Minimum inhibitory concentration and postantibiotic effect of amikacin for equine isolates of methicillin-resistant Staphylococcus aureus in vitro.

Objective: To report the minimum inhibitory concentration (MIC) of amikacin sulfate for equine clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and characterize the initial kill and duration of the postantibiotic effect (PAE) for selected strains.

Study Design: Experimental study.

Methods: Isolates of MRSA (n=35) had their amikacin MIC determined using the E-test agar diffusion method. Two isolates with MICs>256 microg/mL limit were further characterized using broth macrodilution. Six distinct isolates with amikacin MICs of 32, 48, 128 (2 isolates) and 500 (2 isolates) microg/mL had PAE determinations made over a range of amikacin concentrations from 31.25-1000 microg/mL using standard culture-based techniques.

Results: Median MIC of the 35 isolates was 32 microg/mL (range 2 to >256 microg/mL). Mean PAE of selected MRSA strains had an overall mean (all amikacin doses) of 3.43 hours (range 0.10-9.57 hours). PAE for MRSA exposed to amikacin at 1000 microg/mL was 6.18 hours (range 3.30-9.57 hours), significantly longer than that for all other concentrations (P<.0001). There was no statistically significant effect of isolate MIC on PAE.

Conclusions: Isolates had a wide range of MIC; however, growth of all 6 selected strains were inhibited within the range of concentrations tested, including 2 strains with MICs of 500 microg/mL. PAE duration was not influenced by the MIC of amikacin but was significantly longer with treatment at 1000 microg/mL than at lower concentrations.

Clinical Relevance: Clinical isolates of MRSA are susceptible to amikacin at concentrations achieved by regional perfusion: however, the modest duration of PAE observed suggest that further laboratory and in vivo evaluation be conducted before recommending the technique for clinical use.