MRSA bacteria, coloured scanning electron micrograph (SEM). Methicillinresistant Staphylococcus aureus (MRSA) is a Grampositive, 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. Magnification: x5400 when printed at 10 centimetres wide.