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Different Strains of Methicillin Resistant Staphylococcus Aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is an antibiotic-resistant variant of a common bacterium, Staphylococcus aureus. Since MRSA is resistant to multiple antibiotics and is quickly becoming more common, it constitutes a fast-growing challenge for healthcare providers. According to the Centers for Disease Control (CDC), in 2005 a total of 94,360 cases of invasive MRSA infection occurred in the United States, 18,650 of which were fatal. Different strains of MRSA are found, some of which are more virulent or more resistant than others.

  1. USA100-USA800

    • In 2003 the Centers for Disease Control divided more than 900 samples of Staphylococcus aureus into eight major groupings, or patterns, using a technique similar to DNA fingerprinting in humans. These strains are named USA100-USA800. Strains can also be classified using multilocus sequence typing, a procedure in which researchers compare the DNA sequence of seven genes from the bacterium and categorize strains based on sequence using ST followed by a number. for example, ST36:USA200 designates a particular strain common in hospital-acquired infections in the United States.

    USA100 Strains

    • Most hospital-acquired infections are caused by strains with the USA100 pattern. Like all MRSAs, USA100s are resistant to beta-lactam antibiotics (a class of antibiotics that includes penicillin, penicillin derivatives and cephalosporins). USA100s commonly exhibit multidrug resistance and are often resistant to erythromycin and clindamycin as well.

    USA300 Strains

    • Many MRSA infections are contracted in hospitals or other healthcare facilities; MRSA infections acquired elsewhere are called community-acquired infections (CA-MRSA). Most of these fall into the USA300 pattern. These CA-MRSA strains are unusually virulent since they express a gene for a protein toxin that destroys some white blood cells. This feature makes USA300 strains especially aggressive and potentially more dangerous to otherwise healthy individuals, although CA-MRSA strains tend to be more susceptible to antibiotics than the USA100 strains. USA300 now causes some hospital-acquired infections as well.

    USA200

    • ST36:USA200 is the second most common strain in hospital-acquired infections. Like USA100, USA200 is typically erythromycin-resistant and clindamycin-resistant, in addition to its resistance to beta-lactam antibiotics. USA200 is identical to a strain circulating in the United Kingdom, EMRSA16.

    VISA and VRSA Strains

    • Vancomycin has been the traditional drug of last resort to treat Staphylococcus aureus infections. Recently, however, some MRSA strains have emerged that are resistant to vancomycin as well. While these are still quite rare, they could very well become more common in the near future and pose a serious threat to public health since comparatively few alternatives are available to treat them. From 2002 to 2009, nine VRSA strains were reported in the United States; although all were resistant to vancomycin, they remained susceptible to other more seldom used drugs.

    Considerations

    • The rapid rise of antibiotic resistance in Staphylococcus aureus over the past 60 years is a striking demonstration of evolution in action, albeit one that from our point of view is highly undesirable. Taking preventative measures to limit the spread of infection in hospitals and curbing the overuse of antibiotics are vital to ensuring that the remaining antibiotics in medicine's arsenal maintain their potency in the years ahead.

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