A Study to Learn About the Study Medicine Aztreonam-Avibactam (ATM-AVI) in Infants and Newborns Admitted in Hospitals With Bacterial Infection (CHERISH)

Inclusion Criteria Participants must meet the following key inclusion criteria to be eligible for enrollment into the study: 1. Hospitalized with age from birth \<9 months, including preterm birth 2. Part A: Receiving IV antibiotics for treatment of suspected or confirmed bacterial infection, including but not limited to cIAI, cUTI, HAP/VAP, BSI, or sepsis. 3. Part B: Suspected or confirmed gram-negative bacterial infection requiring IV antibiotics, including but not limited to cIAI, cUTI, HAP/VAP, BSI, or sepsis. Participants with any of the following characteristics/conditions will be excluded: 1. Received any other investigational medicinal product within the longer of 30 days or 5 half-lives before enrollment. 2. Any medical or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study. 3. Severe renal impairment or known significant renal disease, as evidenced by elevated serum creatinine at screening, or urinary output \<0.5 mL/kg/h for 6 consecutive hours or requirement for dialysis. 4. Part B Only: Received \>24 hours of systemic antibiotic treatment for gram-negative organisms at time of enrollment, unless documented treatment failure or lack of improvement in at least one objective sign or symptom of infection after ≥48 hours of antibiotics.