Mupirocin molecule. Computer illustration showing the molecular structure of mupirocin. Mupirocin is an antibiotic with activity against methicillin-resistant Staphylococcus aureus (MRSA). It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. Atoms are represented as spheres: carbon (black), hydrogen (white), oxygen (red) and bonds between the atoms by cylinders.