MRSA Infection

Reviewed on 9/30/2022

What Is a MRSA Infection?

MRSA Infection
Because MRSA is so antibiotic-resistant (drug-resistant), it is termed a "superbug" by some investigators.

MRSA is the abbreviation for methicillin-resistant Staphylococcus aureus. Staphylococcus is a group of bacteria, familiarly known as staph or staph bacteria (pronounced "staff"), that can cause a multitude of diseases as a result of infection of various tissues of the body. The distribution of S. aureus is worldwide, and therefore many people have these bacteria in their bodies, meaning they are carriers or "colonized." However, in 1959, methicillin, an antibiotic closely related to penicillin, was introduced to treat Staphylococcus and other bacterial infections. Within 1-2 years, Staphylococcus aureus bacteria (S. aureus) started to be isolated that were resistant to methicillin. These S. aureus bacteria were then termed methicillin-resistant or MRSA. MRSA bacteria usually show resistance to many antibiotics.

Because MRSA is so antibiotic-resistant (drug-resistant), it is termed a "superbug" by some investigators. This superbug is a variation of an already recognized human pathogen, S. aureus, gram-positive bacteria that occur in grape-like clusters termed cocci. The bacteria are usually found in the human armpit, groin, nose (most frequently), and throat. Fortunately, only a few people are colonized by MRSA, usually in the nose, according to the U.S. Centers for Disease Control and Prevention (CDC). In the majority of cases, the colonizing bacteria do not cause disease. However, damage to the skin or other injury (abrasion, cut, spider bite, for example) may allow the bacteria to overcome the natural protective mechanisms of the body and lead to infection; because of its ability to destroy skin, it is also one of the types of bacteria that have been termed a "flesh-eating bacterium." Unfortunately, these organisms can infect anyone, including infants, children, and adults.

MRSA is not a VRE organism (VRE means vancomycin-resistant Enterococcus species). Enterococci are bacteria that occur in the intestine. However, a strain of MRSA can be resistant to the antibiotic vancomycin (Lyphocin, Vancocin HCl, Vancocin HCl Pulvules) and these strains are termed VRSA (vancomycin-resistant Staphylococcus aureus). Plasmids (extra-chromosomal genetic material) that code for antibiotic resistance can be transferred between these two bacterial types and other types of bacteria such as Escherichia (E. coli). Also, the lay press has occasionally labeled MRSA as a virus. This is a mistake, but people still report it from time to time. Don't be confused if the term MRSA virus reappears, as it will be corrected in most instances.

Even without antibiotic resistance, S. aureus has effective means to cause infections. Bacterial strains of S. aureus can produce:

  • proteolytic enzymes (enzymes that break down proteins resulting in pus production),
  • enterotoxins (proteins that cause vomiting, diarrhea, and in some cases, shock),
  • exfoliative toxin (a protein causing skin disruption, and blisters), and
  • an exotoxin TSST-1 (a protein that can cause toxic shock syndrome).

Adding antibiotic resistance to this long list of pathogenic mechanisms (ways to cause infection) makes MRSA a formidable superbug.

How Common Is MRSA?

  • Less than 2% of the U.S. population is colonized with MRSA, and these people are called MRSA carriers.
  • The proportion of healthcare-associated staphylococcal infections that are due to MRSA (known as hospital-associated MRSA or HA-MRSA) rapidly increased from 2% in intensive care units in 1974 to 64% in 2004.
  • Approximately 126,000 hospitalizations are due to MRSA yearly.
  • Recent data suggest that MRSA causes a large percentage of all skin and soft-tissue infections.
  • Invasive (serious) MRSA infections occur in approximately 94,000 people each year and are associated with approximately 19,000 deaths, reportedly more deaths than HIV per year.
  • Of these MRSA infections that cause death, about 86% are HA-MRSA and 14% are CA-MRSA (also termed community-acquired MRSA or community-associated MRSA because these MRSA infections are acquired outside health care settings).
  • The CDC recently reported a decline in reported MRSA infections; HA-MRSA has dropped by about 28%, and CA-MRSA had dropped about 17%. These drops may be due to increased public awareness and utilization of methods to avoid transmitting these bacteria to other people.

What Causes a MRSA Infection?

MRSA bacteria can be transmitted by direct (though skin and body fluids) and indirect contact (from towels, diapers, and toys) to uninfected people.

Also, some individuals have MRSA on their body (on their skin or in their nose or throat) but show no symptoms of an infection; these people are termed MRSA carriers (see above) and can transmit MRSA to others.

Statistics show that CA-MRSA is the predominant MRSA type found in the population. Most carriers are best detected by culturing MRSA from nasal swabs.

Is MRSA Contagious?

MRSA is contagious both directly (by person-to-person contact, usually skin-to-skin contact) and indirectly (when a contaminated person touches objects such as towels, toys, or other surfaces and leaves MRSA bacteria that can be transferred to uninfected individuals):

  • Some MRSA bacteria may survive for weeks on surfaces like doorknobs, towels, furniture, and many other items.
  • Although MRSA bacteria can be included in secretion droplets put forth by infected individuals, direct contact is the usual way MRSA bacteria spread (are transmitted) to others.

The incubation period for MRSA varies from about 1-10 days; the contagious period may include the incubation period and the time it takes to eliminate an individual's MRSA infection.

Some individuals that are carriers of MRSA bacteria may be weakly contagious (meaning it is possible, but far less likely to transmit MRSA to others than people with active infection) as long as they carry the bacteria.

What Are MRSA Infection Risk Factors?

Risk factors for getting MRSA staph infections in healthy people include:

  • Playing contact sports
  • Sharing towels or other personal items
  • Having any condition that suppresses immune system function (for example, HIV, cancer, or chemotherapy)
  • Unsanitary or crowded living conditions (dormitories or military barracks)
  • Being a health care worker
  • Age

Almost anything that leads to breaks in the skin (for example, scratches, abrasions, or punctures) will increase infection risk. MRSA carriers (people colonized by MRSA bacteria but who are not symptomatic) can pass the bacteria without knowing it. Hospitalized patients are at risk of having healthcare workers accidentally transfer MRSA between patients. Unfortunately, hospitalized patients usually have sites (for example, IV lines, and surgical incision sites) that are easily contaminated with MRSA. Consequently, direct contact with MRSA organisms on surfaces or on infected people is the highest risk factor for getting MRSA infections.

What Are MRSA Infection Symptoms?

Picture of a MRSA infection on the leg. SOURCE: CDC
Figure 1: Picture of an MRSA infection on the leg. SOURCE: CDC

Symptoms of MRSA infections are variable; however, pus production is often found in the infected area. Classic examples of fluid-filled or pus-containing areas in patients are boils (pus in hair follicles), abscesses (collections of pus), carbuncles (large abscesses with pus draining), sty (pus in an eyelid gland), and impetigo (pus in blisters on the skin).

Cellulitis (infection under the skin or fatty tissue) usually does not have pus but begins with small red bumps on the skin, sometimes with itching, and also may be due to MRSA. Children and adults have many of the same symptoms. Groups such as family members, close friends, children in a daycare center, or members of an athletic team may develop these symptoms within a short time span.

The symptoms mentioned above are most often found in CA-MRSA but can also be found in HA-MRSA. When any antibiotic therapy fails, CA- and HA-MRSA should be considered as potential causes of infection.

HA-MRSA infections are usually suspected when the hospitalized patient develops signs of sepsis (fever, chills, low blood pressure, weakness, and mental deterioration), even if the patient is being treated with an antibiotic.

CA-MRSA patients who develop sepsis or pneumonia (lung infection) need immediate hospitalization. However, hospitalized patients do not need to have a primary site of MRSA infection, only a site where MRSA can invade (invasive or serious MRSA) and proliferate (for example, any surgical site, IV site, or site of an implanted device). Consequently, symptoms of pus production or signs of sepsis in any hospitalized patient, especially those with immune compromise (for example, HIV, cancer, or the elderly) could be due to MRSA.

Consequently, the symptoms and signs of an MRSA infection in or on the skin are as follows:

  • Redness and/or rash
  • Swelling
  • Pain at the site
  • Fever or warmth at the site
  • Pus and/or draining pus
  • Some patients may have itching
  • Some patients may develop fever
  • The site may present as a sore, boil, abscess, carbuncle, cellulitis, sty, or impetigo-like crusty lesions on the face or other areas
  • Antibiotic treatment does not reduce symptoms
  • More serious infections may have red streaks that progress from the site
  • Ulceration with draining pus
  • Necrotizing fasciitis (a rapidly progressing infection that destroys the tissue under the skin)

A summary of possible symptoms of a hospital-acquired MRSA infection is as follows:

When Should Someone Seek Medical Care for a MRSA Infection?

When any of the symptoms described above (boils, abscesses, carbuncles, cellulitis, sty, impetigo, or sepsis) develop, seek medical care. The CDC clearly states, "Do not attempt to treat an MRSA skin infection by yourself; doing so could worsen or spread it to others. This includes popping, draining, or using disinfectants on the area. If you think you might have an infection, cover the affected skin, wash your hands, and contact your health care provider." Readers are urged to follow this advice.

How Do Health Care Professionals Diagnose a MRSA Infection?

This Kirby-Bauer plate shows variable-sized areas (clear areas) of points at which 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

The diagnosis of MRSA is established by the 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 anybody 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 of being MRSA is when the symptoms worsen and seem unresponsive to antibiotic treatment.

The definitive laboratory studies to diagnose 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.

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.

What Are Treatments for MRSA Infections?

Antibiotic Therapy

Antibiotic therapy is still the mainstay of medical care for MRSA, but antibiotic therapy is complicated by MRSA's antibiotic resistance. Consequently, laboratory determination of MRSA antibiotic resistance and susceptibility is important in establishing effective antibiotic treatment. Definitive antibiotic therapy depends on using those antibiotics shown in microbiological tests (using Kirby-Bauer antibiotic discs on agar plates [see above diagnosis section]) to effectively reduce and stop MRSA growth. Once the antibiotic sensitivities of the patient sample are determined, the patient can be treated appropriately. Unfortunately, these tests take time (usually several days) before results are available.

If a patient has been diagnosed with an MRSA infection, as with all antibiotic therapies, it is important for them to take all antibiotics as directed; do not stop the antibiotic even if the symptoms seem to resolve before the prescribed dose is finished. Early stoppage of antibiotics can allow MRSA to survive and develop further antibiotic resistance. If initial medical care (especially antibiotic therapy) does not help to reduce or eliminate the symptoms, do not wait until the symptoms get worse; go back to a health care provider for further care.

The majority of serious MRSA infections are treated with two or more antibiotics that, in combination, often still are effective against MRSA (for example, vancomycin, linezolid [Zyvox], rifampin [Rifadin], sulfamethoxazole and trimethoprim [Bactrim, Bactrim DS, Septra, Septra DS, SMZ-TMP DS, Sulfatrim Pediatric], and others).

Minor skin infections, however, may respond well to topical mupirocin (Bactroban). The earlier the appropriate diagnosis and therapy are instituted for MRSA, the better the prognosis.

The CDC suggests that a number of different antibiotic regimens may work to help patients based on the type of infection, its severity, and the state of the patient (child, adult, pregnant, or compromised with health problems); the CDC recommends the following the guidelines published by the Infectious Diseases Society of America in 2011, which are still recommended to date.

Surgical Intervention

Drainage of pus is the main surgical treatment for MRSA infections. Items that can serve as sources of infection (tampons, intravenous lines) should be removed. Other foreign bodies present that are likely sources of infection (for example, artificial grafts, artificial heart valves, or pacemakers) may need to be removed if appropriate antibiotic therapy is unsuccessful. Other areas that can harbor MRSA and may need surgical interventions are joint infections (natural or prosthetic), postoperative abscesses, and infection of the bone (osteomyelitis). This is not an all-inclusive list; any site that continues to harbor MRSA and is not adequately treated by antibiotic therapy should be considered for surgical intervention. Drainage of pus needs to be followed by appropriate antibiotic therapy as discussed above.

Unfortunately, patients can still die from MRSA infection, even with appropriate antibiotic therapy, if the infection overwhelms the patient's defense mechanisms (immune system).

What Doctors Usually Treat MRSA Infections?

Many mild infections of MRSA can be treated by a primary care physician. However, more serious infections may require specialists in infectious disease, pulmonary care, and critical care medicine; some individuals may need a surgeon to drain deep pockets of pus and/or remove dead or dying tissue.

Is It Possible to Prevent a MRSA Infection?

The best way to avoid MRSA infection is not making direct contact with skin, clothing, or any items that come in contact with either MRSA patients or MRSA. This is often not possible because MRSA-infected individuals are not immediately identifiable, and MRSA carriers typically have no symptoms and do not know they harbor these bacteria.

A first step is excellent hygiene practices (for example, handwashing with soap after personal contact or toilet use, washing clothes potentially in contact with MRSA patients or carriers, and using disposable items such as gloves when treating MRSA patients). Hand washes like an alcohol-based hand sanitizer or rub were more effective than soap. Antiseptic solutions like Hibiclens and antiseptic wipes are available at most stores to both clean hands and surfaces that may contact MRSA. These are useful at home, in gyms, or in almost any public place such as a public restroom. As long as the infected person has viable MRSA in or on the body, they are considered contagious.

Another prevention method is to treat and cover (for example, antiseptic cream and a Band-Aid) any skin breaks.

Pregnant women need to consult with their doctors if they are infected or are carriers of MRSA. Although MRSA is not transmitted to infants by breastfeeding unless the nipple(s) are infected, there have been a few reports that infants can be infected by their MRSA-positive mothers, but this seems to be an infrequent situation. Some pregnant MRSA carriers have been successfully treated with the antibiotic mupirocin cream (Bactroban).

Caregivers of MRSA patients usually can avoid getting infected by good hygiene (handwashing, using towels, linens, and clothing that may contact the patient only once, and then washing). Disposable gloves should be used when changing dressings or when one is likely to contact body fluids, including saliva.

General screening of people is only recommended for high-risk patients who are being admitted to the hospital according to CDC guidelines. This is usually done by the infection-control group in hospitals. Some hospitals have already instituted this practice. Because MRSA infections have begun to decrease, investigators suggest this practice, along with good home care (after diagnosis and treatment), is responsible for the recent decreases in MRSA infections in the U.S.

What Is the Prognosis of MRSA Infections?

According to the U.S. National Institutes of Health, the outcome (prognosis) of MRSA infection varies according to the severity of the infection and the general condition of the person who has the infection. People with good general health who have mild CA-MRSA that is appropriately treated recover in almost every case. Mild skin infections and even some moderate infections (boils, small abscesses) can have an excellent prognosis if treated early and effectively. Other more serious or extensive MRSA infections have a range of prognoses (outcomes) from good to poor. MRSA pneumonia and sepsis (blood poisoning) have high death rates. The calculated death rate of invasive MRSA is about 20%. MRSA infections can be life-threatening.

Data are sparse on the recurrence of MRSA infections. The recurrence rate of MRSA infection in mild cases is thought to be very low, but some investigators report that patients may be carriers for up to 30 months, so it is possible for a carrier to have a contagious period for this length of time. One group of investigators reports a 21% recurrence rate in HIV patients 9 months after the initial diagnosis. Other investigators report a recurrence rate of 41% in individuals with MRSA skin infections. Most investigators agree that strict hygiene helps reduce the risk of recurrent infections.

As mentioned above, complications of MRSA can be serious and include sepsis, pneumonia, organ damage, tissue loss, and scarring due to necessary surgery. Additionally, a serious complication of antibiotic treatment is intestinal infection by the anaerobic organism Clostridium difficile. This organism and the problems it causes merit another article (see reference 4); it, too, is treatable but it may markedly extend the recovery time for an MRSA-infected patient.

MRSA and Pregnancy

If a pregnant woman is an MRSA carrier, there is no research evidence that her pregnancy will be compromised. In general, MRSA screening is not done routinely during pregnancy. However, if a woman has been diagnosed previously with MRSA and is having a planned C-section, she is at high risk for complications, has an MRSA-positive household member, or has been hospitalized in the last 3 months, she may be screened for MRSA.

Some clinicians will offer treatment to suppress the bacteria; other clinicians may not, depending on the mother's circumstances. Pregnant women who get MRSA infections are treated with antibiotics; if they pass MRSA to their infant, the baby can also be treated. Fortunately, serious MRSA infections in infants are rare. Pregnant women with MRSA infections should be treated by specialists, usually, a team consisting of an ob-gyn and infectious disease consultant, since careful choices in antibiotics and close follow-up yield the best outcomes for the mother and baby.

MRSA Infection Quiz

Methicillin-resistant Staphylococcus aureus, or MRSA, is a superbug that is resistant to several antibiotics. MRSA infections send more than 100,000 people to the hospital each year.

Take our quiz and discover you MRSA IQ.

Reviewed on 9/30/2022
Liu, C., A. Bayer, S.E. Cosgrove, et al. "Clinical Practice Guidelines by the Infectious Diseases Society of America for the Treatment of Methicillin-Resistant Staphylococcus aureus Infections in Adults and Children." Clin. Infect. Dis. 52 (2011): 285-292.

United States. Centers for Disease Control and Prevention. "Methicillin-Resistant Staphylococcus aureus (MRSA): General Information." Feb. 1, 2019. <>.

United States. Centers for Disease Control and Prevention. "Methicillin-Resistant Staphylococcus aureus (MRSA) Infections." Aug. 4, 2015. <>.

United States. Centers for Disease Control and Prevention. "Methicillin-Resistant Staphylococcus aureus (MRSA) Infections (Photos of MRSA Infections)." Sept. 10, 2013. <>.

United States. Centers for Disease Control and Prevention. "MRSA Statistics." Apr. 8, 2011. <>.

Welte, T., and M. Pletz. "Antimicrobial Treatment of Nosocomial Methicillin-Resistant Staphylococcus aureus (MRSA) Pneumonia: Current and Future Options." Int J Antimicrob Agents 36.5 (2010): 391-400.