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Medical Author:

Charles Patrick Davis, MD, PhD

Charles Patrick Davis, MD, PhD

Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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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.

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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