Antimicrobial susceptibility testing of Serpulina hyodysenteriae

The macrobroth dilution technique was used to test the in-vitro effectiveness of 4 commonly used antimicrobial agents against 23 Australian isolates and 7 overseas strains of Serpulina hyodysenteriae. Minimum inhibitory concentrations and minimum bactericidal concentrations were determined. The growth of 90% of isolates was inhibited by dimetridazole at a concentration of 4 micrograms/mL, and by tiamulin at 8 micrograms/mL. Australian isolates resistant to both antimicrobial agents were identified. Lincomycin was less effective than these antimicrobial agents, with 90% of isolates requiring a concentration of 128 micrograms/mL for inhibition of growth, and 54% being susceptible at 64 micrograms/mL. Tylosin did not prevent the growth of the majority of S hyodysenteriae isolates tested, and 90% were resistant to concentrations of > or = 128 micrograms/mL. Resistant isolates came from different geographical areas. Resistance was not related to overall genetic background of the spirochaetes, and was not correlated with the presence of plasmids or the serogroup of the isolates.