MRSA bacteria, coloured scanning electron micrograph (SEM). Methicillin-resistant Staphylococcus aureus (MRSA, yellow) is a Gram-positive, round (coccus) bacterium, shown here on the microscopic fibres of a wound dressing. It is resistant to many commonly prescribed antibiotics. S. aureus is carried by around 30 per cent 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.