Investigation of macrolide resistance genotypes in Mycoplasma bovis isolates from Canadian feedlot cattle

Mycoplasma bovis is associated with bovine respiratory disease (BRD) and chronic pneumonia and polyarthritis syndrome (CPPS), which pose significant welfare and economic concerns. No efficacious vaccines for M. bovis exist; therefore, macrolides are commonly used in feedlots for BRD metaphylaxis. The study assessed the concordance between genotypes and phenotypes for antimicrobial resistance to macrolides. Whole genome sequences of 126 M. bovis isolates derived from 96 feedlot cattle over 12 production years were determined. Antimicrobial susceptibility testing (AST) of five macrolides (gamithromycin, tildipirosin, tilmicosin, tulathromycin and tylosin) was determined using the microbroth dilution method. The AST phenotypes were compared to the genotypes generated from mutations within 23S rRNA and the L4 and L22 ribosomal proteins. Mutations in two domains of the 23S rRNA gene (delta748delta2059 or delta748delta2060) were associated with resistance to tulathromycin, gamithromycin, and tylosin. Non-synonymous mutations were present in L4 and L22 ribosomal proteins in regions proximal to the macrolide binding site. Regardless of genotype, >99 % of all isolates were resistant to tildipirosin and tilmicosin. Generally isolates with combined mutations in macrolide binding sites were resistant to the five macrolides tested.