Antimicrobial Susceptibility Test and Detection of LZD Resistance Genes

The susceptibilities of E. faecalis isolates to clinically relevant antimicrobials were tested by the Phoenix 100 automated microbiology system. The minimum inhibitory concentrations (MICs) of LZD and RZD were determined by the agar dilution method according to Clinical and Laboratory Standards Institute (CLSI) guidelines. While no CLSI interpretive criteria existed for RZD against enterococci, the MICs of RZD for the quality control strain ATCC29212 were observed to range from 0.06 to 0.5 mg/L. Therefore, to analyze the distribution of MICs for RZD in these clinical isolates, the MICs were categorized into the following four levels: ≤ 0.125, 0.25, 0.5, and ≥ 1 mg/L. The four copies of the V domain of the 23S rRNA gene, as well as the rplC and rplD genes, were amplified by PCR and sequenced. The cfr, cfr(B), optrA, and poxtA genes were also amplified by PCR. The primers used for PCR are listed in Supplementary Table S1.