What are methicillin-resistant staphylococcus aureus (MRSA) infections?

Quick Answer
An infection caused by a virulent and destructive bacteria that is resistant to common antibiotics and difficult to treat. It may be localized to one area of the body or may become systemic (spread throughout the body).
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Causes and Symptoms

Staphylococcus aureus is a bacterium commonly found on skin, especially in the nose, axilla (underarm), groin, and rectal areas. Methicillin resistance is the genetically acquired ability of some strains to withstand exposure to a class of antibiotics designed to treat staphylococci. Methicillin-resistant Staphylococcus aureus (MRSA) infections occur when a visible or microscopic break in the skin allows these bacterial organisms to enter the body. Such points of entry may be the result of injury, such as an abrasion sustained wrestling or playing football, or surgical, as in a cesarean section or a joint replacement. Needles used to inject medication or illicit drugs can also introduce these bacteria. Sometimes the source is as innocuous as a hair follicle. Occasionally the source of a bloodstream infection is never identified.

Many strains of MRSA are virulent and destructive. Infections are characterized by fever and pain as well as redness and swelling at the site. Pus may drain from the area or may build up as an abscess within infected tissue. MRSA may be invasive, meaning that it spreads deep into tissues and into the bloodstream and travels through the body infecting other sites. The most common site of infection is the skin where MRSA may cause cellulitis (infection of the skin layers), folliculitis (infection of hair follicles), or boils (abscesses complicating folliculitis). It is a common cause of foot and leg infections in patients with diabetes. Osteomyelitis (bone infection) can occur by direct invasion from an overlying skin infection or through trauma (fracture or foreign body such as shrapnel). When bacteria enter the bloodstream they may find a focus in any organ or tissue and infect that area, sometimes even forming abscesses in these secondary sites. The spine, spleen, and kidneys are common secondary sites. Bacterial endocarditis, or infection of the heart valves and linings, is a particularly dangerous form of MRSA infection and is very difficult to treat. MRSA infections can complicate surgical procedures, causing infection of the incision and sometimes invading deeper tissues or spreading systemically. MRSA infection of surgically placed foreign bodies (artificial joints, bone plates and pins, heart valves) sometimes occurs. MRSA pneumonia can occur in hospitalized patients who require respirators and is a rare but sometimes fatal complication of influenza.

Treatment and Therapy

Treatment of MRSA infections is challenging both because of the organism’s resistance to antibiotics and because of its aggressive and persistent nature. When possible, laboratory testing of the infecting organism can help guide the selection of antibiotics most likely to be effective. Specimens from the infected site (pus, infected tissue, blood) can be cultured for bacterial growth and the bacteria subjected to testing for susceptibility to a variety of antibiotics. Antibiotics that are shown in the laboratory to kill or inhibit growth of the cultured bacteria are the ones most likely to be effective in treating the infection.

The antibiotics most commonly used for treating MRSA infections are vancomycin, linezolid, daptomycin, quinupristin/dalfopristin, clindamycin, and various forms of sulfa drugs and tetracyclines. Intravenous (IV) treatment is necessary for severe infections. Some infections such as bacterial endocarditis and osteomyelitis require antibiotic treatment for six weeks or more, and relapse is not unusual. Surgical debridement to remove diseased or dead tissue or surgical drainage of an abscess may be necessary in addition to antibiotics. MRSA infection involving a foreign body (for example, a prosthetic joint) is particularly difficult to eradicate; removal of the foreign body is often recommended in addition to antibiotics.

Perspective and Prospects

Since the discovery of microorganisms as a cause of disease, Staphylococcus aureus, a common inhabitant of normal skin, has been shown capable of causing severe, life-threatening infection under some conditions. The introduction of antibiotics proved it able to adapt rapidly through mutation, as some strains became resistant first to penicillin and later to the semi-synthetic penicillins (such as methicillin) designed to treat penicillin-resistant staphylococci. Initially, these antibiotic-resistant strains were found almost entirely in hospitals and nursing homes where exposure to antibiotics and evolutionary pressure favored their development. Since the 1990s, MRSA has become widespread outside these settings. Some of these “community-acquired” strains, or CA-MRSA (as distinguished from hospital-acquired or HA-MRSA), have caused severe infections and even death in young, otherwise healthy people who became infected through minor sports injuries or following influenza. Public health authorities have instituted more stringent guidelines for cleaning athletic equipment, requiring athletes to immediate shower after practices and competitions, excluding infected athletes from participation, covering open wounds with bandages, and so on. Education and public awareness of appropriate hygiene and infection control measures are important in reducing the spread of the infection through communities. Efforts to reduce unnecessary or inappropriate antibiotic use (overprescription or premature discontinuation of a prescription) may help to prevent the further development of antibiotic-resistant bacteria. Research and development of more effective antibiotics are essential for improving cure rates for severe MRSA infections.


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