Staphylococci are hardy, prevalent microbes. More than 30 species of staphylococci have been identified, but only a few cause human infections. Staphylococcus aureus (S. aureus) is the most common cause of staphylococcal infections. It causes conditions ranging from boils to food poisoning to toxic shock syndrome. Unique characteristics of staphylococcus make it recognizable as the cause of specific types of infections and aid in its laboratory identification.
Gram Stain Positive Cocci
In their natural state, bacteria are see-through. Therefore, they are dyed in a procedure known as a Gram stain to make them more easily visible under the microscope. Different bacteria pick up the dyes of a Gram stain differently and have distinctive shapes. Staphylococci take up the purple dye of the Gram stain in their cell walls, a feature referred to as Gram stain positivity. Staphylococci are spherically shaped and tend to group together; under the microscope, these bacteria appear as tiny grape-like clusters.
Stability in the Environment
Staphylococci have a thick cell wall--a protective layer that surrounds the bacterium. This characteristic imparts the bacteria with the capacity to survive well in the environment. Hardiness is an advantage for the bacteria, but a disadvantage in terms of controlling the spread of staphylococcal diseases. These bacteria are able to stay alive for prolonged periods outside the human body providing them ample opportunities to spread in the community.
Penicillin Resistance
Most staphylococci are resistant to penicillin, meaning the antibiotic does not kill the bacteria. Penicillin resistance is the result of beta-lactamase production. The bacteria secrete this enzyme, which inactivates penicillin before it has a chance to act against the bacteria.
Methicillin Resistance
Methicillin-resistant Staphylococcus aureus, known as MRSA, is a serious public health concern. MRSA bacteria are resistant to all penicillins and cephalosporins, another large group of antibiotics; this characteristic makes MRSA difficult to cure. The Centers for Disease Control and Prevention reports that in 2004, 63 percent of all staph infections were caused by MRSA.
Catalase Production
Staphylococci produce an enzyme called catalase, which breaks down hydrogen peroxide. This enzyme helps the bacteria counter the actions of white blood cells trying to fight the bacteria in the body. Catalase production distinguishes staphylococci from streptococci in the laboratory; staphylococci are catalase producers but streptococci are not.
Cytolytic Toxin Production
S. aureus produces and secretes toxins that dissolve and kill human cells. Alpha, beta, delta and gamma toxins are nonspecific variants that act against red blood cells, white blood cells and other cell types. Other staphylococcal toxins called leukotoxins and leukocidin act specifically on certain types of white blood cells that have a prominent role in defending the body against bacterial invasion.
Epidermolytic Toxin Production
Some S. aureus bacteria produce epidermolytic toxins (also known as exfoliatins), which cause the clinical condition staphylococcal scalded skin syndrome (SSSS). The production of epidermolytic toxin causes extensive blistering of the upper layer of the skin, the epidermis. The epidermis peels off in large pieces; the skin underneath is tender, moist and red giving it a scalded appearance. This infection is most commonly seen in infants and young children.
Enterotoxin Production
Enterotoxin-producing S. aureus is a major cause of food poisoning. The enterotoxin causes nausea, vomiting, diarrhea and intense abdominal cramps beginning within 6 hours after eating the contaminated food. The illness typically resolves within 48 hours of the onset of symptoms.
Toxic Shock Syndrome Toxin Production
Some types of S. aureus produce and secrete toxic shock syndrome toxin, which causes the illness of the same name. The toxin circulates through the bloodstream causing low blood pressure, high fever, rash, and possibly liver or kidney failure. Life-threatening cardiovascular shock may develop.
References
- "Principles and Practice of Infectious Diseases, Sixth Edition"; Gerald L. Mandell, M.D., et al, Editors; 2004
- Todar's Online Book of Bacteriology: Staphylococcus aureus and Staphylococcal Disease
- Microbiology and Immunology Online, University of South Caroline School of Medicine: Streptococcus pneumoniae and Staphylococci
- MicrobiologyBytes: Staphylococcus aureus and Escherichia coli
- Merck Manual: Staphylococcal Infections
