 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 MeropenemColistin MERCOL 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 