Methicillin-resistant Staphylococcus aureus, known as MRSA, is a subtype of the bacterium Staphylococcus aureus that is resistant to antibiotics such as methicillin and oxacillin. Because MRSA infections can be life-threatening and can spread quickly, prompt diagnosis is crucial. However, according to "Mackie and McCartney's Manual of Practical Medical Microbiology," it is not possible to recommend one single method that will reliably detect all the strains of MRSA. So, a physician should select a combination of methods based on the common bacteria prevalent in the area.
Strip Test
A medical practitioner will collect a patient's sample from the site of infection, and send it to a microbiology laboratory. There, a technician will pace a small portion of the sample on growth medium plates, which have gels that promote growth of bacteria. The plates are incubated overnight at 37 degrees Celsius. If bacteria appear on the plates the next morning, and tests show they are staphylococcus, the lab will then test for methicillin resistance.
A technical will take a small amount of Staphylococcus from the initial growth plates and place them on special growth medium plates. The technician will the place thin paper strips impregnated with the antibiotic methicillin on the plates. According to "Mackie and McCartney's Manual of Practical Medical Microbiology," the plates should be incubated at 30 degrees Celsius for 18 hours. Presence of staphylococcal growth around the methicillin strip indicates MRSA.
Latex Agglutination Test
The latex agglutination test looks for penicillin-binding protein 2, or PBP 2, which is a protein molecule found in the cell membrane of MRSA. This PBP 2 molecule is responsible for its methicillin resistance.
During this test, the technician will mix small latex particles combined with antibodies against PBP 2 with extracts of staphylococcus isolated from the primary growth medium plates of the first test. If the latex particles clump, the test results are positive.
This test can provide results in 15 minutes. A study published in the November 2001 edition of the "Journal of Clinical Microbiology" states that this latex agglutination test can rapidly and accurately determine the presence of oxacillin and methicillin resistance in Staphylococcus aureus and related species.
Mueller-Hinton Agar
A technician will place staphylococcal colonies isolated from the patient's sample on growth medium plates containing a gel known as Muller-Hilton agar supplemented with the antibiotic oxacillin and sodium chloride. The plates are incubated at 35 degrees Celsius for 24 hours. According to "Mackie and McCartney's Manual of Practical Medical Microbiology," all bacteria that grow on these plates are MRSA.
An evaluation study published in the February 2005 edition of "Journal of Antimicrobial Chemotherapy" states that although this agar screening test is less sensitive, it is easy and cheap.
References
- "Mackie and McCartney's Manual of Practical Medical Microbiology"; J. G. Collee, A. G.Fraser, B. P. Marmion and A. Simmons; 1996
- "Journal of Clinical Microbiology"; Evaluation of a Latex Agglutination Test (MRSA-Screen) for Detection of Oxacillin Resistance in Coagulase-Negative Staphylococci; November 2001.
- "Journal of Antimicrobial Chemotherapy"; Evaluation of different methods for detecting methicillin (oxacillin) resistance in Staphylococcus aureus; February 2005.
