Antibiotic susceptibility patterns of Crossiella equi and Amycolatopsis species causing nocardioform placentitis in horses

Nocardioform actinomycetes are significant causes of placentitis and abortions in horses. In the current study, antimicrobial susceptibility patterns of 38 Amycolatopsis spp. and 22 Crossiella equi isolates, the most common nocardioform actinomycetes causing placentitis in horses, were evaluated. Antimicrobial susceptibilities of these isolates were tested by broth microdilution method in a commercial system, which was designed for Nocardia spp., fast-growing Mycobacterium spp., and other aerobic actinomycetes. The minimum inhibitory concentration required to inhibit the growth of 90% of organisms (MIC90) of the following antibiotics tested for Amycolatopsis spp. were: 4 μg/ml for linezolid, trimethophrim-sulfametaxazole (TMP-SMX), and ciprofloxacin; 8 μg/ml for ceftriaxone, doxycycline, and minocycline; 16 μg/ml for amoxicillin-clavulanic acid, clarithromycin, and imipenem; >16 μg/ml for tobramycin; 32 μg/ml for amikacin and cefepime; and 128 μg/ml for cefoxitin. The MIC90 levels for C. equi were 0.25 μg/ml for doxycycline; ≤1 μg/ml for minocycline; 2 μg/ml for linezolid and TMP-SMX; 4 μg/ml for ciprofloxacin; 8 μg/ml for amoxicillin-clavulanic acid, ceftriaxone, and imipenem; 16 μg/ml for clarithromycin; >16 μg/ml for tobramycin; 32 μg/ml for cefepime; >64 μg/ml for amikacin; and 128 μg/ml for cefoxitin.