ANTIMICROBIAL SUSCEPTIBILITY TESTING OF SERPULINA-HYODYSENTERIAE

The macrobroth dilution technique was used to test the in-vitro effect iveness of 4 commonly used antimicrobial agents against 23 Australian isolates and 7 overseas strains of Serpulina hyodysenteriae. Minimum i nhibitory concentrations and minimum bactericidal concentrations were determined. The growth of 90% of isolates was inhibited by dimetridazo le at a concentration of 4 mug/mL, and by tiamulin at 8 mug/mL. Austra lian 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 mug/mL for inhibition o f growth, and 54% being susceptible at 64 mug/mL. Tylosin did not prev ent the growth of the majority of S hyodysenteriae isolates tested, an d 90% were resistant to concentrations of greater-than-or-equal-to 128 mug/mL. Resistant isolates came from different geographical areas. Re sistance was not related to overall genetic background of the spirocha etes, and was not correlated with the presence of plasmids or the sero group of the isolates.