“What antibiotics treat endocarditis?”

It depends on the type of infection; there are several different bacteria which respond to different antibiotics. So, the first step is to do a blood culture to identify the bug and its sensitivity to a range of antibiotics; then choose the best one or two based on what is likely to work with a minimum of side effects. Generally, intravenous antibiotics are used initially, but the course of treatment may be for 6 weeks or so (depending on the bug) and it may be possible to switch to an oral antibiotic for the later part of the treatment.