The N-heterocyclic carbine-silver complex SBC3 inhibits Escherichia coli by targeting TDRS

Abstract:

N-Heterocyclic carbine-silver (Ag-NHC) complexes possessing excellent stability, water solubility, and bactericidal activity are antimicrobial candidates with great potential. Objective To study the inhibitory activity and mechanism of a novel Ag-NHC complex 1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene silver (I) acetate (SBC3), synthesized by Matthias Tacke’s team against Escherichia coli. Methods Visible spectrophotometry was employed to examine the antimicrobial activity of SBC3. Transmission microscopy was employed to observe the morphological changes of DHB4 cells post SBC3 treatment. Flow cytometry was performed to detect the effect of SBC3 on the intracellular reactive oxygen species (ROS) content. The 5,5′- dithiobis-(2-nitrobenzoic acid) (DTNB) assay was used to determine the intracellular thioredoxin (Trx) and thioredoxin reductase (TrxR) activities and the content of glutathione (GSH) post SBC3 treatment. Dithiothreitol (DTT), the ROS scavenger, was added to rescue DHB4 from ROS. SBC3-resistant strains (SRSs) were obtained by successive passaging in the laboratory. The obtained strains showed the minimal inhibitory concentrations (MICs) against SBC3 being 24, 32, and 56 μg/mL, respectively, which were 3, 4, and 7 folds of the MIC of WT. The three strains were named SRS3, SRS4, and SRS7 and then used to retest the above indicators. Western blotting was performed to determine the expression levels of Trx1 and S-glutathionylated proteins ( S-PSSG) post SBC3 treatment. Results The MIC values of SBC3 against tested pathogens were 8.0–30.0 μg/mL. The DHB4 cells treated with SBC3 underwent swelling, which was accompanied by contents leakage. SBC3 significantly inhibited the Trx and TrxR activities, reduced the GSH content, and elevated the ROS level in DHB4. SBC3 treatment decreased the Trx and TrxR activities, reduced the GSH content, and down-regulated the expression of S-PSSG in SRS3, SRS4, and SRS7. However, all the above indicators were increased to different extent compared with those in DHB4. Conclusion SBC3 can target the thiol-dependent redox system (TDRS) of E. coli to exert antibacterial effects. This study provides a new idea for the design of SBC3 as a novel antimicrobial agent.