Infection: Staph Infection
Infection: Staph Infection
Staphylococcus is the name of a group of bacteria responsible for a number of serious illnesses, although most species are harmless to humans. Staphylococci are widespread in all parts of the world. They are commonly found in the soil as well as on the bodies of humans and domestic animals. Staphylococci can live on or in humans without necessarily causing harm. They can, however, cause disease in humans and other animals either by direct destruction of tissue or by releasing toxins into the digestive tract or bloodstream. Under a microscope, these bacteria look like clumps or clusters of grapes.
There are four species of staphylococci that cause various types of infections in humans. The two most important of these are Staphylococcus
aureus, a golden-yellow bacterium, and Staphylococcus epidermidis, a species that causes skin infections. S. aureus is generally considered the most dangerous species of staphylococcus, causing a range of infections from pneumonia and endocarditis to food poisoning and eye infections. Methicillin-resistant S. aureus,or MRSA, has become a major public health concern since the 1990s (see sidebar).
The other two types of staph that cause disease in humans are responsible for urinary tract infections in sexually active women and infections of the bones and joints.
Staphylococci are commonplace organisms found on the scalp, skin (particularly the armpits and genital areas), or outer nasal passages of humans. Biologists refer to the formation of groups or clumps of bacteria on a human or animal as colonization. Staph is found in 80 percent of the general population from time to time and 20–30 percent of the population on an ongoing basis. People who harbor staphylococci most of the time are called “staph carriers.” An estimated 2 billion people are colonized by some form of S. aureus; of these persons, as many as 53 million, or 2.7 percent of carriers, are thought to carry MRSA, the drug-resistant form of S. aureus. It is possible for a person to carry staphylococci for many years without becoming sick. In addition, such domestic animals as cats, dogs, chickens, and horses can carry MRSA strains as well as less powerful staphylococci.
As far as is known, people of either gender, any age group, or any race are equally likely to carry staphylococci. Newborns may be colonized by staphylococci from the mother during childbirth. Some groups, however, are more susceptible than others to staph infections, including diabetics, African Americans, gay men who practice anal intercourse, very young children, elderly adults, and persons with artificial joints or heart valves.
Methicillin-resistant Staphylococcus aureus (MRSA)
Methicillin-resistant Staphylococcus aureus (MRSA) is a strain of the bacterium responsible for severe and potentially fatal skin and soft-tissue infections. There are two major subgroups of MRSA, named for the locations where people can get infected: community-acquired MRSA (CAMRSA) and hospital-acquired (or healthcare-acquired) MRSA (HA-MRSA).
MRSA was first identified as a particular strain of S. aureus in 1961. Methicillin, an 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 uncommon until the 1990s, however, when their rate shot upward, particularly in hospitals.
CA-MRSA causes 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 ooze pus. CA-MRSA can be much more powerful than hospital-acquired MRSA, however, and can lead to sepsis (generalized infection of the entire body), bacteremia (infection of the bloodstream), or pneumonia. 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 pneumonia, infected joints, or urinary tract infections than skin infections.
MRSA infections cannot be treated with the antibiotics used for most staph infections. Newer drugs like vancomycin, linezolid, or tigecycline must be used instead. As of 2008 some 94,000 serious MRSA infections were reported in the United States annually and 19,000 deaths—more than are caused by AIDS.
The causes and symptoms of staph infections vary somewhat depending on the species of staph involved and the specific tissues or organs affected:
- Skin and soft tissue infections. These may be caused by either S. aureus or S. epidermidis. Skin infections caused by staph often look like spider or other insect bites. Infected wounds or surgical incisions typically ooze pus or another discharge. Many of these skin infections develop from scratching insect bites or patches of eczema. After the skin is broken, the bacteria can enter the tissues beneath the surface and form large pus-filled abscesses. In patients with weakened immune systems, the staphylococci can then enter the bloodstream.
- Endocarditis. Endocarditis is an inflammation of the valves and other tissues lining the heart, caused when staphylococci form colonies on the surface of the valves. It is more common in patients
who have damaged or artificial heart valves. A patient with staphylococcal endocarditis will run a fever, and will have a heart murmur or some other abnormality of blood flow in the heart that can be detected on an echocardiogram.
- Pneumonia. Most cases of bacterial pneumonia are caused by streptococci, but about 3 percent of cases of pneumonia acquired outside hospitals are caused by S. aureus. These patients have the typical symptoms of pneumonia: fever, chest pain, cough, and production of sputum (mucus or phlegm from the lungs).
- Joint infections. Staphylococcal infections of the joints are common complications of patients who have artificial joints.
- S. aureus and S. epidermidis are the species usually involved in joint infections. The most common symptom of a joint infection is sudden swelling and pain due to pus and tissue fluid building up in the affected joint. The patient may or may not have a fever.
- Eye infections. People can get staph infections of the tissues on the inside of the eye following eye surgery or an injury to the eye. This type of eye infection is called endophthalmitis. The early symptoms of staphylococcal endophthalmitis include pain, swelling, and redness in the affected eye and partial loss of vision within a week of the injury or operation. The patient may also have a headache and be sensitive to bright light.
- Food poisoning. Staphylococcal food poisoning is the result of toxins secreted by the organisms rather than by tissue damage caused by the bacteria themselves. The foods most likely to be contaminated are those made by hand, those that require little or no cooking—such as sandwiches, cold cuts, and certain types of pastry—and those that have not been refrigerated. The symptoms of staphylococcal food poisoning include nausea, vomiting, and diarrhea, beginning between one and six hours after eating the contaminated food. Most people with staphylococcal food poisoning feel better in one to two days.
The diagnosis of a staphylococcal infection 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 a specific staphylococcus species.
Samples for a staph 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 if pneumonia is suspected. Although a standard blood culture for a staph infection 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. Although staph can be identified in stool samples or vomit from a patient with food poisoning, doctors do not usually test for the organism unless there is an outbreak involving several people. The diagnosis of staphylococcal food poisoning is usually made on the basis of the patient's symptoms.
Treatment of a staph infection depends on its specific type and location. In most cases the doctor will start antibiotic therapy when a staphylococcal infection 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 may make an incision to drain the pus and other infected fluid out of the wound. In some cases an antibiotic cream or lotion may be applied after the wound has been cleansed, or the patient may be given oral or intravenous antibiotics.
- Staphylococcal pneumonia following influenza is usually treated with intravenous antibiotics following hospitalization. People who are seriously ill may need to be given supplemental oxygen in an intensive care unit (ICU).
- If the infection is located in a joint with a prosthetic appliance, the artificial joint must be removed and the patient given a four- to six-week course of antibiotics. Infected joints without a prosthetic appliance are usually drained of fluid and the patient is given a four-week course of antibiotic therapy to clear the infection.
- Infected artificial heart valves may or may not require removal. Endocarditis does, however, require long-term antibiotic therapy, particularly if the patient is over fifty-five.
- Staphylococcal eye infections require emergency treatment. An ophthalmologist (specialist in eye disorders) usually injects antibiotics
into the tissues around the eye and also gives antibiotics by mouth or intravenously. In extreme cases the entire eye may need to be removed.
Staphylococcal food poisoning is treated with bed rest, plenty of fluids, and anti-nausea drugs prescribed by the doctor. Antibiotics cannot be used to treat food poisoning caused by staphylococci because the toxins that cause the nausea and vomiting are not affected by these drugs. Severely ill patients may need to be hospitalized and given intravenous fluids.
The prognosis of staphylococcal 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 antibiotic-resistant staphylococci 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. Elderly people with staphylococcal pneumonia have a worse prognosis than younger adults. In patients over the age of seventy, community-acquired staph infections are associated with a mortality rate of 21 percent in the year following diagnosis.
Most patients with staphylococcal food poisoning or staphylococcal urinary tract infections recover completely; fatalities are rare except in the elderly or people with AIDS.
People can help to prevent staphylococcal infections by taking the following precautions:
- Avoid scratching insect bites or other areas of irritated skin; see a doctor about a boil filled with pus or a similar skin lesion that will not heal.
- Wash hands carefully before and after preparing food.
- Cover infected skin or skin draining pus with waterproof dressings, and dispose of soiled dressings carefully. Clean cuts and scratches promptly and keep them bandaged.
- People with infections on their hands or wrists should avoid preparing or serving food until the infection has been cleared.
- Avoid sharing such personal items as combs, brushes, cosmetics, cell phones, razors, and towels. Be particularly careful in gyms and health clubs, as staphylococci prefer warm, moist environments.
- Wipe down kitchen countertops, athletic equipment, and hospital equipment with alcohol-based sanitizers.
- Avoid direct contact with other people's wounds or injuries whenever possible.
Staph infections are likely to be common for the foreseeable future because these bacteria are widespread in all countries. Research at present is focused on finding new drugs that will be effective against MRSA and improving techniques to prevent its spread outside as well as inside hospitals.
SEE ALSO Pneumonia; Toxic shock syndrome
WORDS TO KNOW
Bacteremia: The presence of bacteria in the bloodstream.
Colonization: The process by which bacteria form colonies in or on the bodies of humans and other animals.
Endocarditis: An inflammation of the tissues lining the inside of the heart and its valves.
Endophthalmitis: Inflammation of the tissues inside the eyeball.
Sepsis: The presence of bacteria or their toxic products in the bloodstream or other tissues, leading to inflammation of the entire body.
Sputum: Matter from the lungs or throat that is brought up by coughing.
Strain: A genetic variant or subtype of a bacterium.
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