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Understanding Methicillin-Resistant Staphylococcus aureus

Author(s): Mary Pat Bolton, MA, RD, LD
Understanding Methicillin-Resistant Staphylococcus aureus

Bacterial cells of Staphylococcus aureus, which is one of the causal agents of mastitis in dairy cows. Its large capsule protects the organism from attack by the cow's immunological defenses.
Courtesy of USDA

What is Methicillin-Resistant Staphylococcus aureus (MRSA)?

MRSA is a strain of a common bacterium, Staphylococcus aureus ("staph"). Over time, MRSA staph bacteria have developed resistance to a family of antibiotics known as beta lactams, which includes methicillin, penicillin, oxacillin, and amoxicillin. During the past four decades, several forms of MRSA that have evolved from the more easily controlled staph microbe have become major public health problems.

Staphylococcus aureus was first identified in the 1880s as a cause of boils, impetigo, and other minor skin disorders. Indeed, S. aureus bacteria are common on the skin and in the nasal passages, where they usually are harmless. However, if staph bacteria invade the body, they can cause infections. The introduction of penicillin in the 1940s gave doctors a way to treat staph infections successfully, but by the end of that decade, the S. aureus microbe had begun to develop resistance to the new antibiotic. Methicillin, a kind of penicillin, was introduced in 1959 to combat the growing problem, but British scientists identified the first methicillin-resistant strain only two years later. Methicillin is no longer used to treat staph infections, but antibiotic-resistant strains of the bacteria still are referred to as methicillin-resistant.

The first case of MRSA in the United States was reported in 1968. Since then, MRSA has become a serious cause of infections in hospitals, primarily affecting patients with weakened immune systems and those who have had recent surgeries or other medical procedures. These infections are referred to as hospital-associated MRSA (HA-MRSA) to distinguish them from infections caused by newer strains of antibiotic-resistant S. aureus that originate in community settings.

Scanning electron micrograph (SEM) of two Staphylococcus epidermis bacteria.
Photo Credit: USDA

Community-associated MRSA (CA-MRSA) developed in the 1990s, striking healthy people with no history of surgery or hospitalization. It usually appears as a skin infection, but can progress rapidly to a bloodstream infection or a serious illness called necrotizing pneumonia, in which healthy lung tissue is destroyed. This type of pneumonia occurs in only 2% of MRSA infections, but it is fatal in 75% of cases.

A recent study by the Centers for Disease Control and Prevention (CDC) found that hospital-associated strains of S. aureus are still responsible for about 85% of MRSA infections. However, aggressive community-associated strains are evolving quickly and now account for most of the skin and soft tissue MRSA infections seen in emergency rooms. The CDC study estimated that 94,360 people developed serious, invasive MRSA infections in 2005, and approximately 18,650 patients died during a hospital stay related to these infections.

Researchers are beginning to unravel the inner mechanisms of MRSA infections. For example, recent studies show that CA-MRSA secretes higher amounts of a peptide (compared with other strains of staph) that causes neutrophils (immune cells) to burst, thereby debilitating the immune system. It also produces proteins that make the microbe stickier, so it can invade tissue more easily.

How do people become infected with MRSA?

Staph bacteria cause illness when they enter the body through skin wounds, and most MRSA infections occur through direct contact with people or surfaces that carry the germ. Even people who are not made ill by the staph bacteria on their skin or in their nasal tissue can pass the bacteria to others. Scientists estimate that 25-30% of the human population is "colonized" with S. aureus, meaning that the bacteria are present, but are not causing illness. Only about 1% of carriers have methicillin-resistant strains.

Not all staph infections involve MRSA strains. In fact, in healthy people, a staph infection may be minor. But serious MRSA disease can strike anyone, regardless of age, health or environment. Outbreaks have occurred among young athletes participating in contact sports and among people living in close quarters, such as prisons, military facilities, nursing homes, and childcare centers.

Older adults and people with chronic illnesses are at heightened risk of developing serious staph infections because their immune systems are no longer strong, but the disease also can strike children and young adults whose immune systems aren't fully developed. Health care workers also have an increased risk of developing MRSA infections.

How is MRSA treated?

Many S. aureus infections can be treated without antibiotics, but this requires draining the wound, which should be done only by a health care professional. Before prescribing an antibiotic, a doctor must determine if MRSA bacteria are present. Accurate diagnosis is critical, because prescribing the wrong medication will delay the start of effective treatment, and it also can encourage the development of more resistant bacteria. New tests that can diagnose MRSA in a matter of hours are becoming more widely available.

Vancomycin is one of the few remaining treatments for hospital-associated strains of MRSA, but even this antibiotic is no longer effective in every case. Several antibiotics still work against CA-MRSA, but new strains are evolving rapidly.

How can MRSA be prevented?

  • Good hygiene is the best defense against staph infections. Wash hands frequently with soap and water, scrubbing for at least 15 seconds. Carry a hand sanitizer containing at least 62 percent alcohol for use when soap and water are not available.2
  • Pay attention to pimples, insect bites, cuts, and abrasions, especially in children and young adults. Get medical care promptly if a skin problem becomes infected. The site of a typical staph infection is red, swollen, and painful, often with pus or other drainage.2
  • Keep skin cuts clean and covered with a bandage until healed, especially if they are draining. Avoid touching another person's skin wound or bandage.2
  • If your doctor prescribes an antibiotic, take all the doses even if the infection seems to improve before the medication is gone. Do not share antibiotics or save them for later use.1
  • Avoid sharing personal items, including towels, washcloths, and razors.1,2

Visit for information about controlling MRSA infections in school settings.


  1. Centers for Disease Control and Prevention. (2005). Community-associated MRSA information for the public. Retrieved 11/1/2007, from
  2. National Institute of Allergy and Infectious Diseases. (2007). Antimicrobial (drug) resistance: methicillin-resistant Staphylococcus aureus. Retrieved 11/1/2007, from
  3. Ledform, H. (2007). Research highlights nastier form of MRSA. Nature News, Published online 1/18/2007. Retrieved 11/1/2007, from
  4. Klevins, R.M., Morrison, M.A., Nadle, J., Petit, S., Gershman, K., Ray, S., et al. (2007). Invasive methicillin-resistant Staphylococcus aureus infections in the United States. Journal of the American Medical Association,298,1763-1771.
  5. Mayo Foundation for Medical Education and Research. (2007). MRSA infection. Retrieved 11/1/2007, from