Brachyspira hyodysenteriae is the aetiological agent of swine
dysentery, a globally distributed disease that causes profound economic loss, impedes the free trade and movement of animals, and has significant impact on pig health.
Infection is generally treated with
antibiotics of which
pleuromutilins, such as
tiamulin, are widely used for this purpose, but reports of resistance worldwide threaten continued effective control. In Brachyspira hyodysenteriae
pleuromutilin resistance has been associated with mutations in chromosomal genes encoding ribosome-associated functions, however the dynamics of resistance acquisition are poorly understood, compromising stewardship efforts to preserve
pleuromutilin effectiveness. In this study we undertook whole genome sequencing (WGS) and phenotypic susceptibility testing of 34 UK field isolates and 3 control strains to investigate
pleuromutilin resistance in Brachyspira hyodysenteriae. Genome-wide association studies identified a new
pleuromutilin resistance gene, tva(A) (
tiamulin valnemulin antibiotic resistance), encoding a predicted ABC-F transporter. In vitro culture of isolates in the presence of inhibitory or sub-inhibitory concentrations of
tiamulin showed that tva(A) confers reduced
pleuromutilin susceptibility that does not lead to clinical resistance but facilitates the development of higher-level resistance via mutations in genes encoding ribosome-associated functions. Genome sequencing of
antibiotic-exposed isolates identified both new and previously described mutations in chromosomal genes associated with reduced
pleuromutilin susceptibility, including the
23S rRNA gene and rplC, which encodes the
L3 ribosomal protein. Interesting three
antibiotic-exposed isolates harboured mutations in fusA, encoding
Elongation Factor G, a gene not previously associated with
pleuromutilin resistance. A longitudinal molecular epidemiological examination of two episodes of swine
dysentery at the same farm indicated that tva(A) contributed to development of
tiamulin resistance in vivo in a manner consistent with that seen experimentally in vitro. The in vitro studies further showed that tva(A) broadened the mutant selection window and raised the mutant prevention concentration above reported in vivo
antibiotic concentrations obtained when administered at certain doses. We show how the identification and characterisation of tva(A), a new marker for
pleuromutilin resistance, provides evidence to inform treatment regimes and reduce the development of resistance to this class of highly important
antimicrobial agents.