Methicillin-resistant Staphylococcus aureus (MRSA) is a strain of bacteria that creates abscesses under the skin or inside the body, typically in a joint. Traditional MRSA testing is a multi-step process that involves Gram staining, testing for Staphylococcus aureus, and testing for methicillin resistance. Newer methods streamline this process: chromogenic agar plating and some kinds of genetic tests can identify MRSA directly after the Gram stain.
The first step in MRSA testing is collection. If there is an active infection, doctors take a sample of the bacteria or swab the wound. They may also take blood samples if they suspect that the bacteria have entered the bloodstream. If the person being tested is asymptomatic, doctors typically perform a nasal swab because the nasal passages are the most common location of MRSA colonies.
Next, laboratory scientists treat the sample with a Gram stain and examine it under a microscope. If the sample exhibits Gram-positive cocci in clusters, it could be MRSA. The next step is testing the sample to determine if it contains Staphylococcus aureus.
Staphylococcus aureus identity testing is done with a tube coagulase test or a latex agglutination test. In a tube coagulase test, the sample is added to a tube containing rabbit plasma and free coagulase. Staphylococcus aureus produces a firm clot in the tube within 24 hours. In a latex agglutination test, latex beads are coated with Staphylococcus aureus antibodies and introduced to the sample. They form a clump if the sample contains Staphylococcus aureus.
If the sample tests positive for Staphylococcus aureus, it is then tested for methicillin resistance to determine whether or not the sample is MRSA. Most tests involve adding an antibiotic to the culture and observing its effect. Originally, methicillin was used to test for MRSA. Methicillin is no longer produced, so laboratory scientists use oxacillin or cefoxitin to test for resistance. If a culture contains MRSA, it grows at the same rate in the presence of the antibiotics as a control sample that is left untreated.
The cultures required for the conventional Staphylococcus aureus identification test and the methicillin resistance test each take up to 24 hours to process. Alternative methods can eliminate much of the time needed for MRSA testing. These methods are more expensive and may require specialized equipment.
In chromogenic agar plating, for example, the sample is added directly to an agar medium. The agar is produced specifically for MRSA identification. If MRSA is present, blue colonies will be visible on the agar plate within 18 hours.
Additionally, genetic tests use molecular methods to detect the gene, called mecA, that gives MRSA its methicillin resistance. In blood samples, this testing can be done after the Gram staining step. Other samples may require the conventional Staphylococcus aureus test as well. Molecular methods may then be used to test for methicillin resistance. Genetic tests can be completed within hours.
The MRSA testing method that is used depends on the preferences of the laboratory scientists. While alternative methods may provide faster results, scientists who manage laboratories in which a large number of tests are run may find them prohibitively expensive. At the same time, a small number of tests may not justify the purchase of additional equipment. In other cases, health care professionals want to wait to invest in alternative testing methods until more research has confirmed their reliability. Conventional cultures are the most common tests for the identification of MRSA because they are relatively inexpensive and their reliability is established.
Testing for MRSA is critical in determining the treatment for a suspected infection. Infections can look similar, so testing is the only way to be certain that an infection is caused by MRSA. Once the diagnosis is confirmed, doctors can prescribe appropriate treatments. They may choose to test for MRSA again after the patient has completed the prescribed treatment course to test whether or not MRSA has colonized the patient.
MRSA testing is also done to prevent further infections. Multiple MRSA diagnoses in a group, such as a school or sports team, may signal an outbreak. It is important to note that the source, or the member of the group who infected the others, may not exhibit symptoms. Doctors may choose to test the entire group to try to determine the source of the outbreak. If the source is found, he or she can be treated or educated about ways to prevent the spread of the disease.
