Modulation of antibiotic resistance and induction of a stress response in Pseudomonas aeruginosa by silver nanoparticles

Abstract

The objective of this study was to characterize the effects of silver nanoparticles on Pseudomonas aeruginosa. Their interactions with several conventional antibiotics and ability to induce a stress response were examined. Interactions between silver nanoparticles (AgNPs) and antibiotics against free-living cells and biofilm of P. aeruginosa were studied using the chequerboard method and time-kill assays. The ability of AgNPs to induce a stress response was determined by evaluation of cellular levels of the DnaK and HtpG chaperones using SDS-PAGE and Western blot analysis. Synergistic activity against free-living P. aeruginosa between AgNPs and ampicillin, streptomycin, rifampicin and tetracycline, but not oxacillin, ciprofloxacin, meropenem or ceftazidime, was demonstrated by the chequerboard method. No such interactions were observed against P. aeruginosa biofilm. The results of time-kill assays confirmed synergy only for the AgNPs-streptomycin combination. AgNPs induced the expression of chaperone DnaK. No induction of the HtpG chaperone was detected. In conclusion, AgNPs not only display potent bactericidal activity against P. aeruginosa, but also act synergistically with several conventional antibiotics to enhance their effect against free-living bacteria as determined by the chequerboard method. The time-kill assay proved synergy between AgNPs and streptomycin only. The ability of AgNPs to induce the major chaperone protein DnaK may influence bacterial resistance to antimicrobials.

Synergy of Silver Nanoparticles and Aztreonam against Pseudomonas aeruginosa PAO1 Biofilms

“…The authors hypothesized this may potentiate the activity of a broad range of antibiotics against Gram-negative bacteria in different metabolic states, as well as restoring antibiotic susceptibility to resistant bacterial strains. This synergistic effect and the results for the uncapped AgNPs reported by Markowska et al (24), as described above, are consistent with the findings we observed through imaging of preformed P. aeruginosa biofilms. In our study, the selected combination of 10-nm citrate-capped AgNPs and aztreonam caused significant defects in biofilm architecture, membrane permeability, and ultrastructure, in comparison with all other antimicrobial treatments evaluated ( Fig.…”

“…Once again, 10-nm and 20-nm AgNPs demonstrated more-significant reductions in biofilm recovery when combined with lower concentrations of aztreonam than did 40-nm and 60-nm nanoparticles. Next, we used the Bliss independence model (22,37,54) and FIC index (24,39,40) to calculate synergy between AgNPs and aztreonam in the inhibition of P. aeruginosa biofilm biomass ( Fig. 6 and Table 3), and we confirmed that 10 nm was the optimal AgNP size to use in the antimicrobial combination.…”

“…A recent study by Markowska et al described synergistic inhibitory effects against P. aeruginosa ATCC 10145 of mixtures of uncapped AgNPs of different sizes (2 nm to 35 nm) and a range of antibiotics (24). The authors found that their AgNP-antibiotic combinations were highly effective against planktonic cells, but they observed no synergy against P. aeruginosa biofilms.…”

“…The authors concluded that the inability of the uncapped AgNP-antibiotic combinations to effectively inhibit biofilms was primarily due to a lack of antimicrobial exposure. They argued that this lack of exposure was caused by both the inherent protection of the extracellular matrix that accompanies P. aeruginosa biofilm development and aggregation of the uncapped AgNPs, which led to an increase in size and therefore a decrease in ability to penetrate the biofilms (24). A recent study by Morones-Ramirez et al described the effects of silver ions and their synergy with antibiotics to treat a range of infections both in vivo and in vitro (22).…”

“…The antimicrobial effects of silver and silver-containing compounds have been known for decades (13)(14)(15); however, despite their historical use and recent resurgence in consumer products, little is known about their precise mode of antimicrobial action (15)(16)(17)(18)(19)(20). Recent studies have revealed the antimicrobial effects of silver ions and silver nanoparticles (AgNPs) in combination with commonly used antibiotics (21)(22)(23)(24). These findings, although largely derived from use against planktonic bacterial cultures, highlight the finding that antimicrobial synergies exist when silver is combined with traditional treatments, with the combined effects being greater than the sum of the individual effects.…”