The purpose of this study is to evaluate how Aztreonam (ATM) and Avibactam (AVI) are processed in pediatric participants. This study also aims to understand participant safety and effects in pediatric participants. The study is seeking participants who are: * 9 months to less than 18 years of age * Hospitalized * Suspected/known to have a gram-negative infection * Receiving intravenous (iv, given directly into a vein) antibiotics * Being treated for complicated infections of various body parts that includes the abdomen, urinary tract, blood stream, and lungs. * Participants will receive either ATM-AVI or best available therapy (BAT). * Both therapies will be given through a vein. * Participants with complicated abdominal infections will also receive iv Metronidazole (MTZ). Patients with cIAI and Cockayne Syndrome are excluded due to a risk of severe hepatotoxicity with the use of MTZ. - Participants on ATM-AVI treatment who have anaerobic infections will also receive iv MTZ at the study doctor's discretion. * The iv dose of ATM-AVI will be based on the participant's weight and kidney function. * The study doctor will determine the iv dose of BAT. * During the first 2 study days, participants on ATM-AVI therapy will have 5 blood draws in small quantities. * Starting on day 4, the study doctor will decide if participants may be switched to oral therapy. * Participants will receive a maximum of 14 days of ATM-AVI treatment. * After discharge from the hospital, 1 study visit may be required. * Depending on the participant's response, the study duration will be from 33 to 50 days. * The investigator will contact participants by phone 28 to 35 days after the last study treatment to check participants health status.