Detection of Tylosin Resistance in Fusobacterium necrophorum subspecies necrophorum

In-feed tylosin, a macrolide, is widely used to prevent liver abscessation in feedlot cattle by repressing growth of ruminal Fusobacterium necrophorum. Although tylosin has been used for almost five decades, no resistant F. necrophorum subsp. necrophorum strain has ever been isolated. Here, we report two strains (FN37 and FN38) previously isolated from abscessed livers containing several antibiotic resistance genes: cfr(C), tet(O), ant(6)-Ia, and erm(B), the latter of which confers resistance to macrolides via modification of the ribosome. To evaluate if erm(B) conferred a phenotypic advantage, four strains (deposited strain ATCC 25286, ruminal isolate FNC, and abscess isolates FN37 and FN38) were tested for their responses to tylosin. The two erm(B)-harboring strains showed resistance at concentrations commonly found within the ruminal compartment under current dosing guidelines, and in the case of FN38, up to 100 μg/mL tylosin was tolerated. Tylosin susceptibility varied depending on the growth phase (stationary vs logarithmic) and preconditioning (growth in medium containing tylosin at a concentration of 1 μg/mL) of the inoculum in all four strains, but the two harboring the erm(B) gene demonstrated robust resistance. This discovery along with whole genome sequencing and preliminary annotation indicates horizontal gene transfer and acquisition of resistance genes, highlighting the need to revisit antimicrobial strategies for the feedlot cattle industry.