Study of 2 Medicines (Aztreonam and Avibactam) Compared to Best Available Therapy for Serious Gram-negative Infections

Inclusion Criteria Participants must meet the following key inclusion criteria to be eligible for enrollment into the study: 1. Participants ≥9 months to \<18 years of age at Screening; Female (post-menarchal) participants must have a negative serum/urine pregnancy test (β hCG sensitivity ≥25 mIU/mL). 2. Suspected/confirmed cIAI, cUTI, HAP/VAP, or BSI with gram-negative pathogens. 3. Require hospitalization and IV antibiotic treatment. Exclusion Criteria Participants with any of the following characteristics/conditions will be excluded: 1. Other medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study. 2. Gram-negative species not expected to respond to ATM AVI ≤14 days. 3. Pregnant or breastfeeding; fertile male/female unwilling/unable to use effective contraception for at ≥7 days (males) or ≥28 days (females) after last ATM-AVI infusion. (HAP/VAP only): 4. Microbiologically known or high likelihood of monomicrobial infection with a gram-positive organism, lung abscess, pleural empyema, or post-obstructive pneumonia, lung or heart transplant. 5. Received \>24 hours of systemic antibiotics during the 48 hours before randomization unless participant has documented treatment failure after at least 48 hours of antibiotic therapy. 6. Current use of any prohibited concomitant medication(s) or unwilling/unable (Cockayne Syndrome patients with cIAI are excluded) to use MTZ or having received previous investigational drug(s) or vaccine ≤30 days or 5 half-lives before randomization (whichever is longer). 7. CrCL ≤15 mL/min/1.73 m2 (eCrCl or eGFR calculation based on age). 8. Non-infectious related screening ALT or AST \>3 x ULN, ALP \>3 x ULN and/or TBili \>2 x ULN (\> 3 x ULN for Gilbert's syndrome). 9. Investigator site staff directly involved in the conduct of the study and their family members, site staff otherwise supervised by the investigator, and sponsor and sponsor delegate employees directly involved in the conduct of the study and their family members.