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Staphylococcus aureus Infections

(Staph Infections)

By Larry M. Bush, MD, Affiliate Professor of Clinical Biomedical Sciences;Affiliate Associate Professor of Medicine, Charles E. Schmidt College of Medicine, Florida Atlantic University;University of Miami-Miller School of Medicine

Staphylococcus aureus is the most dangerous of all of the many common staphylococcal bacteria.

  • 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.

  • Thoroughly washing the hands can help prevent spread of infection.

  • Antibiotics are chosen based on whether they are likely to be effective against the strain causing the infection.

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.

People who have the bacteria but do not have any symptoms caused by the bacteria are called carriers. People most likely to be carriers include those whose skin is repeatedly punctured or broken, such as the following:

  • People who have diabetes mellitus and have to regularly inject insulin

  • People who inject illegal drugs

  • People who are being treated with hemodialysis or chronic ambulatory peritoneal dialysis

  • People with skin infections, AIDS, or previous staphylococcal bloodstream infections

People can move the bacteria from their nose to other body parts with their hands, sometimes leading to infection.

Carriers can develop infection if they have surgery, are treated with hemodialysis or chronic ambulatory peritoneal dialysis, or have AIDS.

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.

Types of staph infections

Staphylococcus aureus infections range from mild to life threatening. The bacteria tend to infect the skin (see Overview of Bacterial 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:

  • Endocarditis: When people inject illegal drugs, have an infected blood vessel catheter, or have an artificial heart valve

  • Osteomyelitis: If Staphylococcus aureus spreads to the bone from infection in the bloodstream or from infection in adjacent 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 drugs that suppress the immune system (immunosuppressants), or when they are hospitalized because they need tracheal intubation and mechanical ventilation (see Hospital-Acquired and Health Care–Associated Pneumonia)

There are many strains of Staphylococcus aureus. Some strains produce toxins that can cause the symptoms of staphylococcal food poisoning, toxic shock syndrome, and scalded skin syndrome.

Risk factors for staph infections

Certain conditions increase the risk of getting a staphylococcal infection:

  • Influenza

  • Chronic lung disorders (such as cystic fibrosis emphysema)

  • Leukemia

  • Tumors

  • A transplanted organ, an implanted medical device, or a catheter inserted into a blood vessel for a long time

  • Burns

  • Chronic skin disorders

  • Surgery

  • Diabetes mellitus

  • Drugs, such as corticosteroids, drugs that suppress the immune system (immunosuppressants), cancer chemotherapy, or injected illegal drugs

  • Radiation therapy

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 all antibiotics that are related to penicillin (called beta-lactam antibiotics). Strains of bacteria that are resistant to beta-lactam antibiotics are called methicillin-resistant Staphylococcus aureus (MRSA).

MRSA strains are common if infection is acquired in a health care facility, and more and more infections acquired in the community, including mild abscesses and skin infections, are caused by MRSA strains.

Did You Know...

  • Staphylococcal infections may be difficult to treat because many of the bacteria have developed resistance to antibiotics.

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.

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 (causing pleurisy) 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) 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.

Prevention of Staph Infections

People can help prevent the spread of these bacteria by always thoroughly washing their hands with soap and water or with antibacterial hand sanitizer gels.

The bacteria can be eliminated from the nose by applying the antibiotic mupirocin inside the nostrils. However, because overusing 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.

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 of getting an 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 are detected, people are isolated to prevent spread of the bacteria.

Treatment of Staph Infections

  • Antibiotics

  • Sometimes surgical removal of infected bone and 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.

Infection that is acquired in a hospital is treated with antibiotics that are effective against MRSA: vancomycin, linezolid, tedizolid, quinupristin plus dalfopristin, ceftaroline, telavancin, or daptomycin. If results of testing later indicate that the strain is susceptible to methicillin and the person is not allergic to penicillin, a drug related to methicillin, such as nafcillin, is used. Depending on how severe the infection is, antibiotics may be given for weeks.

MRSA infection can be acquired outside of a health care facility. The community-acquired MRSA strains are usually susceptible to other antibiotics, such as trimethoprim-sulfamethoxazole, clindamycin, minocycline, or doxycycline, as well as to the antibiotics used to treat MRSA infections acquired in the hospital.

Mild skin infections due to MRSA, such as folliculitis, are usually treated with an ointment, such as one that contains bacitracin, neomycin, and polymyxin B (available without a prescription) or mupirocin (available by prescription only). If more than an ointment is required, antibiotics effective against MRSA are given by mouth or intravenously. Which antibiotic is used depends on the severity of the infection and the results of susceptibility testing.

If an infection involves bone or foreign material in the body (such as heart pacemakers, artificial heart valves and joints, and blood vessel grafts), rifampin is sometimes added to the antibiotic regimen. Usually, infected bone and foreign material has to be removed surgically to cure the infection.

Abscesses, if present, are usually drained.

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 in 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 pacemakers or artificial heart valves and joints).

These bacteria are often resistant to many antibiotics. Vancomycin, which is effective against many resistant bacteria, is used, sometimes with rifampin. Medical devices, if infected, often must be removed.

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