Methicillin-resistant Staphylococcus aureus (MRSA) is a drug-resistant bacterium responsible for severe and potentially fatal skin and soft-tissue infections (sometimes categorized as SSTIs). It is sometimes called a superbug in newspapers and television reports. The organism is also known as multiple-resistant Staphylococcus aureus or oxacillin-resistant Staphylococcus aureus (ORSA). There are two major subgroups of the organism, named for the way in which it is acquired: community-acquired MRSA (CA-MRSA) and hospital-acquired (or healthcare-acquired) MRSA, or HA-MRSA.
Staphylococcus aureus itself is a spherical bacterium that was first identified in 1880 by a Scottish surgeon named Alexander Ogston in pus taken from infected surgical incisions. S. aureus has a golden-yellow color when grown in colonies on agar plates; its name comes from Greek and Latin words that mean “golden seed cluster.” It is a Gram-positive bacterium, which means that it retains a violet dye during the Gram staining process and looks blue or violet under a microscope. There is some evidence that the more brightly-colored strains of S. aureus are more virulent than those that are pale yellow or almost white in color. A team of experimenters in California found in 2005 that blocking the compounds that give the bacterium its golden-yellow color make it more susceptible to antibiotics and lower its ability to survive in the human bloodstream.
S. aureus can be spread by direct contact with the pus from an infected wound or by contact with towels, bed linens, clothing, or sports equipment used by an infected person. The organism can live on a dry surface for several hours after contact. It is most dangerous when it is introduced into the body through an open cut or wound, surgical procedure, indwelling catheter, or prosthetic joint. Seniors are at higher risk for MRSA infections than younger adults because they often have weakened immune systems as well as a higher likelihood of entering a hospital for tests or undergoing surgery to replace heart valves or arthritic joints. S. aureus can cause boils , rashes, and other skin infections; in addition, it can cause infections of the bones and joints, urinary tract infections, toxic shock syndrome, endocarditis (inflammation of the heart valves), food poisoning, meningitis , and pneumonia .
MRSA was identified as a particular strain of S. aureus in 1961. The bacterium's growing resistance to penicillin had been noted in the 1950s, when 40 percent of patients diagnosed with S. aureus infections were found to be difficult to treat with penicillin. By 1960, 80 percent of S. aureus strains were resistant to penicillin. Methicillin, another antibiotic similar to penicillin, was introduced in 1959 to treat penicillin-resistant strains of S. aureus, but only two years later, the first strains of MRSA were reported in the United Kingdom. MRSA infections were relatively uncommon until the 1990s, however, when their rate shot upward, particularly in hospitals.
Community-acquired MRSA, or CA-MRSA, takes the form of a boil or skin infection in about 75 percent of cases and is easily mistaken for a spider bite. The affected area is red, swollen, and may be oozing pus or a watery discharge. CA-MRSA is much more virulent than hospital-acquired MRSA, however, and can lead to sepsis (generalized infection of the entire body), bacteremia (infection of the bloodstream). or pneumonia. The reason for the virulence of CA-MRSA is that these strains of S. aureus secrete toxins that destroy white blood cells. One of these toxins is called Panton-Valentine leukocidin (PVL) and the other is called phenol-soluble modulin or PSM.
Hospital-acquired MRSA, or HA-MRSA, is most commonly found in patients in healthcare settings, particularly those in dialysis centers, nursing homes , or other hospital settings. Patients with HA-MRSA are more likely to develop such internal disorders as pneumonia, infected joints, or urinary tract infections than the skin infections that characterize CA-MRSA.
S. aureus is a commonplace organism. In humans, it can be found on the scalp, skin (particularly the armpits and genital areas), or outer nasal passages. Staph is found in 80 percent of the general population intermittently and 20–30 percent of the population on an ongoing basis. People who harbor the organism most of the time are called “staph carriers.” An estimated 2 billion people around the world carry some form of S. aureus on their bodies; of these persons, as many as 53 million, or 2.7 percent of carriers, are thought to carry MRSA. It is possible for a senior (or younger adult) to carry MRSA bacteria for many years without becoming sick; in addition, such domestic animals as cats, dogs, chickens, and horses can carry MRSA strains.
As far as is known as of 2008, people of either sex, any age group, or any race are equally likely to carry MRSA bacteria. Many newborns acquire the bacteria during delivery as they pass through the mother's birth canal. Some groups, however, are more susceptible than others to MRSA infections, including diabetics, African Americans, gay men who practice anal intercourse, and very young children as well as seniors and persons with prosthetic devices.
MRSA infections are becoming a major public health concern in the United States and Canada, as the organism is developing resistance to more and more newer antibiotics as well as such older drugs as penicillin and amoxicillin. The Centers for Disease Control and Prevention (CDC) estimates that MRSA strikes 32 out of every 100,000 Americans each year and is responsible for 95,000 serious infections and 19,000 deaths. The CDC also reported in 2007 that the number of MRSA infections treated in hospitals doubled over a recent six-year period, from approximately 127,000 infections in 1999 to 278,000 in 2005, while at the same time deaths increased from 11,000 to more than 17,000. These figures suggest that MRSA causes more deaths in the United States each year than AIDS .
The economic costs of MRSA infections are also considerable. A group of researchers at Northwestern University in Chicago reported in 2005 that “Inpatients with S. aureus infection had, on average, 3 times the length of hospital stay (14.3 vs 4.5 days), 3 times the total charges (48,824 US dollars vs 14,141 US dollars), and 5 times the risk of in-hospital death (11.2 percent vs 2.3 percent) than inpatients without this infection.”
Causes and symptoms
MRSA infections are caused by various strains of S. aureus that are resistant to methicillin (and other antibiotics) and that secrete toxins that attack white blood cells or other body tissues. There are three basic types of toxins produced by these bacteria:
- Pyrogenic toxic superantigens (PTSAgs). These toxins cause the symptoms of toxic shock syndrome and staphylococcal food poisoning.
- Exfoliative toxins. These toxins cause the person's skin to peel away in such disorders as scalded-skin syndrome, found primarily in young children.
- PVL and other white cell-killing toxins. PVL not only kills white blood cells but can also cause necrotizing pneumonia, a lung infection that can cause death within three days.
MRSA infections, as noted earlier, are usually spread by contact with materials infected by the bacteria or by contact with people who are carrying the bacteria on their bodies even though they may not be sick. Common modes of transmission include:
- The hands of health care workers.
- Entry into the body through catheters, dialysis equipment, or artificial joints or heart valves.
- Anal intercourse.
- Intravenous drug use.
- Traumatic injury leading to open cuts or scrapes, including sports injuries. There have been fatal cases of MRSA infection in athletes who suffered skinned knees or similar injuries on artificial turf.
Other risk factors for MRSA transmission include poor personal hygiene and living in overcrowded conditions.
Diagnosis of MRSA in seniors is based on a combination of the patient's medical history, symptoms, an examination of the skin or other affected body parts, and a blood culture that is positive for S. aureus. Samples for the culture may be obtained from a skin injury, from drawing a blood sample, from a urine sample, or by having the patient cough up sputum (matter from the lungs) if pneumonia is suspected. Although a standard blood culture for MRSA takes a day or two to yield results, rapid diagnostic methods using amplification and probe-based molecular techniques provide results in hours, thus allowing treatment to be started earlier and improving the patient's chances of recovery.
A complete blood cell count (CBC) will usually show an abnormally low level of white blood cells. The CDC has established the following criteria to distinguish CA-MRSA from HA-MRSA:
- The diagnosis of some form of MRSA was made by a positive blood culture in an outpatient setting or within 48 hours of admission to the hospital.
- The patient has no history of MRSA infections or of being a staph carrier.
- The patient has no history within the past year of surgery, dialysis, hospitalization, or admission to a nursing home or hospice.
- The patient has no indwelling catheters or other medical devices that pass through the skin into the body.
The doctor may order a test known as transesophageal echocardiography (TEE) for a senior who is suspected of having endocarditis caused by MRSA. TEE uses ultrasound to detect murmurs or other abnormalities in the flow of blood through the heart.
Treatment depends on the specific type of MRSA infection. In most cases the doctor will start antibiotic therapy when MRSA is suspected as soon as the sample of tissue, blood, sputum, or urine has been sent to the laboratory. Specific types of infections are treated as follows:
- Skin infections: The doctor will usually make an incision to drain the pus and other infected fluid out of the wound.
- Indwelling catheters and similar devices are removed when MRSA infection is suspected or proved. If the infection is located in a joint with a prosthetic appliance, the artificial joint must be removed.
- Infected artificial heart valves may or may not require removal. Endocarditis does, however, require long-term antibiotic therapy.
The length of the course of antibiotic treatment for MRSA infections varies, as does the doctor's choice of antibiotic. Some antibiotics that are commonly used include vancomycin, trimethoprim-sulfamethoxazole, minocycline, daptomycin, teicoplanin, and tigecycline. Treatment with vancomycin is complicated because the drug must be given intravenously and has a number of side effects. Tigecycline is a newer drug developed to treat MRSA strains that are resistant to vancomycin. Other newer drugs include linozolid, a synthetic antibiotic, and platensimycin, a new antibiotic derived from a natural source.
Nutrition and diet are not a major concern with MRSA infections unless the patient has food poisoning caused by S. aureus. If the doctor determines that the food was contaminated by the bacterium, the patient should be sure to drink plenty of liquids and take any anti-nausea drug that the doctor may prescribe.
Therapy consists of administration of appropriate antibiotics by mouth or intravenously as appropriate, with surgical removal of infected tissue or medical devices as necessary.
The prognosis of MRSA infections varies according to the specific illness. Untreated S. aureus infections of the bloodstream can have a mortality rate as high as 80 percent. Endocarditis and pneumonia caused by MRSA have mortality rates around 11 percent in patients without other diseases or disorders, but the rate may be as high as 44 percent in patients with diabetes, HIV infection, or other disorders that weaken the immune system . In patients over the age of 70, CA-MRSA is associated with a mortality rate of 21 percent in the year following diagnosis.
Most patients with MRSA-related food poisoning recover completely; fatalities are rare except in the very old.
The CDC recommends the following measures to prevent the spread of MRSA infections:
- Careful disposal of bandages or other materials that have covered infected wounds and careful laundering of towels and bed linens.
- Use of alcohol-based hand sanitizers and alcohol-based surface cleansers. MRSA organisms are easily killed by rubbing alcohol. Hospitals sometimes use quaternary ammonium to extend the antibacterial action of the alcohol.
- Testing patients for MRSA infections when they are admitted to a hospital, skilled nursing facility, or hospice.
- Isolating patients diagnosed with a MRSA infection.
- Terminal cleaning of the patient's room after discharge. In terminal cleaning, all detachable objects are removed from the room; the lighting and air duct equipment on the ceiling is sanitized; and the rest of the room is sanitized working downward from the ceiling to the floor. All objects that were removed from the room are disinfected before being returned to it.
QUESTIONS TO ASK YOUR DOCTOR
- What can I do to safeguard my household against MRSA infections?
- Should I be tested to find out whether I am a carrier?
- If I visit a friend in the hospital, am I at risk of carrying a MRSA infection back to the senior in my household?
- What are the side effects of the antibiotics prescribed for MRSA infections?
Caregivers of seniors should be concerned about the following:
- Carefully monitor all indwelling catheters or similar devices and notify the doctor at once if there are signs of infection.
- Look for signs of skin injury and be careful to keep the senior's skin clean and dry. Notify the doctor at once if even a small wound looks infected or the tissue around it seems to be dying.
- Wash hands carefully before and after giving the senior a bath or other body care, and use a hand sanitizer as well. Clean countertops and other hard surfaces with an alcohol-based cleaning agent.
- Do not use or serve food that may be contaminated, and do not prepare food for the senior if you have been diagnosed with a skin infection.
Agar —A gel-like substance derived from red seaweed that is used to make a culture medium for growing bacteria on laboratory plates.
Bacteremia —The presence of bacteria in the bloodstream.
Endocarditis —An inflammation of the tissues lining the inside of the heart and its valves.
Gram-positive —A term that refers to the amount of a crystal violet dye picked up by a bacterium during the Gram stain process. A Gram-positive organism looks blue or violet under a microscope whereas Gram-negative bacteria look red or pink. MRSA organisms are Gram-positive bacteria.
Panton-Valentine leukocidin (PVL) —A toxin produced by a virus integrated into the genetic material of S. aureus that increases the virulence of the bacterium.
Phenol-soluble modulin (PSM) —A protein toxin produced by community-acquired strains of MRSA that destroys white blood cells.
Prophylaxis —A measure intended to preserve health or prevent the spread of disease. Taking an antibiotic before oral surgery to prevent bacteria from entering the bloodstream is an example of prophylaxis.
Sepsis —The presence of bacteria or their toxic products in the bloodstream or other tissues, causing whole-body inflammation. Sepsis is a serious medical condition.
Sputum —Matter from the lungs or throat that is brought up by coughing.
Strain —A genetic variant or subtype of a bacterium (or other microorganism).
Superbug —Informal term for an antibiotic-resistant bacterium.
Virulence —The relative ability of a disease organism to overcome the body's defenses. A highly virulent organism is one that can readily overcome the immune system.
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Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA, 30333, (404) 498-1515, (800) 311-3435, http://www.cdc.gov/.
National Institute of Allergy and Infectious Diseases (NIAID), 6610 Rockledge Drive, MSC 6612, Bethesda, MD, 20892, (301) 496-5717, (866) 284-4107, (301) 402-3573, http://www3.niaid.nih.gov/.
Rebecca J. Frey Ph.D.