*2.3. Antimicrobial Susceptibility Testing*

Susceptibility testing was performed according to the Clinical and Laboratory Standards Institute (CLSI) guidelines or as recommended by the European Committee on Antimicrobial Susceptibility testing (EUCAST) using BD BBLTM, Sensi-Disc™ paper discs (Becton, Dickinson and Company, Sparks, MD, USA) [23,24].

The inhibition zone diameters were interpreted according to EUCAST guidelines with the exception for tetracycline, chloramphenicol and nalidixic acid, which were evaluated in conformity with the Clinical Laboratory Standards Institute (CLSI) guidelines. EUCAST guidelines were chosen as they are the clinical standard for Europe; whenever EUCAST criteria were not available CLSI standards were used.

The following antibiotics were used: amoxicillin/clavulanic acid (20 μg/10 μg), piperacillin/ tazobactam (100 μg/10 μg), cefalexin (30 μg), cefuroxime (30 μg), cefoxitin (30 μg), cefotaxime (5 μg), ceftazidime (10 μg), cefepime (30 μg), imipenem (10 μg), meropenem (10 μg), amikacine (30 μg), gentamicin (10 μg), trimethoprim/sulfamethoxazole (1.25 μg/23.75 μg), ciprofloxacin (5 μg), moxifloxacin (5 μg), tetracycline (30 μg), nalidixic acid (30 μg) chloramphenicol (30 μg).

To determine tigecycline and colistin susceptibility, Etests® (bioMérieux Austria GmbH, Vienna, Austria) according to EUCAST guidelines for tigecycline and colistin were performed as described previously [25,26].

*Escherichia coli* ATCC 25922 and *Pseudomonas aeruginosa* ATCC 27853 were used as control strains in all conducted tests.
