MRSA stands for methicillin-resistant Staphylococcus aureus. These are strains of S. aureus that are resistant to methicillin and to the other β-lactam antibiotics, including penicillin and amoxicillin. Many strains are also resistant to several other groups of antibiotics.How it is resistant. Methicillin was developed because it is not broken down by the β-lactamase enzymes that make many staphylococci resistant to penicillin. MRSA got round this by gaining a gene, mecA, that codes for an altered transpeptidase enzyme (a penicillin-binding protein) in the cell wall. β-lactam antibiotics cannot bind to its active site, so the bacterium keeps building cross-linked peptidoglycan and its wall stays strong.Where it causes problems. Like other S. aureus, MRSA can live harmlessly on the skin or in the nose. It is best known as a hospital-acquired infection, because hospitals bring together:
• patients who are ill, elderly or have weakened immune systems;
• open wounds from surgery, and devices such as catheters and drips that give the bacteria a way in;
• heavy use of antibiotics, which selects for resistant strains;
• many people moving between patients, so bacteria can be spread on hands, clothing and equipment.MRSA causes the same diseases as other S. aureus, from skin and wound infections to pneumonia and septicaemia, but they are harder to treat. Serious infections are treated with antibiotics that still work, such as vancomycin, though some strains have reduced sensitivity even to this.
• patients who are ill, elderly or have weakened immune systems;
• open wounds from surgery, and devices such as catheters and drips that give the bacteria a way in;
• heavy use of antibiotics, which selects for resistant strains;
• many people moving between patients, so bacteria can be spread on hands, clothing and equipment.MRSA causes the same diseases as other S. aureus, from skin and wound infections to pneumonia and septicaemia, but they are harder to treat. Serious infections are treated with antibiotics that still work, such as vancomycin, though some strains have reduced sensitivity even to this.