Comparative evaluation of minocycline susceptibility testing methods in carbapenem-resistant Acinetobacter baumannii

In this study, the performance of two commonly used routine antimicrobial susceptibility testing methods, the automated VITEK(®)2 system and Etest (bioMérieux, Marcy-l'Étoile, France), was compared with the standard broth microdilution (BMD) method on 87 multidrug- and carbapenem-resistant Acinetobacter baumannii clinical isolates. Clinical and Laboratory Standards Institute (CLSI) 2015 breakpoints (susceptible, ≤4 mg/L; intermediate, 8 mg/L; and resistant, ≥16 mg/L) were used. Minocycline showed excellent activity, with 94.3% of isolates susceptible by BMD (VITEK(®)2, 73.6%; Etest, 63.2%). The MIC50/90 values (minimum inhibitory concentrations required to inhibit 50% and 90% of the isolates, respectively) were as follows: BMD, 1/4 mg/L; VITEK(®)2, ≤1/8 mg/L; and Etest, 4/16 mg/L. Etest produced 14.9% major/20.7% minor errors and VITEK(®)2 produced 3.4% major/17.2% minor errors. These data indicate that VITEK(®)2 may be more reliable than Etest for routine susceptibility testing of minocycline for A. baumannii isolates. As both VITEK(®)2 and Etest produced higher minocycline MICs compared with the reference method, BMD may be needed to validate the categorisation of carbapenem-resistant A. baumannii by these assays as minocycline non-susceptible.