Final answer:
Vancomycin is the drug of choice for treating penicillin and methicillin-resistant bacterial infections, including endocarditis, which is an infection of the heart's inner lining.
Step-by-step explanation:
The drug used to treat cases of penicillin and methicillin resistance, particularly when treating endocarditis, is B. vancomycin.
Vancomycin is a glycopeptide antibiotic that is effective against gram-positive bacteria, including those strains that have become resistant to penicillin and methicillin, such as methicillin-resistant Staphylococcus aureus (MRSA). It is often used as a last line of defense, especially for treating serious infections like endocarditis, which is an infection of the heart's inner lining (endocardium). Developing resistance against vancomycin is a major concern, and it usually requires careful use in clinical settings to prevent the spread of resistant bacteria like vancomycin-resistant enterococci (VRE) and vancomycin-resistant S. aureus (VRSA).