corrected transmission electron microscopy to examine the bactericidal effects of silver nanoparticles and the ultrastructural changes in bacteria that are induced by silver nanoparticles. The study revealed that our 1 nm average size silver nanoparticles induced thinning and permeabilization of the cell wall, destabilization of the peptidoglycan layer, and subsequent leakage of intracellular content, causing bacterial cell lysis. We hypothesize that positively charged silver nanoparticles bind to the negatively charged polyanionic backbones of teichoic acids and the related cell wall glycopolymers of bacteria as a first target, consequently stressing the structure and permeability of the cell wall. This hypothesis provides a major mechanism to explain the antibacterial effects of silver nanoparticles on Staphylococcus aureus. Future research should focus on defining the related molecular mechanisms and their importance to the antimicrobial activity of silver nanoparticles.
Highlights
Bacterial infections are a major reason of morbidity and mortality globally [1], and most infections can be attributed to species of the genus Staphylococcus [2]
HAADF-STEM images of treated methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA) cells demonstrate the affinity between silver and osmium that can generate electron-dense particles around the bacterial cell wall, as seen in Figure 4a; specific cases of AgNPs binding to cell walls are shown in Figure 4, Figure 5a and Figure 6
Zeta potential measurements suggest that the positively charged particles are silver nanoparticles with adsorbed ions (Figure 2)
Summary
Bacterial infections are a major reason of morbidity and mortality globally [1], and most infections can be attributed to species of the genus Staphylococcus [2]. WTAs play an important role in antibiotic resistance in MRSA, and they increase bacterial vulnerability to cationic antimicrobials, peptides, ions and metals. The presence of three species in an AgNP solution is important, as we propose a new mechanism for how each of these three species interacts with the bacterial cell [42].
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