MRSA bacteria, computer artwork. Some of the bacteria are dividing. Methicillin-resistant Staphylococcus aureus (MRSA) is a Gram-positive round (coccus) bacterium. It is resistant to many commonly prescribed antibiotics. S. aureus is carried by around 30% of the population without causing any symptoms. However, in vulnerable people, such as those that have recently had surgery, it can cause wound infections, pneumonia and blood poisoning. The spread of MRSA can be prevented by health workers washing their hands between patients and before procedures.