Antimicrobial susceptibility of U.S. porcine Brachyspira isolates and genetic diversity of B. hyodysenteriae by multilocus sequence typing.

Swine dysentery, caused by and the newly recognized in grower-finisher pigs, is a substantial economic burden in many swine-rearing countries. Antimicrobial therapy is the only commercially available measure to control and prevent -related colitis. However, data on antimicrobial susceptibility trends and genetic diversity of species from North America is limited. We evaluated the antimicrobial susceptibility profiles of U.S. isolates recovered between 2013 and 2022 to tiamulin, tylvalosin, lincomycin, doxycycline, bacitracin, and tylosin. In addition, we performed multilocus sequence typing (MLST) on 64 isolates. Overall, no distinct alterations in the susceptibility patterns over time were observed among species. However, resistance to the commonly used antimicrobials was seen sporadically with a higher resistance frequency to tylosin compared to other tested drugs. was more susceptible to the tested drugs than and . MLST revealed 16 different sequence types (STs) among the 64 isolates tested, of which 5 STs were previously known, whereas 11 were novel. Most isolates belonged to the known STs: ST93 ( = 32) and ST107 ( = 13). Our findings indicate an overall low prevalence of resistance to clinically important antimicrobials other than tylosin and bacitracin, and high genetic diversity among the clinical isolates from pigs in the United States during the past decade. Further molecular, epidemiologic, and surveillance studies are needed to better understand the infection dynamics of on swine farms and to help develop effective control measures.