Cultured bacteria tested for MRSA. Bacterial culture growing on mannitol salt agar (pink) in a Petri dish. Mannitol salt agar is a selective medium for growing MRSA (Methicillin Resistant Staphylococcus aureus), which turns yellow with the presence of the bacteria. Here, the antibiotic cefoxitin (small circle, upper centre) has been added to the gel. As MRSA is sensitive to cefoxitin, growth of the bacteria has been inhibited in a circle around the drug, showing that the bacteria on the gel is likely to be MRSA.