A Selective Culture Medium for Screening Aztreonam-Avibactam Resistance in Enterobacterales and Pseudomonas aeruginosa.

Aztreonam/avibactam (ATM/AVI) has been recently approved drug for clinical use in the European Union. The aim of this study was to develop and evaluate a novel selective medium for the isolation of ATM/AVI-resistant strains (Super ATM/AVI selective medium) to help to control their spread. Minimum inhibitory concentrations of ATM/AVI were determined using the broth microdilution method for 77 Gram-negative isolates, including 62 Enterobacterales and 15 Pseudomonas aeruginosa. The Super ATM/AVI selective medium was elaborated using optimal final concentrations of ATM at 5 mg/L and AVI at 4 mg/L, being supplemented with amphotericin B and vancomycin to prevent growth of yeasts and Gram-positive bacteria and with ZnSO4 to optimize the expression of metallo-β-lactamase producers. Super ATM/AVI showed high sensitivity (94.6%) and specificity (100%) at a detection limit of 103 CFU/mL.