Genomic alterations and altered cell wall structure in daptomycin-resistant Enterococcus faecium revealed by comparison of isolates before and after exposure to daptomycin.

Daptomycin is a cyclic lipopeptide used in the treatment of vancomycin-resistant Enterococcus faecium (VREfm). However, the development of daptomycin-resistant VREfm challenges the treatment of nosocomial VREfm infections. Resistance mechanisms of daptomycin are not fully understood. Here, we analyzed the genomic changes leading to a daptomycin-susceptible VREfm isolate becoming resistant after 50 days of daptomycin and linezolid combination therapy. A total of seven isogenic VREfm isolates from the same patient (daptomycin-susceptible and daptomycin-resistant) were analyzed using Illumina whole genome sequencing, and two isolates were further characterized with Nanopore sequencing. One nonsynonymous SNP in the rpoC gene previously shown to harbor mutations in daptomycin-resistant VREfm was identified in the daptomycin-resistant isolates. Whole genome comparative analysis identified the loss of a 46.5 kb fragment, duplication of a 29.7 kb fragment, and integration of two plasmids upon acquisition of daptomycin resistance. Transmission electron microscopy showed similar alterations in cell morphology and cell wall structure as have previously been described in daptomycin-resistant E. faecalis.