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What causes necrotizing fasciitis?
Bacteria cause most cases of necrotizing fasciitis; only rarely do other organisms such as fungi cause this disease. Group A
Streptococcus and Staphylococcus, either alone or with other bacteria, cause many cases of necrotizing fasciitis although
Clostridium bacteria should be considered as a cause especially if gas is found in the infected tissue. Because of better microbial isolation techniques for anaerobic bacteria, bacterial genera such as
Bacteroides, Peptostreptococcus, and Clostridium are often cultured from the infected area. Frequently, culture of tissue involved by necrotizing fasciitis also yields a mixture of other
nonanaerobic bacterial types such as
E. coli, Klebsiella, Pseudomonas, and others. Many investigators conclude that
nonanaerobic organisms damage tissue areas enough to cause local areas of hypoxia (reduced oxygen) where anaerobic organisms then can thrive and extend the infection further. This results in polymicrobial infection in which one type of bacteria aids the survival and growth of another type of bacteria (synergy). Infrequently,
Vibrio vulnificus causes the disease when a person, usually someone with liver function problems (for
example, alcoholics or immunosuppressed patients), eats contaminated seafood or a wound gets contaminated with seawater containing
Other organisms may rarely cause necrotizing fasciitis, but when they do, the resulting infections are often difficult to treat successfully. For example,
Aeromonas hydrophila (a Gram-negative rod-shaped bacterium) has recently been the source of this disease in a 24-year-old who cut her thigh in a homemade zip line accident in the U.S. The organism established itself and caused the otherwise healthy young woman to have her leg amputated, and she may suffer further complications over time. Although
Aeromonas hydrophila is usually associated with warm brackish water and causes infections in fish and amphibians, gastroenteritis is the disease it causes most often in humans when the water sources are swallowed. Because it is often resistant to multiple antibiotics, it is difficult to eradicate if it infects human tissues. In addition, once it infects tissues, its enzymes and toxins allow a rapid entrance of the organisms to the bloodstream, causing sepsis and infection of other body organs.
In general, the bacteria that cause necrotizing fasciitis utilize similar methods to cause and advance the disease. Most produce toxins that inhibit the immune response, damage or kill tissue, produce tissue hypoxia, specifically dissolve connective tissue, or do all of the above. In polymicrobic infections, one bacterial genus may produce one toxic factor (for
E. coli causing tissue hypoxia) while different types of coinfecting bacteria may produce other toxins that lyse
(disintegrate) damaged tissue cells or connective tissue. In general, this disease is not contagious, but the organisms that may lead to its development are contagious, usually by direct contact between people or items that can transfer the bacteria. People usually need a break in their skin (cut, abrasion) for these flesh-eating bacteria to cause disease.