MicrobiologyBytes

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Staphylococcus aureus is an extremely common bacterium which colonises human skin and mucosal surfaces, particular in the nose. It is estimated that around 2 billion people, some 30 percent of the world’s population, carry Staphylococcus aureus somewhere on or in their bodies. Although Staphylococcus aureus can cause disease, this is rare in healthy people. After all, many of us are walking around covered in this bacterium! “Staph aureus” can sometimes cause food poisoning, but the real problems start if the bacterium finds an opportunity to penetrate the skin or membranes and enter the body. Once inside, they cause wound infections, urinary tract infections, pneumonia, and may infect the blood.

Fortunately, most strains of this bacterium are sensitive to many different antibiotics, and these infections can be effectively treated and so do not pose life-threatening problems for heathy people. A few strains of S. aureus can cause necrotizing fasciitis, popularly known as “flesh-eating disease”. In fact, toxins released by the bacteria cause the destruction of skin and muscle and the bacteria do not actually eat the tissues. Although this can be very serious, if diagnosed quickly, this condition is treatable with a range of antibiotics, although surgery may be required and there may be lasting damage to affected areas of the body.
The real problems start when the bacteria become resistant to not one, but most of the antibiotics commonly used to treat these infections. Methicillin-resistant Staphylococcus aureus (MRSA) is a specific strain of the Staphylococcus aureus bacterium that has developed antibiotic resistance many antibiotics, especially penicillins frequently used against this organism, including methicillin. MRSA was first discovered in the UK in 1961 and is now widespread, particularly in hospitals, where it is commonly known as a “superbug”. Microbiologists have so far discovered 17 strains of MRSA, which differ in their resistance to various antibiotics. Two particular strains, clones 15 and 16, are thought to be more transmissible than the others, and account for 96% of MRSA bloodstream infections in the UK.
Due to overuse of antibiotics, drug-resistant strains of Staphylococcus aureus have become more and more common since they were first spotted in the early 1960s. Poor hygiene and overcrowding might also be factors in the increased prevalence of this bug in hospitals. Busy medical staff may skip handwashing, so placing alcohol disinfectant hand-rubs next to each patient is being tried to encourage better hygiene.
MRSA infections are typically treated with vancomycin. This is the only antibiotic which is still effective against these organisms. Vancomycin is a glycopeptide antibiotic, but it has a low oral absorption and so must be injected. It’s also less effective that the standard antistaphylococcal penicillins. VRSA (vancomycin resistant Staphylococcus aureus) has also recently appeared, but is thankfully not very common at the present time.
In healthy people MRSA infections are usually asymptomatic, and individuals may carry the bacteria without any disease for anything from a few weeks to many years. This silent carriage makes it easy for the resistant forms to spread. When they encounter an environment with little competition, for example a patient whose normal bacterial flora has been disturbed by antibiotic treatment, then they can really take off.

More Information:

• Panton-Valentine Leukocidin (PVL) MRSA
• UK Health Protection Agency
• Wikipedia: MRSA

This entry was posted on Monday, December 18th, 2006 at 10:29 am and is filed under Bacteria, Biology, Health, Medicine, Microbiology, Podcast, Science. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.