Mutations in the 50S ribosomal subunit of Brachyspira hyodysenteriae associated with altered minimum inhibitory concentrations of pleuromutilins

Brachyspira hyodysenteriae, the causative agent of swine dysentery, is responsible for severe mucohaemorrhagic colitis with considerable financial loss to worldwide swine production. Antimicrobial resistance against macrolides and lincosamides is widespread and the mechanisms are well known. Currently, the most common treatment for swine dysentery is the use of pleuromutilins and resistance to these drugs also is increasingly being reported. Although resistance mechanisms against pleuromutilins are less clear than for other drugs, they seem to involve alterations of the peptidyl transferase centre (PTC), including ribosomal RNA and the ribosomal protein L3. The present study was conducted to examine molecular mechanisms of resistance on a representative set of B. hyodysenteriae field strains with different resistance patterns. In total, we identified 24 single nucleotide polymorphisms (SNPs) in the 23S rRNA gene and genes of the ribosomal proteins L3, L4, L2 and L22. The SNP in the ribosomal protein gene L3 at position 443 led to an amino acid substitution of asparagine (Asn) by serine (Ser) at position 148, significantly associated with MICs for pleuromutilins. Based on this SNP a correct assignment of 71% of the strains with respect to a threshold of >0.625 μg tiamulin/ml was reached. Unexpectedly low MICs in some of the Asn-strains were explained by a second SNP at position 2535 of the 23S rRNA.Our results clearly show the associations between MICs for pleuromutilins and mutations in their binding site. A complete list of SNPs that influence MICs of B. hyodysenteriae strains is needed to enable the interpretation of future molecular susceptibility testing.