Staphylococci and staphylococci infections

Staphylococci are a group of Gram-positive bacteria that are members of the genus Staphylococcus. Several infections are caused by staphylococci. In particular, infections associated with methicillin-resistant Staphylococcus aureus are an increasing problem in hospitals.

The name staphyloccus is derived from Greek (staphyle—a bunch of grapes). The designation describes the typical grape-like clustered arrangement of staphylococci viewed under a light microscope . Staphylococci are divided into two groups based on the presence or absence of the plasma-clotting enzyme called coagulase . The coagulase-positive staphylococci consist mainly of Staphylococcus aureus and the coagulase-negative group consists primarily of Staphylococcus epidermidis and Staphylococcus saprophyticus. Because the treatment of infections caused by these bacteria can be different, the coagulase test provides a rapid means of indicating the identity of the bacteria of concern.

Staphylococci are not capable of movement and do not form spores. They are capable of growth in the presence and absence of oxygen. Furthermore, staphylococci are hardy bacteria, capable of withstanding elevated conditions of temperature, salt concentration, and a wide pH range. This hardiness allows them to colonize the surface of the skin and the mucous membranes of many mammals including humans.

Staphylococcus aureus is the cause of a variety of infections in humans. Many are more of an inconvenience than a threat (e.g., skin infection, infection of hair follicles, etc.). However, other infections are serious. One example is a skin infection known as scalded skin syndrome. In newborns and burn victims, scalded skin syndrome can be fatal. Another example is toxic shock syndrome that results from the infection of a tampon with a toxin-producing strain (other mechanisms also cause toxic shock syndrome). The latter syndrome can overwhelm the body's defenses, due to the production by the bacteria of what is called a superantigen. This superantigen causes a large proportion of a certain type of immune cells to release a chemical that causes dramatic changes in the physiology of the body.

Staphylococci can also infect wounds. From there, the infection can spread further because some strains of staphylococci produce an arsenal of enzymes that dissolve membranes, protein, and degrade both DNA and RNA . Thus, the bacteria are able to burrow deeper into tissue to evade the host's immune response and antibacterial agents such as antibiotics . If the infection spreads to the bloodstream, a widespread contamination of the body can result (e.g., meningitis , endocarditis, pneumonia , bone inflammation ).

Because staphylococci are resident on the skin of the hands, the bacteria can be easily transferred to objects or people. Within the past few decades the extent to which staphylococci infection of implanted devices is a cause of chronic diseases has become clear. For example, contamination of implanted heart valves and artificial hips joints is now recognized to be the cause of heart damage and infection of the bone.

Additionally, the ready transfer of staphylococci from the skin is an important reason why staphylococci infections are pronounced in settings such as hospitals. Staphylococcus aureus is an immense problem as the source of hospital-acquired infections. This is especially true when the strain of bacteria is resistant to the antibiotic methicillin and other common antibiotics. This resistance necessitates more elaborate treatment with more expensive antibiotics. Furthermore, the infection can be more established by the time the antibiotic resistance of the bacteria is determined. These so-called methicillin-resistant Staphylococcus aureus (MRSA) are resistant to only a few antibiotics currently available. The prevalence of MRSA among all the Staphylococcus aureus that is isolated in hospitals in the United States is about 50%. The fear is that the bacteria will acquire resistance to the remaining antibiotics that are currently effective. This fear is real, since the MRSA is prevalent in an environment (the hospital) where antibiotics are in constant use. Development of a fully resistant strain of Staphylococcus aureus would make treatment of MRSA infections extremely difficult, and would severely compromise health care.

Staphylococci are also responsible for the poisoning of foods (e.g., ham, poultry, potato salad, egg salad, custards). The poisoning typically occurs if contaminated food is allowed to remain at a temperature that allows the staphylococci to grow and produce a toxin. Ingestion of the toxin produces an intestinal illness and can affect various organs throughout the body.

The need for more effective prevention and treatment strategies for staphylococcal infections is urgent, given the wide variety of infections that are caused by staphylococci and the looming specter of a completely resistant staphylococcus.

See also Bacteria and bacterial infection; Infection and resistance