Genomic Analysis of Reduced Susceptibility to Tigecycline in Enterococcus faecium

Abstract

ABSTRACTTigecycline (TIG) is approved for use for the treatment of complicated intra-abdominal infections, skin and skin structure infections, as well as pneumonia. Acquired resistance or reduced susceptibility to TIG has been observed in Gram-negative rods, has seldom been reported in Gram-positive organisms, and has not yet been reported inEnterococcus faecium. Using the serial passage method,in vitromutant AusTig andin vitromutants HMtig1 and HMtig2 with decreased TIG susceptibility (MICs, 0.25 μg/ml) were obtained from strainsE. faeciumAus0004 and HM1070 (MICs, 0.03 μg/ml), respectively. In addition, two vancomycin-resistantE. faeciumclinical isolates (EF16 and EF22) with reduced susceptibility to TIG (MICs, 0.5 and 0.25 μg/ml, respectively) were studied. Compared to the wild-type strains, thein vitromutants also showed an increase in the MICs of other tetracyclines. An efflux mechanism did not seem to be involved in the reduced TIG susceptibility, since the presence of efflux pump inhibitors (reserpine or pantoprazole) did not affect the MICs of TIG. Whole-genome sequencing of AusTig was carried out, and genomic comparison with the Aus0004 genome was performed. Four modifications leading to an amino acid substitution were found. These mutations affected therpsJgene (efau004_00094, coding for the S10 protein of the 30S ribosomal subunit),efau004_01228(encoding a cation transporter),efau004_01636(coding for a hypothetical protein), andefau004_02455(encoding thel-lactate oxidase). The four other strains exhibiting reduced TIG susceptibility were screened for the candidate mutations. This analysis revealed that three of them showed an amino acid substitution in the same region of the RpsJ protein. In this study, we characterized for the first time genetic determinants linked to reduced TIG susceptibility in enterococci.