A novel method for early detection of MIC value – Broth dilution using indicator solution versus agar dilution: an original article

Introduction: The adequate protocol for treatment of an infection is often determined on the basis of the minimum inhibitory concentration (MIC) of the causative organism. Traditional methods (agar dilution, microbroth dilution, and gradient diffusion) are labour intensive and time consuming (they are usually take over 48 hours to report the results). On the other hand, automated systems (VITEK™, Phoenix™, MicroScan WalkAway™) and rapid methods of MIC detection (using dielectrophoresis (DEP), magnetic bead rotation sensors and microfluidic incubation) require expensive instruments. This study is aimed to develop a rapid MIC detection method with the ability to applied to a resource limited setting.

Methods: Agar dilution method and a novel broth dilution method (containing indicator solution) were simultaneously performed using amikacin, ceftriaxone, piperacillin-tazobactam, imipenem, cefoxitin and azithromycin.

Results: Isolates of Escherichia coli, Enterobacter spp, Klebsiella spp, Staphylococcus aureus, Proteus mirabilis, Acinetobacter spp and Pseudomonas spp were used. The MIC values for Enterobacteriaceae and S. aureus isolates for each antibiotic were obtained within 4 to 5 hours by a novel broth dilution method. The obtained MIC values were corresponded with the MIC shown on the following day by agar dilution method.

Conclusion: Broth dilution method with indicator solution is effective in rapid determination of the MIC for cephalosporins, penicillin, carbapenems, cephamycin, aminoglycosides and macrolides for most isolates of Enterobacteriaceae and S. aureus. Unfortunately this method did not work for the non-fermenter group of organisms like Pseudomonas spp and Acinetobacter spp, as their results could not be obtained before 24 hours. The method is time saving, relatively inexpensive and is applicable to resource limited settings.