Antimicrobial susceptibility pattern of Brachyspira intermedia isolates from European layers

A broth microdilution method was used to determine the antimicrobial susceptibility of 20 Brachyspira intermedia isolates obtained from different layer flocks in Belgium and The Netherlands between 2008 and 2010. The antimicrobial agents used were tylosin, tilmicosin, tiamulin, valnemulin, doxycycline, and lincomycin. The minimal inhibitory concentration (MIC) distribution patterns of tylosin, tilmicosin, lincomycin, and doxycycline were bimodal, demonstrating acquired resistance against doxycycline in three strains, against the macrolides in two strains, and against lincomycin in one strain. The MICs of tiamulin and valnemulin showed a monomodal distribution, but with tailing toward the higher MIC values, possibly suggesting low-level acquired resistance in six isolates. Sequencing revealed a G1058C mutation in the 16S rRNA gene in all doxycycline-resistant strains. The strain resistant to tylosin, tilmicosin, and lincomycin had an A2058T mutation in the 23S rRNA gene.