The purpose of this study is to learn about the safety and effects of ATM-AVI for the possible treatment of infections caused by a type of bacteria called gram-negative bacteria. The study medicine is a combination of an antibiotic, aztreonam (ATM), and another medicine, avibactam (AVI), which is used to help stop bacteria from being resistant to antibiotics. Antibiotics are medicines that fights bacteria and infections. The study will include newborns and infants up to 9 months of age who are admitted in the hospital. The study is conducted in 2 parts: Part A and Part B. In Part A, all participants will receive a single intravenous (injected directly into a vein) infusion of ATM-AVI. This is to study the safety and effects of a single amount. In Part B, all participants will receive multiple intravenous infusions of ATM-AVI as treatment for a possible or confirmed infection with gram-negative bacteria.