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Methicillin Resistant Staphylococcus aureus screening
Este artigo foi revisto pela última vez em
Este artigo foi modificado pela última vez em 10 de Julho de 2017.
At a Glance
Why Get Tested?
To detect MRSA carrier status
When To Get Tested?
When your doctor wants to determine if you are a MRSA carrier (you have no evidence of an active infection) or to determine if MRSA are still present after an infection has been treated with appropriate antibiotic therapy
Sample Required?
Swab of nose; occasionally swab of wound infection site or skin lesion
What is being tested?
This collection of tests detects the presence of, and sometimes evaluates the genetic characteristics of, Methicillin Resistant Staphylococcus aureus (MRSA). MRSA are strains of Staphylococcus aureus, or “staph,” bacteria that are resistant to the antibiotic methicillin and to related “beta lactam” antibiotics such as oxacillin. First appearing in the early 1960s, MRSA cases and outbreaks have been a problem in confined populations such as hospitals, prisons, and nursing homes ever since. MRSA strains have caused a significant number of severe skin, lung, bone, and heart-related staph infections that have proven difficult to treat and in some cases proven fatal. Most hospitals have instituted measures to attempt to eradicate MRSA and to control the spread of MRSA from person to person. This has been a challenge as staph is a common bacterium that colonizes the skin and in the nose of about 25-30% of the population. In the past, only about 0.8 % of the colonizing staph was MRSA, but in the last decade this has risen above 1%, and a few studies of select populations have shown MRSA colonization rates in those studied as high as 9%.

Since the 1960’s, there have been occasional outbreaks of MRSA outside of the hospital setting, but in the last few years the number of cases has greatly increased and concern among doctors and other healthcare workers is growing. In the community, MRSA is causing infections in young, previously healthy people with no apparent risk factors. Studies of these cases and outbreaks are showing that the bacteria is being spread in the community by MRSA colonized or infected people through close contact (such as sports or a day care) and through contact with contaminated objects (such as sports equipment, shared towels, razors, etc.) Often the infection it causes will be a lingering skin infection that may be first reported as a “spider bite.” Sometimes it is a lung infection that persists.

Studies have also shown that the community acquired strains of MRSA were frequently genetically distinct from those found in the hospital setting (indicating that they developed separately), resistant to antibiotics routinely prescribed to treat skin infections, and in some cases have proven to be especially virulent, producing toxins and causing an invasive infection. These strains of MRSA are now being found in the hospital as well, as infected and/or colonized patients and healthcare workers bring them into this setting.

How is the sample collected for testing?

A nasal swab is collected by rotating a swab inside of each nostril. Occasionally, a swab of a wound infection site or of a skin lesion is collected.
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Common Questions
  • How is it used?
    A MRSA screen is a test that looks solely for the presence of MRSA and no other pathogens. It is primarily used to identify the presence of MRSA in a colonized patient, or to detect if these resistant bacteria remain at a wound site after the patient has been treated for a MRSA infection. On a community level, screening may be used to help determine the source of an outbreak and on a national level used to evaluate the genetic characteristics of an identified MRSA strain.

    The most widely used test to identify MRSA colonization is the culture. This test is definitive but takes time, usually 1 to 2 days. A nasal swab is collected from the nares (nostrils) of an asymptomatic person and cultured - put into a special nutrient broth, or spread onto a nutrient gel, incubated, and then examined for the growth of characteristic MRSA colonies. Occasionally, a swab is collected from the wound site or skin lesion of a person who has been previously treated for a MRSA infection and cultured similarly.

    Some hospitals have instituted measures to control the spread of MRSA by screening those patients they feel are at risk of being a carrier (everyone or specific target groups). Healthcare workers and the family members of carriers may also be screened for MRSA. When an outbreak of MRSA occurs in the community, numerous MRSA screens may be performed to help identify the source of the infection. In some settings, such as nursing homes, a large number of people may be screened to evaluate the spread of colonization in a specific population.

    Faster methods of MRSA screening by molecular methods have been developed. These new methods test for certain genetic components of MRSA, such as the mecA gene, in the samples of possible carriers. The mecA gene confers resistance to the antibiotics methicillin, oxacillin, nafcillin, and dicloxacillin. When a staphylococcus is resistant to these antimicrobial agents, other similar antibiotics, such as the cephalosporins or amoxicillin/clavulante, are not effective. While this testing for mecA is not yet widely used, it does have the potential to detect nasal or wound carriage within hours instead of days required by culture. Research is underway to determine the utility of the rapid and more expensive molecular test.

  • When is it ordered?
    One or more MRSA screening tests may be ordered when a doctor, hospital, community health department, or researcher wants to evaluate potential MRSA colonization in an individual, their family members, and/or a group of people in the community as the source of a MRSA infection. Specific populations that have close contact, such as a soccer team, residents of a nursing home, health care workers, etc. may be tested for MRSA carrier status when an increased number of infections occur within their close group. Occasionally, a person who has been treated for MRSA infection or for MRSA colonization may be screened to determine whether MRSA is still present.
  • What does the test result mean?
    If a screen is positive for MRSA, then the patient is a carrier. If a wound site culture of a person treated for MRSA is still positive, then the bacteria are still present. If the nasal screen or wound site culture is negative, then MRSA is either not present or is present in very low numbers.

    If a molecular test confirms the presence of mecA in S. aureus, then the organism is classified as MRSA.

  • Is there anything else I should know?
    A sampling of positive MRSA tests may be subjected to further testing to help investigate the spread of MRSA within a community or region but are not often used in the treatment of an individual patient. They include pulsed-field gel electrophoresis (PFGE), which can identify the type and subtype of S. aureus strains, and DNA testing, which can be used to look at the genetic material of the microorganism and to detect the presence or absence of mecA, which confers resistance to methicillin, oxacillin and nafcillin.

    In a research setting, DNA testing is also being performed to detect the presence of the Panton-Valentine leukocidin (PVL) gene. This gene is associated with the production of a toxin that can greatly exacerbate the complications associated with MRSA infections and can frequently prove fatal.

    Public awareness of MRSA and efforts to contain its spread are growing. Doctors are being encouraged to order MRSA screening on their patients with skin infections and in any cases where they suspect a MRSA carrier. Standard courses of antibiotics and/or over-the-counter triple antibiotic ointments may be adequate to treat regular staph infections but often lead to treatment failure in patients with MRSA.

  • Can I get MRSA more than once?
    Yes, being successfully treated for MRSA colonization does not prevent you from getting it again.
  • Can I be colonized with MRSA and not know it?
    Yes, those who are carriers are frequently healthy and will not know that they have it.
  • Are different strains of MRSA always identified?
    No, not routinely. In the community and research setting, strains may be identified to evaluate their genetic characteristics, to track outbreaks, and to monitor their geographical spread.
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