What are staphylococcal infections?

Causes and Symptoms

Under the microscope, staphylococci bacteria are observed to grow in irregular, grapelike clusters from which they derive their name. Individually, the organisms are spherical and appear purple (positive) when stained with Gram technique. Staphylococci are found predominantly living on the skin and mucous membranes of mammals and birds and usually exist in a benign symbiotic relationship with their hosts. When the barrier imposed by either the skin or mucous membranes is breached, however, staphylococci may then cause disease, assisted by a variety of enzymes and toxins that they are able to manufacture. Many strains of staphylococci, such as Staphylococcus aureus, produce coagulase, which catalyzes the formation of a clot from fibrinogen proteins in the blood. Hyaluronidases, lipases, and other proteolytic enzymes carve out a cavity within the clot, which is covered by a coagulase-generated fibrin coat, and an abscess is formed. Abscess formation is one of the hallmarks of
staphylococcal infection and can be found in virtually any organ in the body as a result of local invasion or spread to the site via the bloodstream.

Staphylococcal strains are able to produce a variety of proteins called exotoxins that have significant roles in determining the type of illness that results. Superantigens, such as toxic shock syndrome
toxin-1 (TSST-1), target the circulatory system and can markedly lower blood pressure. TSST-1 producing strains have caused disease not only by growing in vaginal tampons but also while simultaneously causing postsurgical wound infections. Food poisoning
results after ingesting food that is contaminated with staphylococcal strains that produce enterotoxins. The toxins are produced in the food after contamination from the colonized noses or infected skin of food handlers through sneezing or direct contact. The enterotoxins are relatively heat-stable and do not result in any unusual taste, odor, or appearance of the food. Abdominal cramps, nausea, vomiting, and diarrhea occur one to six hours after ingestion of the contaminated food containing the toxin. This is not a true infection, as it is the preformed toxin that produces the illness. Exfoliative toxin is produced by the
strains causing scalded skin syndrome. The illness is manifested by fever and reddened skin that subsequently peels off.

Staphylococci are able to form a variety of cytotoxins that damage the membranes of bodily tissues. These toxins are able to destroy red and white blood cells as well as organ cells. Leukocytolytic activity from staphylococci was first reported in 1932 and the Panton-Valentine leukocidin (PVL) was named in honor of these scientists. PVL is produced by strains of S. aureus causing skin and soft tissue infection and pneumonia in the community (outside the hospital).

S. aureus is the preeminent pathogen causing infection in hospitalized patients. It is the most common cause of surgical wound infections. It is joined by another species, S. epidermidis, which can produce slime enabling the bacteria to adhere to the surfaces of medical devices such as vascular catheters, central nervous system
shunts, artificial heart valves, and prosthetic joints. Together, these two species account for a large number of hospital-acquired infections.

Infection of skeletal muscle and contiguous tissues is called pyomyositis or necrotizing fasciitis. This type of infection has been common in developing countries with tropical climates, but since the 1970s it has been seen more frequently in modern countries with temperate climates. The pathogenesis of pyomyositis is not completely understood. Some cases follow trauma, and there are a number of risk factors, such as intravenous drug abuse, human immunodeficiency virus (HIV) infection, and skin diseases. Other cases occur in healthy individuals without any apparent risk factor. Recently, virulent strains of community-acquired methicillin-resistant S. aureus (MRSA) have caused pyomyositis in the United States.

S. saprophyticus is an important cause of urinary tract infections in young women. The infection may be manifested by cloudy or blood-tinged urine and dysuria. It is associated with sexual intercourse or swimming. Various adhesions and the enzyme urease contribute to its ability to produce infection of the urinary tract.

Treatment and Therapy

Specific treatment of a staphylococcal infection hinges on administering an effective antibiotic. MRSA is a type of S. aureus that is resistant to antibiotics called beta-lactams, which include methicillin and other related, commonly used antibiotics. Vancomycin, an antibiotic developed in the 1950s, has remained effective for nearly all strains, but it must be given intravenously and has some serious potential side effects. Some newer antibiotics, daptomycin and linezolid, are effective against MRSA and have been used successfully to treat infections.

Antibiotics are not the only measure necessary to cure these infections. Surgical or percutaneous catheter drainage of abscesses, surgical debridement of dead tissue, and the removal of medical devices or protheses are often necessary. Other medical supportive modalities, such as fluid replacement, vasopressors, or mechanical ventilation, may be required.

Perspective and Prospects

The Centers for Disease Control and Prevention (CDC) is collaborating with other medical organizations to develop and promote strategies to reduce the transmission of staphylococci, primarily MRSA, in both health care and community settings. The CDC has also launched a campaign to prevent antimicrobial resistance
by educating both the public and health care providers about unnecessary and inappropriate antibiotic usage. Legislative efforts have been directed at eliminating the use of antibiotics in animal feed to assist in preventing resistance.

Newer antimicrobial agents continue to be developed, but the stream has slowed. Molecular-based treatments directed toward toxins and other virulence factors are being actively pursued.

Bibliography

Crossley, Kent B., and Gordon L. Archer, eds. The Staphylococci in Human Disease. 2d ed. Hoboken, N.J.: Wiley-Blackwell, 2010.

Kasper, Dennis L., et al., eds. Harrison’s Principles of Internal Medicine. 16th ed. New York: McGraw-Hill, 2005.

Koneman, Elmer W. The Other End of the Microscope: The Bacteria Tell Their Own Story. Washington, D.C.: ASM Press, 2002.

Mayo Clinic. "Staph Infections." Mayo Clinic, June 9, 2011.

McCoy, Krisha. "Methicillin-Resistant Staph Infection." Health Library, September 30, 2012.

MedlinePlus. "Staphylococcal Infections." MedlinePlus, April 19, 2013.