Methicillin-resistant Staphylococcus aureus (MRSA)

MRSA stands for methicillin-resistant Staphylococcus aureus. It is a type of bacterium commonly found on the skin and/or in the noses of healthy people. Although it is usually harmless at these sites, it may occasionally get into the body (e.g. through breaks in the skin such as abrasions, cuts, wounds, surgical incisions or indwelling catheters) and cause infections. These infections may be mild (e.g. pimples or boils) or serious (e.g. infection of the bloodstream, bones or joints).

The treatment of infections due to Staphylococcus aureus was revolutionized in the 1940s by the introduction of the antibiotic penicillin. Unfortunately, most strains of Staphylococcus aureus are now resistant to penicillin. This is because Staphylococcus aureus has 'learnt' to make a substance called β-lactamase (pronounced beta-lactamase), that degrades penicillin, destroying its antibacterial activity.

Some related antibiotics, such as methicillin and flu cloxacillin, are not affected by β -lactamase and can still be used to treat many infections due to β -lactamase-producing strains of Staphylococcus aureus. Unfortunately, however, certain strains of Staphylococcus aureus, known as MRSA, have now also become resistant to treatment with methicillin and flu cloxacillin.

Although other types of antibiotics can still be used to treat infections caused by MRSA, these alternative drugs are usually not available in tablet form and must be administered through a drip inserted into a vein.

MRSA infections most often occur in patients in hospitals and are rarely seen among the general public. As with ordinary strains of Staphylococcus aureus, some patients harbors MRSA on their skin or nose without harm (such patients are said to be 'colonised'), whereas other patients may develop infections.

Some patients are at increased risk of developing infection. They include those with breaks in their skin due to wounds (including those caused by surgery), indwelling catheters or burns, and those with certain types of deficiency in their immune system, such as low numbers of white cells in their blood.

When MRSA spread from an initial site of colonisation to a site where they cause infection in the same patient (eg spread from the colonised nose to a wound), the resulting infection is described as 'endogenous'.

In addition to causing endogenous infections, MRSA can spread between patients, usually by direct or indirect physical contact. For example, hospital staff attending to a colonised or infected patient may become contaminated or colonised with MRSA themselves (perhaps only briefly). They may then spread the bacteria to other patients with whom they subsequently have contact. These patients may in turn become colonised and/or infected. The spread of MRSA (or for that matter other bacteria) between patients is called cross-infection.

Some strains of MRSA that are particularly successful at spreading between patients may also spread between hospitals, presumably when colonised patients or staff moves from one hospital to another. These strains are known as epidemic MRSA (or EMRSA for short)