This research study is testing a new treatment called AP-SA02 for adults with a blood infection caused by the germ Staphylococcus aureus (S. Aureus). The study is in two phases. In Phase 1b, researchers will check the safety and how well people can handle different doses of AP-SA02 given through an IV (intravenous), which means directly into the vein. In Phase 2a, they will also see how well the treatment works in addition to its safety. The participants will be split into two groups: one will get AP-SA02 along with the best antibiotic treatment available, and the other will get just the best antibiotic treatment.

Key Points:

• The study is for hospitalized adults aged 18 and older with a positive blood test for S. Aureus.
• Participants should not have another kind of infection, heart infection, brain abscess, or allergies to phage products.
• Participants should not be pregnant or breastfeeding and should agree to use birth control if they can have children.