Traditional MRSA testing involves multiple steps, including Gram staining, testing for Staphylococcus aureus, and testing for methicillin resistance. Newer methods, such as chromogenic agar plating and genetic testing, can identify MRSA directly after the Gram stain. Testing is vital for diagnosis and treatment, and to prevent further infections and outbreaks. The method used depends on the preferences of the laboratory and the number of tests performed.
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 simplify this process: chromogenic agar plating and some types of genetic testing can identify MRSA directly after the Gram stain.
The first step in MRSA testing is collection. If there’s an active infection, doctors take a sample of the bacteria or swab the wound. They may also take blood samples if they suspect that bacteria has entered the bloodstream. If the person being tested is asymptomatic, doctors typically do a nasal swab because the nasal passages are the most common location for MRSA colonies.
Next, lab scientists treat the sample with a Gram stain and examine it under a microscope. If the sample shows clustered Gram-positive cocci, it could be MRSA. The next step is to test 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 solid clot in the tube within 24 hours. In a latex agglutination test, latex beads are coated with Staphylococcus aureus antibodies and introduced into the sample. They form a lump 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 made, so lab scientists use oxacillin or cefoxitin to test for resistance. If a culture contains MRSA, it grows at the same rate in the presence of antibiotics as an untreated control sample.
The cultures required for the conventional Staphylococcus aureus identification test and the methicillin resistance test take up to 24 hours to process. Alternative methods can eliminate much of the time required 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 manufactured specifically for the identification of MRSA. If MRSA is present, blue colonies will be visible on the agar plate within 18 hours.
Additionally, genetic testing uses molecular methods to detect the gene, called mecA, that gives MRSA its methicillin resistance. In blood samples, this test can be performed after the Gram stain step. Other samples may also require the conventional Staphylococcus aureus test. Molecular methods can then be used to test for methicillin resistance. Genetic testing can be completed within hours.
The MRSA test method used depends on the preferences of the laboratory scientists. While alternative methods may provide faster results, scientists who operate laboratories where large numbers of tests are performed 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, healthcare professionals want to wait to invest in alternative testing methods until further research confirms their reliability. Conventional cultures are the most common tests for identifying MRSA because they are relatively inexpensive and have established reliability.
Testing for MRSA is vital in determining treatment for a suspected infection. Infections can look similar, so testing is the only way to be sure an infection is caused by MRSA. Once the diagnosis is confirmed, doctors can prescribe appropriate treatments. They may choose to retest for MRSA after the patient completes the prescribed course of treatment to see whether or not MRSA has colonized the patient.
The MRSA test is also done to prevent further infections. Multiple MRSA diagnoses in a group, such as a school or sports team, can signal an outbreak. It is important to note that the source, or group member who infected 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, it can be treated or educated in ways to prevent the spread of the disease.
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