MRSA. Computer artwork displaying the term MRSA, with each letter in a petri-dish containing culture that is growing MRSA bacteria. MRSA stands for 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 or those that have weak immune systems, 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.