In vitro antimicrobial activity against reference strains and field isolates of Treponema hyodysenteriae.

The in vitro susceptibilities of eight isolates of Treponema hyodysenteriae from pigs naturally infected with swine dysentery between 1976 and 1983 were determined by an agar dilution technique. Carbadox, olaquindox, tiamulin, metronidazole, furazolidone, and monensin were the most active against these field isolates regardless of the year of recovery. The influence of inoculum size on the MICs against four reference strains of T. hyodysenteriae was studied. Various degrees of activities of ampicillin and lincomycin were found, depending on the inoculum size. The effect of successive in vitro subcultures on the susceptibility of a reference strain of T. hyodysenteriae was examined. The strain resistant to tylosin became susceptible to the drug.