study (within COMBACTE CARE): A Phase 3 prospective, randomized, multicenter, open label, central assessor blinded, parallel group, comparative study to determine the efficacy, safety and tolerability of aztreonam avibactam (ATM AVI) ±metronidazole (MTZ) versus meropenem±colistin (MER±COL) for the treatment of serious infections due to Gram negative bacteria, including metallo β lactamase (MBL) - producing multidrug resistant pathogens, for which there are limited or no treatment options.