By Katja Priester

Disease, Transmission and Etiology

 Methicillin-resistant Staphylococcus aureus or MRSA, pronounced (staff-ill-oh-KOK-us AW-ree-us) is a type of staph bacteria that is resistant to many antibiotics. The etiological agent for MRSA is the root, Staphylococcus aureus.  About 1/3 or people have Staph on their skin and never get an infection.  Persons infected with MRSA are contagious, although the risk of infecting others is relatively low, and can infect other people by touching hands and by fomites (nonliving objects) such as bed-linens, medical equipment and bed-rails.[1] There are two classifications: HC-MRSA (hospital acquired) and CA-MRSA (community acquired.) Places with high public traffic like the subway, have a higher potential for MRSA contamination.[3] 


            MRSA tends to enter the body from contaminated people or surfaces through open cuts or wounds. Staph aureus is both an aerobic (oxygen loving) and anaerobic (oxygen fearing), gram-positive bacteria.[3] Under a microscope, these bacteria appear to look like grape-like clusters. Unlike a common Staph infection, MRSA will not respond to betalactam antibiotics, including methicillin, penicillin, amoxicillin and oxacillin.[2]   The coagulase adhesion produced by S. aureus causes the blood to clot by converting fibrinogen to fibrin.  Some S. aureus  bacteria that are tightly coated in fibrin are more virulent due to the resistance of phagocytosis.  The species are facultative-anaerobic and catalase positive.  Determining methicillin resistance would require a nasal swab or a swab from the infected area and observing colonization of said strains that would lack mecA.[5] MSA plates are incubated at 37 degrees Celsius for 48 hours.  Mannitol fermenting colonies will then be sub-cultured onto trypticase soy agar plates at 37 degrees Celsius.  S. aureus isolates are screened for methicillin resistance using the disc diffusion method.[2] Seen below is a small cluster of MRSA bacteria seen under a microscope:

Key Identification Tests

            Clinical presentation in MRSA is generally in the skin presenting as a fluid-filled (puss) cavity with a white or yellow center (head) that can be drained with a syringe. To obtain a culture, a biopsy or fluid drainage from the affected area would be obtained. Although skin infections are the most common presentations of MRSA, it can also manifest in the lungs in which case a sputnam culture would be necessary through a bronchoscopy or respiratory lavage; blood, in which case a blood sample would be necessary; in the urine, in which case, aseptic techniques would be used to gather a urinalysis.[2]  MRSA is very difficult to detect due to multi-cultures, one resistant to the antibiotic and one not.  Resistant bacteria tend to grow much slower and specifically at 33-35 degrees Celsius in broth or agar-based and for a full 24 hours. Anything above 35 degrees Celsius will not grow.[2]

Historical significance

            Sir Alexander Flemming first discovered the antibiotic in 1929 by, semi-accident, when he noticed that a mold-contaminated agar plate that contained staphylococcus, formed a halo where no Staph could grow.  He named the antibiotic penicillin and, unfortunately, was unable to isolate it. Ten years later, Ernst Chain and Howard Floray were successful in isolating penicillin and revolutionized modern medicine.  Shigella dysenteriae was the first multi-drug resistant microorganism discovered in 1953 and paved the way for many others, including MRSA.[4] MRSA was first discovered in 1961.[9]

Signs and Symptoms

            Some people may carry MRSA with no symptoms.  Signs or infection would usually present itself at the site of infection with redness and swelling.[6]  With skin infections, the affected site may be painful with a puss-filled center becoming worse with different stages of the infection. MRSA can be life threatening, especially if not detected early. Recognizing the signs and symptoms is crucial.[2]


            Studies suggest that CA-MRSA may be more easily spread than HC-MRSA.[2] Panton-Valentine leukocidin or PVL, is a known toxin produced by Staph and is said to be the causative agent for necrosis. CA-MRSA, also known as community acquired MRSA, tends to spread more easily due to the lack of sanitation and fomite contact. HC-MRSA, also known as hospital acquired MRSA, tends to be more controlled in that hospital sanitation guidelines are very strict.  However, those with a low immune system who undergo an operation are still at the greatest risk for developing an infection with MRSA. [7] A MRSA skin infection can spread rapidly to the blood if punctured while draining, if not treated in a timely manner and even when all precautions are taken, a person infected with MRSA may face several complications including amputation and death. S. aureus is coagulase-positive and is said to make the bacteria more virulent as it is resistant to phagocytosis. [2]


            In some cases, antibiotics may be used to treat MRSA.  In most cases, medical attention is required and the patient may need to have their abscess drained.[1]If an infection is suspected, it is critical to cover the wound, wash hands and go to the doctor.  Attempting to treat the infection without medical care can cause the infection to become life threatening and/or spread to others.[2] ]  Typically, with a small skin boil, the doctor will carefully drain it and no further treatment is needed.  This can be complicated if uninfected skin is punctured while draining, it could cause the infection to spread.  MRSA is resistant to many antibiotics.  Some antibiotics still work, such as, the first line of defense drugs,  vancomycin and bactrim.  Typical treatment would be administered intravenously.[9]


            The best personal prevention for MRSA is to keep hands clean and washed, keep wounds that are draining covered, avoid coming into contact with other people’s wounds and avoid sharing personal items (cups, razors, towels.)[2]  Healthcare facilities are to follow the standard precautions  that include wearing gloves (sometimes double), gowns, eyewear, mask, and properly handling the infected patient and all devices/instruments. There is currently no vaccine for MRSA.[1][2] One of the main causes of superbugs like MRSA is patients taking un-prescribed antibiotics, not finishing the full dose of antibiotics prescribed and in general, not following the doctor’s orders when it comes to antibiotics.  People often discontinue taking their prescribed antibiotic when they begin to feel better.  Although, they may feel better, unwanted bacteria may still be present.  Leaving those bacteria may cause them to become resistant to the antibiotics.[10]

Local Cases/Outbreaks

            MRSA is becoming a common skin infection and kills 18,000 people annually in the U.S. As a patient with a previous infection describes “I ended up with losing a third of my stomach, a third of my pancreas, about half of my small bowel, my spleen, my gallbladder and they did an appendectomy while they were in there.”[8] Many people carry Staph and can remain asymptomatic.[1] While precautions among healthcare facilities are being taken, MRSA is still a cause for alarm.  Community infections among athletes and jails remain higher.[1][2]

Global Cases/Outbreaks

            MRSA contributes to 50% of the world’s hospital Staph aureus infections around the world. Isolated cases have been recorded in Europe with an average of 40% but in the Netherlands, only 2% due to their innovative hygiene.[2] Seen below is a photo from the CDC, showing a cutaneous abcess on the hand caused by MRSA.


[1]Texas Department of State Health Services “FAQ’s about MRSA,” last updated on   06/06/2011,,  02/15/2012


[2]Center for Disease Control and Prevention “Methicillin-resistant Staphylococcus Aureus  (MRSA) Infections,” last updated on 04/15/2011,, 02/18/2012


[3] “What are the Characteristics of MRSA?” last updated on 03/12/2010 02/18/2012


[4]Textbook of Bacteriology “The Microbial World,” last updated in 2009,, 02/18/2012


[5]PubMed” Characterization of a Catalase-Negative Methicillin-Resistant Staphylococcus aureus Strain,” last updated in August 2007,, 03/02/2012


[6]New York Department of Health “Methicillin-Resistant Staphylococcus Aureus (MRSA),” Last updated on 09/2004,, 02/18/2012


[7]Webber State University “Virulence Factors of S aureus,” last updated on10/06/2004,, 02/18/2012


[8] CBS Baltimore “Guidelines May Stop MRSA Outbreaks,” last updated on 01/05/2011,, 02/18/2012


[9]WebMD ”Understanding MRSA Infection,” last updated on 03/02/2012,, 03/02/2012


[10]Mayo Clinic “Understanding MRSA—detection and treatment,” last updated on 02/04/2012,, 02/05/2012