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 AgNO 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.