TABLE 2

Antimicrobial susceptibilities of 101 isolates of R. equi to azithromycin, clarithromycin, erythromycin, rifampin, gentamicin, and doxycycline, determined by broth macrodilution, Etest, and disk diffusiona

AntimicrobialTestMIC90 (μg/ml)MIC50 (μg/ml)Range (μg/ml)S (%)I (%)R (%)
AzithromycinBroth1281(0.03–256)69.33.027.7
Etest321.5(0.016–>256)74.31.024.8
DiskNANANA73.32.024.8
ClarithromycinBroth160.125(<0.03–128)75.23.021.8
Etest160.125(0.03–>256)76.22.021.8
DiskNANANA77.22.020.8
ErythromycinBroth640.5(0.06–256)74.3025.7c
Etest160.38(0.09–>256)76.05.019.0
DiskNANANA74.32.023.8
RifampinBroth320.25(<0.03–>256)72.32.025.7d
Etest320.25(0.012–>32)70.3029.7
DiskNANANA68.3031.7
GentamicinBroth10.5(<0.03–>256)93.1b4.03.0
Etest1.51(<0.016–>256)99.001.0
DiskNANANA99.001.0
DoxycyclineBroth21(0.03–4)99.001.0
Etest1.50.5(0.016–32)99.001.0
DiskNANANA98.002.0
  • a S, susceptible; I, intermediate; R, resistant. NA, not applicable.

  • b Significant difference between the percentage of susceptibility obtained by broth macrodilution and that obtained with the Etest or disk diffusion (P = 0.002).

  • c Significant difference between the percentage of resistance obtained by broth macrodilution and that obtained with the Etest (P = 0.029).

  • d Significant difference between the percentage of resistance obtained by broth macrodilution and that obtained with disk diffusion (P = 0.034).