Methicillin resistant staphylococcus aureus laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Laboratory Findings

• Complete blood count and differential count - low red cell count, leukocytosis, thrombocytopenia
• Grams staining - gram positive cocci are seen.
• C reactive protein - elevated
• ESR - elevated

• Aspiration of joint fluid for:

• Cell count
• Grams stain
• Cultures

• Liver function tests - elevated transaminases may be seen (e.g toxic shock syndrome)
• Serum creatinine - elevated

Cultures

Culture of the bacteria is the gold standard test for the diagnosis of MRSA infections. Samples used for culture include blood, urine, synovial fluid, sputum, pus from abscesses and boils. Mannitol Salt Agar is used for culture, which is a selective medium (encourages the growth of a group of certain bacteria while inhibiting the growth of others) with 7–9% NaCl that allows S. aureus to grow. This produces yellow-colored colonies as a result of salt utilization and a subsequent drop in the medium's pH.

Furthermore, for differentiation on the species level, catalase (positive for all species), coagulase (fibrin clot formation), DNAse (zone of clearance on nutrient agar), lipase (a yellow color and rancid odor smell), and phosphatase (a pink color) tests are all done. For staphylococcal food poisoning, phage typing can be performed to determine if the staphylococci recovered from the food is the source of infection.

The National Committee for Clinical Laboratory Standards (NCCLS), now called the Clinical and Laboratory Standards Institute (CLSI), recommends the cefoxitin disk screen test, the latex agglutination test for PBP2a, or a plate containing 6 μg/ml of oxacillin in Mueller-Hinton agar supplemented with NaCl (4% w/v; 0.68 mol/L) as alternative methods of testing for MRSA.

Accurate detection of oxacillin/methicillin resistance can be difficult due to the presence of two subpopulations (one susceptible and the other resistant) that may coexist within a culture of staphylococci. All cells in a culture may carry the genetic information for resistance, but only a small number may express the resistance in vitro. This phenomenon is termed heteroresistance and occurs in staphylococci resistant to penicillinase-stable penicillins, such as oxacillin.

Cells expressing heteroresistance grow more slowly than the oxacillin-susceptible population and may be missed at temperatures above 35°C. This is why CLSI recommends incubating isolates being tested against oxacillin, methicillin, or nafcillin at 33-35° C (maximum of 35°C) for a full 24 hours before reading.