MRSA: the superbug of fear and wonder! What exactly is this MRSA we hear about in the news and why is it such a problem?

What?

MRSA stands for Methicillin Resistant Staphylococcus Aureus or Multiply Resistant Staphylococcus Aureus(S.aureus). It is resistant to the antibiotics usually used for S.aureus. This bacterium (bug) prefers warm, damp conditions like the nose, armpits and groin and is found all around. About 1 in 4 people carry this microbe, which does them no harm in normal circumstances, sitting there as a tenant. The microbe can be eradicated from these sites but in some people it will return - they seem to offer a harbour for S.aureus.

S.aureus is a common cause of disease.S.aureus infections normally produce pus eg boils, abscesses and wound infections. It can also cause pneumonia, infections of the heart valves (infective endocarditis) and particularly infections of artificial body parts eg hip joints. S.aureus produces an enzyme, coagulase, that "walls" off the infection but makes it more difficult for the white blood cells to reach the bugs.

MRSA is a problem because it is difficult to treat but it is no more dangerous (virulent) than "common-or-garden" S.aureus. It requires extended treatment with antibiotics, and until recently the only effective antibiotics had to be given intravenously. This would entail up to 6 weeks of hospital treatment. S.aureus is usually resistant to penicillin because it produces an enzyme, beta-lactamase¹, which destroys penicillin. Methicillin and flucloxacillin are semi-synthetic antibiotics derived from penicillin that are resistant to the enzyme so they are effective against normal S.aureus. MRSA has mutated further so the target of penicillins has altered, so even methicillin does not work. MRSA is usually resistant to multiple antibiotics eg tetracycline, trimethoprim and erythromycin.

Why?

MRSA is created by antibiotic usage. Contrary to news reports, many of the patients with MRSA in hospital have brought the bug in with them - this has been proven by swabbing patients on admission. MRSA is endemic in nursing homes. MRSA rates are more related to overuse of antibiotics than poor hospital hygiene - the USA has a massive problem with resistant bacteria (vancomycin-resistant enterococci as well as MRSA). There is further information about antibiotic resistance on A591374

Treatment

Over the last year new antibiotics have been marketed to treat MRSA. Vancomycin has been the mainstay, largely superceded now by teicoplanin. Now Synercid (quinupristin/dalfopristin), daptomycin and linezolid are all options. Linezolid works well orally, offering outpatient treatment as an option as both vancomycin and teicoplanin can only be given via an intravenous drip. Any artificial parts infected eg hip replacements have to be removed to eradicate the infection.

Phage treatment is used in some research centres but there are practical problems. Bacteriophage viruses are viruses that attack bacteria, but each virus only attacks one type of S.aureus. There are over 100 phage types of S.aureus.

The UK government has recently announced plans for Directors of Infection Control for NHS trusts. This is a cosmetic measure only. The necessary changes required are increased staffing levels to improve compliance with hygiene procedures and public education to reduce the pressure on GPs to prescribe medically unnecessary antibiotics.