Susceptibility was tested by disc diffusion following the CLSI recommendations using Mueller–Hinton agar (Oxoid, Basingstoke, UK) and 10 antimicrobial agents, which are primarily effective against A. baumannii[37] (link). The resistance breakpoints were adjusted according to the known distribution of inhibition zone diameters among A. baumannii strains. These values were identical to those of the CLSI for intermediate susceptibilities except for tetracycline and piperacillin, for which the CLSI values for resistance were used. The agents (µg per disc; resistance breakpoint in mm) included ampicillin+sulbactam (10+10; ≤14), piperacillin (100; ≤17), ceftazidime (30; ≤17), imipenem (10; ≤15), gentamicin (10; ≤14), tobramycin (10; ≤14), amikacin (30; ≤16), ofloxacin (5; ≤15), sulfamethoxazole+trimethoprim (23.75+1.25; ≤15) and tetracycline (30; ≤14) (Oxoid). Multidrug resistance was defined as resistance to at least one representative of three or more of the five classes of antimicrobial agents, i.e. beta-lactams, aminoglycosides, fluoroquinolones, tetracyclines or the combination of sulfonamide and diaminopyrimidine.
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