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

The research study investigates the susceptibility of two bacterial species (Amycolatopsis and Crossiella equi), which contribute to placentitis (an inflammation of the placenta) in horses, to different antibiotics. The objective is to identify the minimum dose of each antibiotic required to inhibit bacterial growth or ‘minimum inhibitory concentration’ (MIC).

Research Methodology

• The study focused on 38 isolates of Amycolatopsis species and 22 isolates of Crossiella equi. These isolates are known contributors to nocardioform placentitis in horses.
• The researchers used a broth microdilution method to test each isolate’s susceptibility to different antibiotics. This test was conducted using a commercial system designed specifically for such bacteria.

Findings

• The researchers provided MIC(90) levels for both bacteria in regards to a variety of antibiotics. MIC(90) levels denote the minimum concentration of an antibiotic required to inhibit 90% of bacterial growth.
• The MIC(90) levels for Amycolatopsis species were highest (>16 µg/ml) for tobramycin, followed by 32 µg/ml for amikacin and cefepime, and 128 µg/ml for cefoxitin. Lower MIC(90) levels were found for other drugs including linezolid, TMP-SMX, and ciprofloxacin (4 µg/ml); ceftriaxone, doxycycline, and minocycline (8 µg/ml); and amoxicillin-clavulanic acid, clarithromycin, and imipenem (16 µg/ml).
• For Crossiella equi, the lowest MIC(90) was 0.25 µg/ml for doxycycline. Other antibiotics exhibited higher MIC(90) levels: minocycline (≤1 µg/ml); linezolid and TMP-SMX (2 µg/ml); ciprofloxacin (4 µg/ml); amoxicillin-clavulanic acid, ceftriaxone, and imipenem (8 µg/ml); clarithromycin (16 µg/ml); tobramycin (>16 µg/ml); cefepime (32 µg/ml); amikacin (>64 µg/ml); and cefoxitin (128 µg/ml).

Conclusion

• The study’s findings provide vital knowledge on how Amycolatopsis species and Crossiella equi respond to different antibiotics, crucial data for veterinarians treating placentitis in horses.
• The variation in MIC(90) levels among different antibiotics underlines the importance of selecting the most effective antibiotic for these types of bacterial infections.