MRSA is a strain of Methicillin-resistant "Staphylococcus aureus". This is a bacterium which causes infections in different parts of the body. It is tougher to treat than other strains of "Staphylococcus aureus" – or 'staph' – because it is resistant to some commonly used antibiotics. In fact, it is now resistant to all forms of penicillin and cephalosporin.