Staphylococcus aureus is the most dangerous of all of the many common staphylococcal bacteria. These gram-positive, sphere-shaped (coccal) bacteria (see figure How Bacteria Shape Up) often cause skin infections but can cause pneumonia, heart valve infections, and bone infections and may be resistant to treatment with some antibiotics.
These bacteria are spread by having direct contact with an infected person, by using a contaminated object, or by inhaling infected droplets dispersed by sneezing or coughing.
Skin infections are common, but the bacteria can spread through the bloodstream and infect distant organs.
Skin infections may cause blisters, abscesses, and redness and swelling in the infected area.
The diagnosis is based on the appearance of the skin or identification of the bacteria in a sample of the infected material.
Antibiotics are chosen based on whether they are likely to be effective against the strain causing the infection.
Thoroughly washing the hands can help prevent spread of infection.
(See also Overview of Bacteria.)
Staphylococcus aureus is present in the nose (usually temporarily) of about 30% of healthy adults and on the skin of about 20%. The percentages are higher for people who are patients in a hospital or who work there.
The bacteria can spread from person to person by direct contact, through contaminated objects (such as gym equipment, telephones, door knobs, television remote controls, or elevator buttons), or, less often, by inhalation of infected droplets dispersed by sneezing or coughing.
Carriers are people who have the bacteria but do not have any symptoms caused by the bacteria. Carriers can move the bacteria from their nose to other body parts with their hands, sometimes leading to infection. People who are hospitalized or work in a hospital are more likely to be carriers.
Types of staph infections
Staphylococcus aureus infections range from mild to life threatening.
The most common staphylococcal infections are
Skin infections, often causing abscesses
However, the bacteria can travel through the bloodstream (called bacteremia) and infect almost any site in the body, particularly heart valves (endocarditis) and bones (osteomyelitis).
The bacteria also tend to accumulate on medical devices in the body, such as artificial heart valves or joints, heart pacemakers, and catheters inserted through the skin into blood vessels.
Certain staphylococcal infections are more likely in certain situations:
Bloodstream infections: When a catheter that is inserted in a vein has remained in place for a long time
Endocarditis: When people inject illicit drugs or have an artificial heart valve or when a catheter inserted in a vein is infected
Osteomyelitis: When Staphylococcus aureus spreads to the bone from an infection in the bloodstream or from an infection in nearby soft tissue, as may occur in people with deep pressure sores or foot sores due to diabetes
Lung infection (pneumonia): When people have had influenza (particularly) or a bloodstream infection, when people are taking corticosteroids or medications that suppress the immune system (immunosuppressants), or when they are hospitalized because they need tracheal intubation and mechanical ventilation (called hospital-acquired pneumonia)
Staph toxins
There are many strains of Staphylococcus aureus. Some strains produce toxins that can cause staphylococcal food poisoning, toxic shock syndrome, or scalded skin syndrome.
Toxic shock syndrome is also caused by toxins produced by some streptococci. This syndrome causes rapidly progressive and severe symptoms that include fever, rash, dangerously low blood pressure, and failure of several organs.
Risk factors for staph infections
Certain conditions increase the risk of getting a staphylococcal infection:
Chronic lung disorders (such as cystic fibrosis or emphysema)
Tumors
A transplanted organ, an implanted medical device (such as artificial heart valves, joints, or heart pacemakers), or a catheter inserted into a vein for a long time
An open wound or sore
Chronic skin disorders
Surgery
A chronic kidney disorder requiring dialysis
Medications, such as corticosteroids, that suppress the immune system (immunosuppressants), or cancer chemotherapy
Radiation therapy
Injection of illicit drugs
Newborns and breastfeeding mothers
Resistance to antibiotics
Many strains have developed resistance to the effects of antibiotics. If carriers take antibiotics, the antibiotics kill the strains that are not resistant, leaving mainly the resistant strains. These bacteria may then multiply, and if they cause infection, the infection is more difficult to treat.
Whether the bacteria are resistant and which antibiotics they resist often depend on where people got the infection: in a hospital or other health care facility or outside of such a facility (in the community).
Methicillin-resistant Staphylococcus aureus (MRSA)
Because antibiotics are widely used in hospitals, hospital staff members commonly carry resistant strains. When people are infected in a health care facility, the bacteria are usually resistant to several types of antibiotics, including almost all antibiotics that are related to penicillin (called beta-lactam antibiotics). Strains of bacteria that are resistant to almost all beta-lactam antibiotics are called methicillin-resistant Staphylococcus aureus (MRSA). Methicillin is a type of penicillin.
MRSA strains are common when infection is acquired in a health care facility (called hospital-acquired infection). Some strains of MRSA cause infections that are acquired outside of a health care facility (called community-acquired infection), including mild abscesses and skin infections. The number of these community-acquired infections is increasing.
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Symptoms of Staph Infections
Skin infections due to Staphylococcus aureus can include the following:
Folliculitis is the least serious. A hair root (follicle) is infected, causing a slightly painful, tiny pimple at the base of a hair.
Impetigo consists of shallow, fluid-filled blisters that rupture, leaving honey-colored crusts. Impetigo may itch or hurt.
Abscesses (boils or furuncles) are warm, painful collections of pus just below the skin.
Cellulitis is infection of skin and the tissue just under it. Cellulitis spreads, causing pain and redness.
Toxic epidermal necrolysis and, in newborns, scalded skin syndrome are serious infections. Both lead to large-scale peeling of skin.
The furuncle (boil) in this photo is a tender, swollen, pus-filled area.
Photo provided by Thomas Habif, MD.
This woman has a red, inflamed furuncle under her eyebrow.
DermPics/SCIENCE PHOTO LIBRARY
In impetigo, clusters of sores rupture and develop a honey-colored crust.
Image courtesy of Thomas Habif, MD.
This child with impetigo has clusters of scabby, yellow-crusted sores.
DR P. MARAZZI/SCIENCE PHOTO LIBRARY
The furuncle (boil) in this photo is a tender, swollen, pus-filled area.
Photo provided by Thomas Habif, MD.
This woman has a red, inflamed furuncle under her eyebrow.
DermPics/SCIENCE PHOTO LIBRARY
In impetigo, clusters of sores rupture and develop a honey-colored crust.
Image courtesy of Thomas Habif, MD.
This child with impetigo has clusters of scabby, yellow-crusted sores.
DR P. MARAZZI/SCIENCE PHOTO LIBRARY
All staphylococcal skin infections are very contagious.
Breast infections (mastitis), which may include cellulitis and abscesses, can develop 1 to 4 weeks after delivery. The area around the nipple is red and painful. Abscesses often release large numbers of bacteria into the mother’s milk. The bacteria may then infect the nursing infant.
Pneumonia often causes a high fever, shortness of breath, and a cough with sputum that may be tinged with blood. Lung abscesses may develop. They sometimes enlarge and involve the membranes around the lungs and sometimes cause pus to collect (called an empyema). These problems make breathing even more difficult.
Bloodstream infection is a common cause of death in people with severe burns. Symptoms typically include a persistent high fever and sometimes shock.
Endocarditis can quickly damage heart valves, leading to heart failure (with difficulty breathing) and possibly death.
Osteomyelitis causes chills, fever, and bone pain. The skin and soft tissues over the infected bone become red and swollen, and fluid may accumulate in nearby joints.
Diagnosis of Staph Infections
For skin infections, a doctor's evaluation
For other infections, culture of blood or infected body fluids
Staphylococcal skin infections are usually diagnosed based on their appearance.
Other infections require samples of blood or infected fluids, which are sent to a laboratory to grow (culture), identify, and test the bacteria. Laboratory results confirm the diagnosis and determine which antibiotics can kill the staphylococci (called susceptibility testing).
If a doctor suspects osteomyelitis, x-rays, computed tomography (CT), magnetic resonance imaging (MRI), radionuclide bone scanning, or a combination is also done. These tests can show where the damage is and help determine how severe it is.
Bone biopsy is done to obtain a sample for testing. The sample may be removed with a needle or during surgery.
Treatment of Staph Infections
Antibiotics
Sometimes surgical removal of infected bone and/or foreign material
Infections due to Staphylococcus aureus are treated with antibiotics. Doctors try to determine whether the bacteria are resistant to antibiotics and, if so, to which antibiotics.
methicillin
Abscesses, if present, are usually drained.
Prevention of Staph Infections
People can help prevent the spread of these bacteria by always thoroughly washing their hands with soap and water or applying an alcohol-based hand sanitizer.
mupirocin can lead to mupirocin resistance, this antibiotic is used only when people are likely to get an infection. For example, it is given to people before certain operations or to people who live in a household in which the skin infection is spreading.
If carriers of staphylococci need to have certain types of surgery, they are often treated with an antibiotic before the surgery.
People with a staphylococcal skin infection should not handle food.
In some health care facilities, people are routinely screened for MRSA when they are admitted. Some facilities screen only people who are at increased risk of getting a MRSA infection, such as those who are about to have certain operations. Screening involves testing a sample taken from the nose with a cotton swab. If MRSA strains are detected, people are isolated to prevent spread of the bacteria.
Other Staphylococcal Infections
Staphylococcus aureus produces an enzyme called coagulase. Other species of staphylococci do not and thus are called coagulase-negative staphylococci. These bacteria normally reside on the skin of all healthy people.
These bacteria, although less dangerous than Staphylococcus aureus, can cause serious infections, usually when acquired in a hospital. The bacteria may infect catheters inserted through the skin into a blood vessel or implanted medical devices (such as heart pacemakers or artificial heart valves and joints).
