Novel chromosome-encoded erm(47) determinant responsible for constitutive MLSB resistance in Helcococcus kunzii.

OBJECTIVES: The aim of the study was to identify the determinant responsible for erythromycin resistance in Helcococcus kunzii clinical isolate UCN99 and to characterize the genetic support and environment of this novel gene. METHODS: MICs were determined using the broth microdilution method according to EUCAST guidelines. The entire genome sequence of H. kunzii UCN99 was determined using a 454/Roche GS Junior sequencer. The fragment encompassing the new resistance gene and its own promoter was cloned into the pAT29 shuttle vector and the recombinant plasmid pAT29Ωerm(47) was expressed in both Staphylococcus aureus and Streptococcus agalactiae. The transcription start site (TSS) was experimentally determined by 5' RACE-PCR. RESULTS: UCN99 exhibited a constitutive macrolide/lincosamide/streptogramin B (MLSB) resistance phenotype, suggesting the presence of an Erm protein. WGS allowed the identification of a novel gene, named erm(47), encoding a protein sharing 44%-48% amino acid identity with known Erm methylases. In both S. aureus and S. agalactiae, the introduction of pAT29Ωerm(47) conferred a significant increase (≥16-fold) in MICs of all macrolides and lincosamides tested, as well as a 4-fold increase in MICs of quinupristin (streptogramin B), confirming the MLSB resistance. The TSS identification revealed the presence of a short leader peptide, potentially implicated in a translational attenuation mechanism. It was also demonstrated that erm(47) was harboured by a 81 kb genomic island integrated into a chromosomal gene. CONCLUSIONS: This is the first description of a novel MLSB resistance determinant, named erm(47). The prevalence of this gene among Gram-positive cocci must be further investigated to determine its clinical significance.

Genomic analysis of reduced susceptibility to tigecycline in Enterococcus faecium.

Tigecycline (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 in Enterococcus faecium. Using the serial passage method, in vitro mutant AusTig and in vitro mutants HMtig1 and HMtig2 with decreased TIG susceptibility (MICs, 0.25 µg/ml) were obtained from strains E. faecium Aus0004 and HM1070 (MICs, 0.03 µg/ml), respectively. In addition, two vancomycin-resistant E. faecium clinical 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, the in vitro mutants 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 the rpsJ gene (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), and efau004_02455 (encoding the l-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.