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Staphylococcus aureus Infections

(Staph Infections)


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Reviewed/Revised Mar 2023 | Modified Sep 2023
Topic Resources

Staphylococcus aureus is the most dangerous of all of the many common staphylococcal bacteria. These gram-positive Overview of Gram-Positive Bacteria Gram-positive bacteria are classified by the color they turn after a chemical called Gram stain is applied to them. Gram-positive bacteria stain blue when this stain is applied to them. (Other... read more , sphere-shaped (coccal) bacteria (see figure ) 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.

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

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:

Staph toxins

Risk factors for staph infections

Certain conditions increase the risk of getting a staphylococcal infection:

Resistance to antibiotics

Many strains have developed resistance Antibiotic resistance Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more 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 Penicillins Penicillins are a subclass of antibiotics called beta-lactam antibiotics (antibiotics that have a chemical structure called a beta-lactam ring). Carbapenems, cephalosporins, and monobactams... read more ). 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.

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:

Some Skin Infections Caused by Staphylococcus aureus

All staphylococcal skin infections are very contagious.

Breast infections (mastitis Mastitis A breast infection ( mastitis) can occur after delivery ( postpartum infection), usually during the first 6 weeks and almost always in women who are breastfeeding. If the baby is not positioned... read more ), 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 Abscess in the Lungs A lung abscess is a pus-filled cavity in the lung surrounded by inflamed tissue and caused by an infection. A lung abscess is usually caused by bacteria that normally live in the mouth and are... read more may develop. They sometimes enlarge and involve the membranes around the lungs and sometimes cause pus to collect (called an empyema Types of fluid Types of fluid ). 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.

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 Testing a Microorganism's Susceptibility and Sensitivity to Antimicrobial Drugs Infectious diseases are caused by microorganisms, such as bacteria, viruses, fungi, and parasites. Doctors suspect an infection based on the person's symptoms, physical examination results,... read more ).

If a doctor suspects osteomyelitis, x-rays, computed tomography (CT), magnetic resonance imaging (MRI), radionuclide bone scanning Radionuclide Scanning Radionuclide scanning is a type of medical imaging that produces images by detecting radiation after a radioactive material is administered. During a radionuclide scan, a small amount of a radionuclide... read more , 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.

Infection that is acquired in a hospital is treated with antibiotics that are effective against MRSA. They include vancomycin, linezolid, tedizolid, quinupristin plus dalfopristin, ceftaroline, telavancin, daptomycin, and other antibiotics. 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 or oxacillin, 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 and possibly another antibiotic are 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.

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.

Some doctors recommend applying the antibiotic mupirocin inside the nostrils to eliminate staphylococci from the nose. 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.

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).

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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