Comparison of testing methods assessing the in vitro efficacy of the combination of aztreonam with avibactam on multidrug-resistant Gram-negative bacilli

Background Aztreonam-avibactam (ATM-AVI) combination shows promising effectiveness on most carbapenemase-producing Gram-negatives, yet standardized antibiotic susceptibility testing (AST) methods for evaluating the combination in clinical laboratories is lacking. We aimed to evaluate different ATM-AVI AST approaches. Methods 96 characterized carbapenem-resistant clinical isolates belonging to 9 Enterobacterales (EB; n = 80) and P. aeruginosa (PA; n = 16) species, including 90 carbapenemase producers and 72 strains resistant to both CAZ-AVI and ATM, were tested. Paper disk elution (DE; Bio-Rad) and E-test gradient strips stacking (SS; bioMérieux) were performed for the ATM + CAZ-AVI combination. MIC Test Strip (MTS; Liofilchem) was evaluated for ATM-AVI MIC determination. Results were interpreted applying ATM clinical breakpoints of the EUCAST guidelines to assess the combinations and compared to the broth microdilution (BMD) method using Sensititre panels (Thermofisher). Results According to the reference BMD, 93% of EB and 69% of PA were tested susceptible to ATM-AVI. The synergistic effect of ATM-AVI was of 95% for EB, but of only 17% for PA. Higher categorical and essential agreement (CA / EA) rates were obtained with the MTS method for both EB (89% / 91%) and PA (94% / 94%) compared to those of SS (87% / 83% for EB; 81% / 81% for PA). MTS and SS yielded 2 and 3 major discrepancies (MD), respectively, while 3 very major discrepancies (VMD) each were observed for both methods. Concerning the DE method, CA reached 91%, for EB and 81% for PA, but high number of VMD were observed for EB (n = 6; 8%) and for PA (n = 3; 19%). Conclusions The ATM-AVI association displayed excellent in vitro activity against highly resistant clinical Enterobacterales strains. MTS method offers accurate ATM-AVI AST results, while the SS method might serve as better alternative then DE method in assessing the efficacy of ATM + CAZ-AVI combination. However, further investigation is needed to confirm the methods' ability to detect ATM-AVI resistance.