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.