Emerging Diseases

Methicillin-resistant Staphylococcus aureus (MRSA)

During the past four decades, methicillin-resistant Staphylococcus aureus, or MRSA, has evolved from a controllable nuisance into a serious public health concern. MRSA is largely a hospital-acquired infection, in fact, one of the most common. Recently, however, new strains have emerged in the community that are capable of causing severe infections in otherwise healthy people.

Staphylococcus aureus (MRSA)


During the past four decades, a type of bacteria has evolved from a controllable nuisance into a serious public health concern. This bacterium is known as methicillin-resistant Staphylococcus aureus, or MRSA. About one-third of people in the world have S. aureus bacteria on their bodies at any given time, primarily in the nose and on the skin. The bacteria can be present without causing an active infection. Of the people with S. aureus present, about 1 percent has MRSA, according to the Centers for Disease Control and Prevention (CDC).

MRSA can be categorized according to where the infection was acquired: hospital-acquired MRSA (HA-MRSA) or community-associated MRSA (CA-MRSA).

Hospital-acquired MRSA (HA-MRSA)

HA-MRSA is acquired in the hospital setting and is one of many hospital-acquired infections exhibiting increased antimicrobial resistance. HA-MRSA has increased during the past decade due to a number of factors including an increased number of immunocompromised and elderly patients; an increase in the number of invasive procedures, e.g., advanced surgical operations and life support treatments; and failures in infection control measures such as hand washing prior to patient contact and removal of non-essential catheters.

Community-associated MRSA (CA-MRSA)

CA-MRSA is caused by newly emerging strains unlike those responsible for HA-MRSA and can cause infections in otherwise healthy persons with no links to healthcare systems. CA-MRSA infections typically occur as skin or soft tissue infections, but can develop into more invasive, life-threatening infections. CA-MRSA is occurring with increasing frequency in the United States and around the world and tends to occur in conditions where people are in close physical contact, such as athletes involved in football and wrestling, soldiers kept in close quarters, inmates, childcare workers, and residents of long-term care facilities.

MRSA has attracted the attention of the medical research community, illustrating the urgent need to develop better ways to diagnose and treat bacterial infections.


The Staphyloccoccus aureus bacterium, commonly known as staph, was discovered in the 1880s. During this era, S. aureus infection commonly caused painful skin and soft tissue conditions such as boils, scalded-skin syndrome, and impetigo. More serious forms of S. aureus infection can progress to bacterial pneumonia and bacteria in the bloodstream—both of which can be fatal. S. aureus acquired from improperly prepared or stored food can also cause a form of food poisoning

In the 1940s, medical treatment for S. aureus infections became routine and successful with the discovery and introduction of antibiotic medicine, such as penicillin.

From that point on, however, use of antibiotics—including misuse and overuse—has aided natural bacterial evolution by helping the microbes become resistant to drugs designed to help fight these infections.

In the late 1940s and throughout the 1950s, S. aureus developed resistance to penicillin. Methicillin, a form of penicillin, was introduced to counter the increasing problem of penicillin-resistant S. aureus. Methicillin was one of most common types of antibiotics used to treat S. aureus infections; but, in 1961, British scientists identified the first strains of S. aureus bacteria that resisted methicillin. This was the so-called birth of MRSA.

The first reported human case of MRSA in the United States came in 1968. Subsequently, new strains of bacteria have developed that can now resist previously effective drugs, such as methicillin and most related antibiotics.

MRSA is actually resistant to an entire class of penicillin-like antibiotics called beta-lactams. This class of antibiotics includes penicillin, amoxicillin, oxacillin, methicillin, and others.

S. aureus is evolving even more and has begun to show resistance to additional antibiotics. In 2002, physicians in the United States documented the first S. aureus strains resistant to the antibiotic, vancomycin, which had been one of a handful of antibiotics of last resort for use against S. aureus. Though it is feared that this could quickly become a major issue in antibiotic resistance, thus far, vancomycin-resistant strains are still rare.


Today, S. aureus has evolved to the point where experts refer to MRSA in terms ranging from a considerable public health burden to a crisis. The bacteria have been classified into two categories based on where infection is first acquired.

Hospital-Acquired (HA)-MRSA

HA-MRSA has been recognized for decades and primarily affects people in healthcare settings, such as those who have had surgery or medical devices surgically implanted. This source of MRSA is typically problematic for the elderly, for people with weakened immune systems, and for patients undergoing kidney dialysis or using venous catheters or prosthetics.

A study published in 2005 found that nearly 1 percent of all hospital in-patient stays, or 292,045 per year, were associated with S. aureus infection. The study reviewed nearly 14 million patient discharge diagnoses from 2000 and 2001. Patients with diagnoses of S. aureus infection, when compared with those without the infection, had about three times the length of stay, three times the total cost, and five times the risk of in-hospital death. Notably, the S. aureus infections in this hospital study resulted in 14,000 deaths.

Community-Associated (CA)-MRSA

CA-MRSA has only been known since the 1990s. CA-MRSA is of great concern to public health professionals because of who it can affect. Unlike the hospital sources, which usually can be traced to a specific exposure, the origin of CA-MRSA infection can be elusive. CA-MRSA skin infections are known to spread in crowded settings; in situations where there is close skin-to-skin contact; when personal items such as towels, razors, and sporting equipment is shared; when personal hygiene is compromised; and when health care is limited.

Outbreaks of CA-MRSA have involved bacterial strains with specific microbiologic and genetic differences from traditional HA-MRSA strains, and these differences suggest that community strains might spread more easily from person to person than HA-MRSA. While CA-MRSA is resistant to penicillin and methicillin, they can still be treated with other common-use antibiotics.

CA-MRSA most often enters the body through a cut or scrape and appears in the form of a skin or soft tissue infection, such as a boil or abscess. The involved site is red, swollen, and painful and is often mistaken for a spider bite. Though rare, CA-MRSA can develop into more serious invasive infections, such as bloodstream infections or pneumonia, leading to a variety of other symptoms including shortness of breath, fever, chills, and death. CA-MRSA can be particularly dangerous in children because their immune systems are not fully developed.

You should pay attention to minor skin problems—pimples, insect bites, cuts, and scrapes—especially in children. If the wound appears to be infected, see a healthcare provider.

Researchers continue to study information about these cases in an attempt to determine why certain groups of people become ill when exposed to these strains. Researchers also continue to try to understand why high-incidence areas may appear. For example, for unknown reasons, severe outbreaks have occurred in Alaska, Georgia, and Louisiana. 


To diagnose S. aureus, a sample is obtained from the infection site and sent to a microbiology laboratory for testing. If S. aureus is found, the organism should be further tested to determine which antibiotic would be effective for treatment.

Doctors often diagnose MRSA by checking a tissue sample or nasal secretions for signs of drug-resistant bacteria. Current diagnostic procedures involve sending a sample to a lab where it is placed in a dish of nutrients that encourage bacterial growth (a culture). It takes about 48 hours for the bacteria to grow. However, newer tests that can detect staph DNA in a matter of hours are now becoming more widely available. This will help healthcare providers decide on the proper treatment regimen for a patient more quickly, after an official diagnosis has been made.

In the hospital, you might be tested for MRSA if you show signs of infection, or if you are transferred to a hospital from another healthcare setting where MRSA is known to be present. You also might be tested if you have had a previous history of MRSA.


Healthcare providers can treat many S. aureus skin infections by draining the abscess or boil and may not need to use antibiotics. Draining of skin boils or abscesses should only be done by a healthcare provider.

For mild to moderate skin infections, incision and drainage by a healthcare provider is the first-line treatment. Before prescribing antibiotics, your provider will consider the potential for antibiotic resistance. Thus, if MRSA is suspected, your provider will avoid treating you with beta-lactam antibiotics, a class of antibiotic observed not to be effective in killing the staph bacteria.

For severe infection, doctors will typically use vancomycin intravenously.


The best defense against spreading MRSA is to practice good hygiene, as follows:

  • Keep your hands clean by washing thoroughly with soap and water. Scrub them briskly for at least 15 seconds, then dry them with a disposable towel and use another towel to turn off the faucet. When you don’t have access to soap and water, carry a small bottle of hand sanitizer containing at least 62 percent alcohol.
  • Always shower promptly after exercising.
  • Keep cuts and scrapes clean and covered with a bandage until healed. Keep wounds that are draining or have pus covered with clean, dry bandages. Follow your healthcare provider’s instructions on proper care of the wound. Pus from infected wounds can contain S. aureus and MRSA, so keeping the infection covered will help prevent the spread to others. Bandages or tape can be discarded with regular trash.
  • Avoid contact with other people’s wounds or bandages.
  • Avoid sharing personal items, such as towels, washcloths, razors, clothes, or uniforms.
  • Wash sheets, towels, and clothes that become soiled with water and laundry detergent; use bleach and hot water if possible. Drying clothes in a hot dryer, rather than air-drying, also helps kill bacteria in clothes.

Tell any healthcare providers who treat you if you have or had an S. aureus or MRSA skin infection. If you have a skin infection that requires treatment, ask your healthcare provider if you should be tested for MRSA. Many healthcare providers prescribe drugs that are not effective against antibiotic-resistant staph, which delays treatment and creates more resistant germs.

Healthcare providers are fighting back against MRSA infection by tracking bacterial outbreaks and by investing in products, such as antibiotic-coated catheters and gloves that release disinfectants.


NIAID funds basic and translational research with the ultimate goal to develop and promote enhanced diagnostics, better therapeutic treatments, and new vaccines that are effective against methicillin-resistant Staphylococcus aureus (MRSA). Given the increasing prevalence of MRSA in both hospital and community settings, it is important to understand how MRSA spreads, the factors that influence the severity of disease (virulence factors), and how best to treat MRSA infections. Virulence factors can include proteins that allow the bacteria to adhere to and colonize the host, to invade host cells, to inhibit the host immune response, and to poison and damage host cells.

Community-associated (CA) strains are notably effective at causing severe infections in otherwise healthy people and are different from the strains that cause hospital infections. As these strains begin to appear in hospitals where immunocompromised patients are at risk, it becomes increasingly important to understand how CA-MRSA can colonize and invade healthy people. Some current examples of NIAID-supported MRSA research are described below.

Virulence Associated Factors of CA-MRSA

Drs. Michael Otto and Frank DeLeo, and their colleagues at NIAID’s Rocky Mountain Laboratories, recently described the essential role of the phenol-soluble modulin (PSM) protein family in CA-MRSA disease severity. These PSM proteins are able to destroy most immune cells, particularly white blood cells that help people fight off infection. While these proteins may not be the only virulence factors produced by CA-MRSA, they have been identified as major factors in the disease severity of CA-MRSA.

How CA-MRSA Spreads Among Households

Dr. Robert Daum, a researcher at the University of Chicago, is seeking to determine the best methods for containing, preventing, and treating CA-MRSA infections by understanding the circumstances that facilitate the transfer of CA-MRSA among household members. The study will determine how easily CA-MRSA is transferred from the initial infected person to other members within his/her household and at what rate household members become colonized or infected with CA-MRSA. The rate of CA-MRSA spread among household members will be compared to the rate of spread that occurs between a person infected with hospital-acquired (HA) MRSA, and other members within a household.

Strategies to Optimize the Use of Existing Antibiotics for MRSA Therapy

Two clinical trials are underway to define the optimal treatment for skin and soft tissue infections caused by CA-MRSA. CA-MRSA strains have remained more susceptible to commonly available antibiotics than HA-MRSA strains. Therefore, these trials will evaluate how effective off-patent antimicrobials are in treating uncomplicated cases of skin and soft tissue infections caused by CA-MRSA bacteria. Off-patent antimicrobials would be a cost-effective means of treating these infections and would alleviate the use of last-resort antibiotics such as vancomycin, which is essential for the treatment of HA-MRSA.




(2011). Methicillin-resistant Staphylococcus aureus (MRSA). Retrieved from http://www.eoearth.org/view/article/51cbf1ea7896bb431f6a72e7


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