Minimum inhibitory concentration and postantibiotic effect of amikacin for equine isolates of methicillin-resistant Staphylococcus aureus in vitro.
Authors: Caron John P, Bolin Carole A, Hauptman Joseph G, Johnston Kimberly A
Journal: Veterinary surgery : VS
Summary
# Amikacin efficacy against equine MRSA: In vitro susceptibility and postantibiotic effects Methicillin-resistant *Staphylococcus aureus* (MRSA) presents a significant therapeutic challenge in equine surgery and wound management, prompting investigation into aminoglycoside alternatives such as amikacin, particularly for regional limb perfusion where high local concentrations can be achieved. Researchers tested 35 equine MRSA clinical isolates for amikacin susceptibility using E-test methodology and broth macrodilution, then characterised the postantibiotic effect (PAE)—the period during which bacterial growth remains suppressed following drug removal—across six representative strains at concentrations ranging from 31.25–1000 µg/mL. Median minimum inhibitory concentration (MIC) was 32 µg/mL, though considerable variation existed (range 2–>256 µg/mL); crucially, all tested strains were inhibited at achievable perfusion concentrations, including two highly resistant isolates with MICs of 500 µg/mL. Mean PAE across all doses was modest at 3.43 hours overall, but increased significantly to 6.18 hours when amikacin concentration reached 1000 µg/mL compared with lower doses (P<0.0001), with no correlation between isolate MIC and PAE duration. Whilst these findings support amikacin's in vitro efficacy for regional perfusion in MRSA cases, the relatively brief PAE—even at maximum concentrations—necessitates careful clinical evaluation before adoption, as dosing intervals must accommodate this limited window of bacterial suppression.
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Practical Takeaways
- •Amikacin can inhibit equine MRSA at drug concentrations achievable through regional perfusion techniques, offering a potential treatment option for localized infections
- •The relatively short postantibiotic effect (average 3.4 hours) suggests that frequent dosing or alternative delivery methods may be necessary for clinical efficacy
- •Further in vivo studies are needed before recommending regional perfusion with amikacin for routine clinical MRSA management in horses
Key Findings
- •Median amikacin MIC for equine MRSA isolates was 32 µg/mL with a wide range (2 to >256 µg/mL)
- •Mean postantibiotic effect (PAE) across all doses was 3.43 hours, ranging from 0.10 to 9.57 hours
- •PAE at 1000 µg/mL was significantly longer (6.18 hours) compared to lower concentrations (P<0.0001)
- •MIC value did not influence PAE duration, but all tested strains were inhibited within concentrations achievable by regional perfusion