Meropenem

Isolation of E. coli was performed as follows: liquid aspirates were diluted approximately ×10⁶ fold with sterile PBS (phosphate saline buffer). Approximately 0.05 ml volume of feces were placed into 0.5 ml of sterile PBS, vortexed to homogeneity, an aliquot was diluted approximately ×10⁶ fold. Biopsy samples were vortexed in 0.2 ml of sterile PBS. For all samples, 0.1 ml of the resulting liquid was spread onto the Luria-Bertani agar plates. After overnight incubation on 37 °C, isolated colonies were identified with the Matrix Assisted Laser Desorbtion/Ionization (MALDI) Biotyper software (Bruker Daltonics, Germany) using the Microflex LT mass spectrometer (Bruker Daltonics, Germany). For DNA extraction, all E. coli strains were grown in the Luria-Bertani broth at 37 °C with shaking (200 RPM) overnight and collected by centrifugation. Samples and corresponding E. coli isolates are listed in Table 1.
The testing of susceptibility to ampicillin/sulbactam, ceftriaxone, cefotaxime, ceftazidime, cefepime, imipenem, meropenem, gentamicin, levofloxacin, and ciprofloxacin (all from Bio-Rad, USA) was performed by the disc-diffusion method using the Mueller-Hinton agar plates. The E. coli strain ATCC 25922 was used as a control. Current CLSI and EUCAST criteria were used for interpretation.

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.