Antibacterial Effect of Silver and Iron Oxide Nanoparticles in Combination with Antibiotics on E. coli K12

Antibiotic resistance is one of the main public health problems. The increase in the occurrence of multi-resistant pathogenic strains of bacteria due to biofilm formation gradually leads to inefficiency of traditional antibiotics. There are many strategies to combat biofilms, such as nanotechnologies. It is extremely important and relevant for nanomedicine to understand how the exposure of metal nanoparticles affects living organisms. The aim was to study the synergistic antibacterial activity of silver (AgNPs) and iron oxide (Fe 3 O 4 NPs) nanoparticles in combination with tetracycline (Tet) and ampicillin (Amp). The tests were performed against the wild strain E. coli K12. Our results suggest that both Fe 3 O 4 NPs and AgNPs form complexes with Amp and Tet. Fe 3 O 4 NPs slow down and AgNPs suppress the growth of E. coli at high concentrations. Dose-dependent inhibition of E. coli K12 growth is observed for AgNPs and Tet-AgNPs. The synergistic antibacterial effect is likely due to enhanced bacterial binding by AgNPs, which is assisted by Tet, but not by Amp. Fe 3 O 4 NPs do not exhibit bacterial growth inhibitory activity, but in combination with Amp, antagonism is observed. Fe 3 O 4 NPs slightly enhance the antibacterial effect of Tet. The antibacterial effect of Amp and Tet decreases when used in combination with AgNPs. Fe 3 O 4 NPs reduce the inhibitory effect of antibiotics on the formation of microcolonies. AgNPs suppress the growth of microcolonies, and in combination with Tet, the effects are enhanced. We hypothesize that the synergistic antibacterial activity correlates with the complex formation between NPs and the antibiotics. We propose pathways which lead to the synergistic effect.