In vitro susceptibility of Achromobacter spp. isolates: comparison of disk diffusion, Etest and agar dilution methods

In this study, we analysed the antimicrobial susceptibility of 92 strains of Achromobacter spp. isolated from clinical samples to 18 antimicrobial agents. The disk diffusion method and Etest were compared with the agar dilution method, and the breakpoints of susceptibility and resistance for the disk diffusion method for the antimicrobials tested were determined. The most active antibiotics were piperacillin, piperacillin/tazobactam and the carbapenems. By applying the linear least-squares regression method, breakpoints could be established for antibiotics active against this genus such as imipenem, meropenem, ertapenem and trimethoprim/sulfamethoxazole (SXT). Other active antibiotics, such as piperacillin and minocycline, could be tested by the Etest method. The less active antibiotics such as gentamicin, doxycycline and tetracycline could be tested by the disk diffusion method. For the rest of the antimicrobial agents tested, breakpoints could not be established owing to the high percentage of errors and/or the poor linear regression coefficient obtained. Therefore, these antimicrobial agents should be tested by minimal inhibitory concentration determination. In summary, we recommend the following zone diameter breakpoints for resistant and susceptible, respectively: ≤11 mm and ≥22 mm for imipenem; ≤13 mm and ≥24 mm for meropenem; ≤17 mm and ≥24 mm for ertapenem; ≤15 mm and ≥21 mm for gentamicin; ≤27 mm and ≥28 mm for SXT; ≤20 mm and ≥29 mm for tetracycline; and ≤20 mm and ≥24 mm for doxycycline.