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Staph infections are caused by Staphylococcus (staph) bacteria, which can affect many bodily systems. In the human body, these bacteria can infect the skin, lungs, bones and blood. Infections can range from relatively minor (e.g., pimples and boils on the skin) to debilitating or life-threatening (e.g., bacteremia, endocarditis). Staph infections can also cause pneumonia, which could be life-threatening to people with weakened immune systems.

Staphylococcal bacteria are named after the Greek words for a cluster of grapes (staphyle) and berry (kokkus). A staph bacterium was thus referred to as a “cluster-forming berry” because, as the round bacteria reproduce, they form grapelike clusters of microbes.

The bacteria can be found on the bodies of humans, animals and in the environment (e.g., in dust particles). They are present on the skin (including areas around the groin and anus) or mucous membranes of the nose and mouth of up to 30 percent of healthy people, according to the Centers for Disease Control and Prevention (CDC). In these cases, people carry the bacteria without developing an infection.

A healthy immune system typically prevents staph bacteria from causing infection. However, these bacteria may cause infection in people with immunosuppression (a weakened immune system). Staph bacteria usually enter the body through a break in the skin (e.g., due to a cut or scrape), or tear of the mucous membranes, the moist tissue lining cavities and passageways of the body, such as inside the nose or mouth. Bacteria can also enter the bloodstream during certain dental or medical procedures. Though staph infections typically begin in one area, they may spread through the bloodstream to other parts of the body.

After staph has entered the body, the bacteria use a variety of defenses to protect themselves from the immune system. Some types of staph bacteria use enzymes to stick to tissues, foreign bodies and each other to reduce the surface area exposed to antibodies produced by the immune system to fight infection. The bacteria may also produce a layer of slime called biofilm around clumps of bacteria that reduces the effectiveness of the antibodies. Successfully resisting the immune system enables the bacterial infection to cause symptoms.

In some cases, the body may develop more serious symptoms when the staph bacteria release toxins. This can occur when staph bacteria cause more serious conditions such as food poisoning, toxic shock syndrome or scalded skin syndrome, an infection that causes skin to be easily shed.

Staph bacteria themselves are also resilient. They can withstand high temperatures, high salt concentrations and a lack of moisture. This can make it difficult to kill the bacteria (e.g., when on objects, surfaces or in food).

Staph infections most frequently occur in hospitals and other healthcare facilities (e.g., nursing homes), where many people have weakened immune systems. Hospital-acquired staph infections include surgical wound infections, bloodstream infections (bacteremia), urinary tract infections and pneumonia. Staph infections are one of the most common causes of patients developing conditions secondary to the condition for which they were originally admitted (nosocomial infections).

Community-acquired staph infections are those obtained outside of hospitals or healthcare facilities, and among people who have not been recently hospitalized. Staph bacteria are a leading cause of community-acquired infections. They can infect the body through an injury or because of a weakened immune system. Community-acquired staph infections typically involve disorders of the skin, bones or joints.

A growing resistance to antibiotics

Some staph bacteria have developed a resistance to many medications. The best-known example is methicillin-resistant Staphylococcus aureus (MRSA, often pronounced “mursa.”) MRSA, sometimes described as a “superbug,” is resistant to certain antibiotics including penicillin, oxacillin, amoxicillin and methicillin (which is no longer commercially available in the United States), and sometimes to vancomycin.

An estimated 94,360 Americans developed a serious MRSA infection in 2005, with 18,650 related fatalities, more than the number of deaths from AIDS, the Centers for Disease Control and Prevention (CDC) reported in 2007. About 85 percent of the invasive MRSA cases involved transmission at a healthcare facility, and most of these developed later outside the facility.

After several outbreaks occurred in 2007, CDC officials described MRSA as an epidemic in some areas of the United States. However, most of these infections are minor and go away without treatment. People can help prevent MRSA and other staph infections by practicing good hygiene and not misusing antibiotics.