Piperacillin tazobactam

All GNB isolates which were non-naturally colistin-resistant and grown on the LBJMR medium were subjected to an AST according to the current (DD) test method (Kirby-Bauer procedure). The minimal inhibition concentration (MIC) was confirmed by CLSI and EUCAST guidelines [28 ]. AST was performed with a definite turbidity bacterial suspension in NaCl (0.5 McFarland; 1.5 × 10⁸ cells/mL). Antibiogram test included the following sixteen antibiotics: amoxicillin (AMX), amoxicillin-clavulanic acid (AMC), cefepime (FEP), piperacillin/tazobactam (TPZ), cefalotin (KF), ceftriaxone (CRO), ertapenem (ETP), imipenem (IMP), fosfomycin (FF), nitrofurantoin (F), trimethoprim-sulfamethoxazole (SXT), amikacin (AK), ciprofloxacin (CIP), doxycycline (DO), colistin (CT), and gentamicin (GT) (Bio-Rad, Marne-la-Coquette, France). Hierarchical clustering of the antibiotic resistance phenotype was performed using Multi-Experiment Viewer (MeV 4.9.0).
Strains with a narrow diameter zone of inhibition (ZOI) less than 15 mm were picked out to confirm the minimal inhibition concentration value using other complementary tests, namely the E-tests method (BioMérieux) and UMIC test (Biocentric Bandol, France) [29 (link)]. Furthermore, strains were considered to be multidrug-resistant (MDR) if bacteria were resistant to more than three different classes of antibiotics.

Antimicrobial susceptibility was tested by employing disk diffusion, gradient test, and broth microdilution test according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) recommendations, 2021 [43 ]. The disks impregnated with the following antimicrobial agents were tested: imipenem (10 μg), meropenem (10 µg), ciprofloxacin (5 µg), levofloxacin (5 µg), ceftazidime (10 µg), cefepime (30 µg), amikacin (30 µg), piperacillin/tazobactam (30/6 µg), aztreonam (30 µg), ticarcillin (75 µg), ceftazidime-avibactam (10–4 µg), and ceftolozane/tazobactam (38–10 µg) (BioRad, Watford, UK). Minimum inhibitory concentrations (MICs) for colistin, imipenem, and meropenem were evaluated by ComASP Colistin (Liofilchem, Roseto, Italy) and Gradient strip test (Liofilchem, Roseto, Italy), respectively. P. aeruginosa ATCC 27853 was used as the control strain in the antibiotic susceptibility testing. MDR P. aeruginosa were defined as isolates that tested resistant to at least one antimicrobial agent of three or more different classes. Extensively drug-resistant (XDR) P. aeruginosa were defined as a subset of MDR isolates that tested non-susceptible to at least one antimicrobial agent of five different classes. P. aeruginosa was defined as pandrug-resistant (PDR) when the organism was resistant to all tested agents.