Abstract
Staphylococcus aureus is a human pathogen responsible for many antibiotic-resistant infections, for instance burn wound infections, which pose a threat to human life. Exploring possible synergy between various antimicrobial agents, like nanoparticles and plant natural products, may provide new weapons to combat antibiotic resistant pathogens. The objective of this study was to examine the potential of silver nanoparticles (AgNPs) to enhance the antimicrobial activity of selected naphthoquinones (NQs): plumbagin (PL), ramentaceone (RAM), droserone (DR), and 3-chloroplumbagin (3ChPL). We also attempted to elucidate the mechanism by which the AgNPs enhance the antimicrobial activity of NQs. We analyzed the interaction of AgNPs with bacterial membrane and its effect on membrane stability (TEM analysis, staining with SYTO9 and propidium iodide), as well as aggregation of NQs on the surface of nanoparticles (UV-Vis spectroscopy and DLS analysis). Our results demonstrated clearly a synergistic activity of AgNPs and three out of four tested NQs (FBC indexes ≤ 0.375). This resulted in an increase in their combined bactericidal effect toward the S. aureus reference strain and the clinical isolates, which varied in resistance profiles. The synergistic effect (FBC index = 0.375) resulting from combining 3ChPL with silver nitrate used as a control, emphasized the role of the ionic form of silver released from nanoparticles in their bactericidal activity in combination with NQs. The role of membrane damage and AgNPs-NQ interactions in the observed synergy of silver nanoparticles and NQs was also confirmed. Moreover, the described approach, based on the synergistic interaction between the above mentioned agents enables a reduction of their effective doses, thus significantly reducing cytotoxic effect of NQs toward eukaryotic HaCaT cells. Therefore, the present study on the use of a combination of agents (AgNPs-NQs) suggests its potential use as a possible strategy to combat antibiotic-resistant S. aureus.
Highlights
The effort placed on the exploration of new anti-infectious approaches is justified in the near post-antibiotic era that we face (Ventola, 2015)
Noteworthy is the fact that when the bacterial cells were treated with combined AgNPs and NQs, the bactericidal effect was observed at significantly lower concentrations for all of the tested agents
As little is known about the mechanism underlying the synergistic effect of AgNPs-NQs combination, we verified the role of the main mechanistic aspects of the bactericidal activity of AgNPs in the observed synergy when the nanoparticles are used simultaneously with naphthoquinone
Summary
The effort placed on the exploration of new anti-infectious approaches is justified in the near post-antibiotic era that we face (Ventola, 2015). Drug resistance is observed in a growing number of clinical and environmental isolates of bacteria and fungi. The problem of increasing antibiotic resistance concerns most of antibiotic drugs approved to treat infectious diseases (Magiorakos et al, 2012). Staphylococcus aureus is a Gram-positive bacterium belonging to a group of the most troublesome antibiotic resistant pathogens, socalled “ESCAPE” according to Peterson (2009). Most infections caused by S. aureus are associated with strains resistant to β-lactams (and other classes of antibiotics) and most of them are healthcare-acquired like burn wound infections (Tong et al, 2015). The likelihood of anti-infectious treatment failing due to the antibiotic resistance is an increasingly recurring problem.
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