Which = ?

Which refers to the"commensal bacteria" or " the bacteria causing the infections"? Or both?
Context:

Since their discovery last century, antibiotics have served society well by saving tens of millions of lives. Too many individuals—including illustrious composers and writers such as Schubert, Mahler, Mozart, and Wilde—died prematurely in the pre-antibiotic era from bacterial infections that are now treatable. Antibiotics are called miracle drugs because they kill bacteria, and thereby cure people of potentially fatal infectious diseases. Antibiotics are also unique drugs, because they act not only on the bacteria causing the infections, but also on a myriad of commensal bacteria, which can then disseminate widely, creating a reservoir of resistant organisms.