Establishment and characterization of silver-resistant Enterococcus faecalis

Enterococcus faecalis is a Gram-positive facultative anaerobe involved in many fatal or refractory infections of humans. Silver, often used as silver ions (Ag⁺) or nanoparticles (AgNPs), is a strong and broad-spectrum antibacterial agent, but E. faecalis shows resistance against it. Despite this, the knowledge about the resistance of E. faecalis against silver is still lacking. In this study, the silver-resistant E. faecalis strains (AgR and ANR E. faecalis) were established through a serial selection method. Their biological and silver-resistant features as well as the Gene Ontology (GO) in comparison with the original E. faecalis were evaluated. The results showed that the silver-resistant E. faecalis could proliferate as original bacteria and had strong resistance against both Ag⁺ and AgNPs. The minimum bactericidal concentrations (MBCs) of AgNO₃ on original, AgR, and ANR E. faecalis were 400 mg/L, 600 mg/L, and 500 mg/L, and the MBCs of AgNPs on these strains were 80 mg/L, 110 mg/L, and 130 mg/L, respectively. GO analysis revealed significant difference (P < 0.05) in gene expressions of biological process (BP), cellular component (CC), and molecular function (MF) among original, AgR, and ANR E. faecalis. These findings provided a significant basis for further understanding and managing the silver-resistance of E. faecalis in infection-control environments. The mechanism behind Ag⁺/AgNPs resistance of E. faecalis needs to be further investigated.