Comparison of Disk Diffusion and E-Test with the Reference Method of Microbroth Dilution for Susceptibility Testing of Acinetobacter baumannii Isolates to Tetracyclines

Due to the continuous increase of multidrug-resistant Acinetobacter baumannii strains around the world, decision making for an effective treatment of infections caused by this organism depends on the results of antimicrobial susceptibility tests. In this study, the validity of disk diffusion and E-test methods was assessed by their comparison with the reference method of microbroth dilution for three antibiotics of tetracycline, doxycycline and minocycline.
Total of 68 A. baumannii isolates were obtained from patients hospitalized in the burn center of Shahid Motahari Hospital in Tehran, Iran. Susceptibility of the Acinetobacter isolates was evaluated using the disk diffusion, E-test and microbroth dilution methods, according to the guidelines of Clinical and Laboratory Standards Institute.
Among the isolates, 82.3% were tetracycline-resistant (with minimum inhibitory concentration 50 (MIC50) and MIC90 of 32 and more than 32 µg/ml, respectively and 41.2% were doxycycline-resistant (with MIC50 and MIC90 of 4 and more than 32 µg/ml, respectively). Minocycline, with resistance of up to 13.3% (MIC50 and MIC90 of 1 and 8 µg/ml, respectively) showed the highest antimicrobial activity against the A. baumannii isolates. Antimicrobial susceptibility of bacteria was different depending on the type of methods used. No very major error was observed in any of the methods of susceptibility testing. Overall, the level of major and minor errors in the E-test was lower than the disk diffusion method.
The results of this study indicate that minocycline has notably high antimicrobial activity against A. baumannii compared to other antibiotics of the tetracycline group.