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MRSA Infection (cont.)

MRSA Infection Diagnosis

The diagnosis of MRSA is established by culture of the bacteria from an infected area. Any area of the skin with pus, abscesses, or blisters should be cultured for MRSA. Patients with sepsis or pneumonia should have blood cultures drawn. Pus from surgical sites, bone marrow, joint fluid, or almost any body site that may be infected should be cultured for MRSA. Unfortunately, MRSA infections look like almost any staph infection initially, so identification of MRSA strains is important for the patient and doctor to consider. What makes an infection suspicious as being MRSA is when the symptoms worsen and seem unresponsive to antibiotic treatment.

The definitive laboratory studies to diagnose a MRSA are straightforward. S. aureus is isolated and identified from the patient by standard microbiological techniques (growth on Baird-Parker agar plates and a positive coagulase test). The coagulase test is a laboratory test based upon the ability of S. aureus to produce the enzyme coagulase that ultimately leads to the formation of a blood clot. After S. aureus bacteria are isolated, the bacteria are then cultured in the presence of methicillin (and usually other antibiotics). If S. aureus grows in the presence of methicillin, the bacteria are termed MRSA. The Kirby-Bauer method (shown below) shows clear areas where various antibiotics kill bacteria; MRSA bacteria show little or no clear areas to most antibiotics tested.

Picture of a Kirby-Bauer plate showing variable-sized areas (clear areas) of where antibiotics kill bacteria
Figure 2: This Kirby-Bauer plate shows variable-sized areas (clear areas) of points at which antibiotics kill bacteria. SOURCE: CDC/Don Stalons

Carriers of MRSA are detected by swabbing the skin, nasal passages (the most likely area to be positive), or throat of asymptomatic people and performing the culture techniques described above.

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