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

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.

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.

UCN99 exhibited a constitutive macrolide/lincosamide/streptogramin B (MLS_B) 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 MLS_B 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.

This is the first description of a novel MLS_B resistance determinant, named erm(47). The prevalence of this gene among Gram-positive cocci must be further investigated to determine its clinical significance.