MRSA screening

What is being tested?
This group of tests detects the presence of meticillin-resistant Staphylococcus aureus (MRSA) and sometimes assesses its genetic characteristics. MRSA are strains of Staphylococcus aureus, or “staph”, bacteria that are resistant to the antibiotic meticillin and to related beta-lactam antibiotics such as oxacillin. Since MRSA first appeared in the 1960s, cases and outbreaks have been a problem in closed populations such as hospitals, prisons and care homes. MRSA strains have caused a significant number of serious staph infections of the skin, lungs, bones and heart that have proved difficult to treat and have sometimes been fatal. Many hospitals have taken measures to try to eradicate MRSA strains and control their spread from person to person. This has been a challenge, as staph is a common bacterium that colonises the skin and nostrils of about 25-30% of the population. In the past, only about 0.8% of colonising staph were MRSA, but over the last decade there has been an increase of about 1%, and a few studies of selected populations have shown MRSA colonisation rates above 9% in those studied.

Since the 1960s there have been occasional cases of MRSA outside hospitals, but in recent years the number of cases has increased sharply, and concern among doctors and other healthcare professionals 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 show that the bacteria are spreading in the community through people who carry or are infected with MRSA, through close contact (such as in sports facilities or shelters) and through contact with contaminated objects (such as sports equipment, shared towels, razors, etc.). The infection it causes is often a persistent skin infection that may first be reported as a “spider bite”. Sometimes it is a persistent lung infection.

Studies have also shown that community-acquired MRSA strains are genetically distinct from those found in hospitals (indicating that they developed separately), are resistant to antibiotics routinely prescribed for skin infections, and in some cases have proved particularly virulent, producing toxins and causing invasive infection. These MRSA strains are now also being found in hospitals, as infected patients, carriers and healthcare workers bring them into that environment.

What sample is collected for the test?
A nasal swab is collected by rotating the swab in each nostril. Occasionally a swab is taken from an infected wound site or a skin lesion.

How is it used?
When is it requested?
What does the test result mean?
What else should I know?

How is it used?
MRSA screening is a test that looks only for MRSA and not for other pathogens. It is used mainly to identify the presence of MRSA in a patient, or to detect whether these resistant bacteria remain at the wound site after a patient has been treated for an MRSA infection. At community level, screening can be used to help determine the source of an outbreak and, at national level, to assess the genetic characteristics of an identified MRSA strain.

The most widely used test for identifying MRSA colonisation is culture. This test is definitive but takes time, usually 1 to 2 days. A nasal swab is taken from the nostrils of a person without symptoms and inoculated into a selective broth or onto nutrient agar, incubated and then examined for the growth of characteristic MRSA colonies. Occasionally a swab is taken from the wound or skin lesion of a person who has previously been treated for an MRSA infection and cultured in a similar way.

Some hospitals have taken measures to control the spread of MRSA by screening patients thought to be at risk of being carriers (all patients or specific groups). Healthcare workers and family members of carriers may also be checked for MRSA. When an MRSA outbreak occurs in the community, numerous MRSA tests may be carried out to help identify the source of infection. In some settings, such as care homes, large numbers of people may be tested to assess the spread of infection in a specific population.

Faster MRSA screening methods have been developed using molecular techniques. These new methods test for certain genetic components of MRSA, such as the mecA gene, in samples from possible carriers. The mecA gene confers resistance to the antibiotics meticillin, oxacillin, nafcillin and dicloxacillin. When a staph strain is resistant to these antimicrobials, other similar antibiotics, such as cephalosporins or amoxicillin/clavulanic acid, are not effective. Although mecA gene testing is not yet widely used, it can detect nasal or wound carriage within hours instead of the days required for culture. Research is under way to determine how useful these rapid and much more expensive molecular tests are.

When is it requested?
One or more MRSA screening tests may be requested when a doctor, hospital, health department or researcher wants to assess possible MRSA colonisation in an individual, their family members and/or groups of people in the community as a source of MRSA infection. Specific groups in close contact, such as a football team, care home residents or healthcare workers, may be tested for MRSA carriage when an increased number of infections occurs within their group. Occasionally, a person who has been treated for an MRSA infection or MRSA sepsis may be checked to see whether the MRSA strain is still present.

What does the test result mean?
If the MRSA screen is positive, the patient is a carrier. If a wound culture from a person treated for MRSA is still positive, the bacterium is still present. If the culture from the nasal swab or wound site is negative, MRSA strains are either absent or present in low numbers.
If the molecular test confirms the presence of mecA in S. aureus, the organism is classified as MRSA.

What else should I know?
A sample from a positive MRSA test may undergo further testing to help track the spread of MRSA in the community or area, but this is not often used in the treatment of an individual patient. These tests are pulsed-field gel electrophoresis (PFGE), which can identify the type and subtype of S. aureus strains, and DNA testing, which can be used to examine the organism’s genetic material and detect the presence or absence of mecA, confirming resistance to meticillin, oxacillin and nafcillin.
In research, DNA testing is also used to detect the presence of the Panton-Valentine leukocidin (PVL) gene. This gene is associated with the production of a toxin that can seriously worsen the complications associated with MRSA infections and can often prove fatal.

Public awareness of MRSA and efforts to contain its spread are increasing. Doctors are encouraged to request MRSA screening for their patients with skin infections and whenever they suspect MRSA carriage. Standard antibiotics and over-the-counter antibiotic ointments may be adequate for treating ordinary staph infections, but in patients with MRSA they often lead to treatment failure.

1. Can I be infected with MRSA strains more than once?
2. Can I be infected with MRSA without knowing it?
3. Are different MRSA strains always identified?

1. Can I be infected with MRSA strains more than once?
Yes. Having been successfully treated for an MRSA infection does not mean you cannot become ill again.

2. Can I be infected with MRSA without knowing it?
Yes. Carriers are usually healthy and do not know that they carry it.

3. Are different MRSA strains always identified?
No, not routinely. In the community and in research, strains may be identified to assess their genetic characteristics, understand outbreaks and monitor their geographical spread.