Macrolide resistance due to (55).

Unlabelled: Antimicrobial resistance (AMR) is a global threat. The identification and characterization of novel resistance genes is integral to AMR surveillance. The (55) gene was originally identified through whole genome sequencing of macrolide-resistant strains of . The gene was annotated as a ribosomal methyltransferase, but its role as a determinant of macrolide resistance was not formally demonstrated. Three (55) alleles have now been documented. The plasmid-borne (55), the transposon-associated (55), and the chromosomal encoded (55) exhibit ≈82% amino acid sequence identity. Here, we confirm that, when expressed from plasmids in a macrolide-susceptible strain of , all three (55) variants confer resistance to azithromycin and clarithromycin.

Importance: Macrolide antibiotics are often the only oral treatment option for infections with rapidly growing mycobacteria such as and . We previously identified three variants of a newly predicted macrolide resistance gene, (55), in , including the first case of a plasmid-mediated macrolide resistance in mycobacteria. The present study provides experimental evidence that the three (55) variants confer macrolide resistance and that each variant is unique in the degree to which it reduces susceptibility to clinically relevant macrolides.