E48 - Comparison of Microbroth Dilution and Disk Diffusion Methods for Antimicrobial Sensitivity of Equine Bacterial Isolates

Abstract: Background – Broth dilution techniques, where bacterial isolates are grown in liquid growth media with incremental concentrations of antimicrobials, allow precise determination of the concentration of each antimicrobial that will inhibit bacterial growth in vitro (MIC), and hence are better suited to pharmacokinetic/pharmacodynamic approaches to treatment.



Hypothesis/Objectives – Determination of MIC using microbroth techniques for equine clinical isolates, and comparison with conventional disk-diffusion (ADDT) sensitivity testing results.



Methods – Antimicrobial sensitivity of bacteria isolated from equine clinical submissions to veterinary diagnostic laboratories at Charles Sturt University and the University of Adelaide were determined using ADDT and Sensititre® microbroth dilution to determine MIC. Resistance profiles were generated for each antimicrobial agent across the range of concentrations tested, using MIC distribution frequency tables, and results were compared between techniques.



Results – Sensititre® MIC results were available for 229 isolates tested against twenty commonly used antimicrobials. Multiple drug resistance was observed in 79.5% of isolates (95% CI 73.8-84.2%) compared to 51.7% of isolates based on ADDT, but extreme drug resistance was less commonly recognised. Agreement between MIC and ADDT results was observed on 1505 of 1932 (77.9%) occasions, and was better for Gram negative isolates (82.9%) than for Gram positive isolates (74.4%, P < 0.001). Conventional results over-estimated sensitivity for aminoglycosides, but underestimated susceptibility to other antimicrobials tested.



Conclusions and clinical importance – The current study demonstrated the value of MIC determination of antimicrobial susceptibility for equine patients. High error rates considerably restrict the use of ADDT for determination of antimicrobial sensitivity testing.