Synergistic activity of silver nanoparticles and antibiotics: apramycin against Escherichia coli

The combination of silver nanoparticles and conventional antibiotics could be a promising alternative strategy to fight against antimicrobial resistance. The occurrence of synergistic bactericidal activity between these combinations is supported by a large number of studies, but there is still a great lack of information about the action mechanisms. In this study, a direct analytical method for the detection and quantification of silver content in cells based on single cell inductively coupled plasma mass spectrometry (SC-ICP-MS), in combination with microscopy and microbiological techniques, has been used. Quantification of silver accumulated in Escherichia coli bacteria exposed to combinations of silver(i) or silver nanoparticles and apramycin can help elucidate the synergistic mechanisms of silver-antibiotic combinations. The combination of silver with apramycin resulted in the occurrence of synergistic effects, allowing the reduction of silver(i), silver nanoparticle and apramycin concentrations (from 4- to 16-fold) while preserving the individual bactericidal effects of each antimicrobial. Severe damage in bacteria walls, including double membrane rupture and cytoplasm leakage, was observed when the combination of apramycin and silver was used. In general, the presence of apramycin has promoted silver uptake by bacteria.