Background:The ceftazidime-avibactam combination is able to inhibit ESBLs, AmpCs and Class A carbapenemases and has been recommended for the treatment of complicated UTIs and ventilator-associated bacterial pneumonia.Aims:The present study was undertaken to evaluate the in vitro activity of ceftazidime-avibactam and to determine the synergistic activity of aztreonam/ceftazidime-avibactam combination against carbapenem-resistant Gram-negative bacilli.Materials and Methods:Gram-negative isolates that exhibited resistance to at least one of the carbapenems (imipenem or meropenem) by the Kirby-Bauer disc diffusion method were subjected to phenotypic characterisation by the Vitek-2 automated method. Phenotypically confirmed isolates were subjected to ceftazidime/avibactam-aztreonam synergy testing by disc diffusion method.Results:Twenty-two carbapenem-resistant isolates showed a minimum inhibitory concentration of 4-64 mu g/mL for imipenem and 8-64 mu g/mL for meropenem. Out of 22 carbapenem-resistant isolates, 18 (81.82%) isolates showed resistance to ceftazidime-avibactam and aztreonam, and 2 (9.09%) isolates showed intermediate resistance to aztreonam. Nine (40.91%) isolates showed synergy to ceftazidime-avibactam/aztreonam combination by disc diffusion method. An increase in zone diameter of 5-23 mm and 5-16 mm was observed with the ceftazidime-avibactam/aztreonam combination for Klebsiella pneumoniae and Escherichia coli, respectively, when compared to ceftazidime-avibactam and aztreonam disc tested alone. Out of 21 Enterobacterales studied, 21 (100%) isolates showed resistance to amoxicillin-clavulanate (>= 32 mu g/mL) and piperacillin-tazobactam (128 mu g/mL), 12 (57.14%) isolates showed resistance to gentamicin (>= 16 mu g/mL), 5 (23.81%) isolates were resistant to amikacin (>= 64 mu g/mL), 21 (100%) isolates were resistant to ciprofloxacin (>= 4 mu g/mL), 19 (90.48%) isolates were resistant to cotrimoxazole (>= 320 mu g/mL), 21 (100%) isolates were resistant to cefepime (>= 16 mu g/mL) and 6 (28.57%) isolates were resistant to tigecycline (2 mu g/mL).Conclusion:In the present study, 42.86% of Enterobacterales isolates showed synergism to the ceftazidime-avibactam/aztreonam combination. The optimal dosing strategy and in vivo efficacy of this combination need to be evaluated.