Enterobacteriaceae
Enterobacteriaceae
Enterobacteria are bacteria from the family Enterobacteriaceae, which are primarily known for their ability to cause intestinal upset. Enterobacteria are responsible for a variety of human illnesses, including urinary tract infections, wound infections, gastroenteritis , meningitis , septicemia, and pneumonia . Some are true intestinal pathogens; whereas others are merely opportunistic pests which attack weakened victims.
Most enterobacteria reside normally in the large intestine, but others are introduced in contaminated or improperly prepared foods or beverages. Several enterobacterial diseases are spread by fecal-oral transmission and are associated with poor hygienic conditions. Countries with poor water decontamination have more illness and death from enterobacterial infection. Harmless bacteria, though, can cause diarrhea in tourists who are not used to a geographically specific bacterial strain. Enterobacterial gastroenteritis can cause extensive fluid loss through vomiting and diarrhea, leading to dehydration.
Enterobacteria are a family of rod-shaped, aerobic, facultatively anaerobic bacteria. This means that while these bacteria can survive in the presence of oxygen, they prefer to live in an anaerobic (oxygen-free) environment. The Enterobacteriaceae family is subdivided into eight tribes including: Escherichieae, Edwardsielleae, Salmonelleae, Citrobactereae, Klebsielleae, Proteeae, Yersineae, and Erwineae. These tribes are further divided into genera, each with a number of species.
Enterobacteria can cause disease by attacking their host in a number of ways. The most important factors are motility, colonization factors, endotoxin, and enterotoxin . Those enterobacteria that are motile have several flagella all around their perimeter (peritrichous). This allows them to move swiftly through their host fluid. Enterobacterial colonization factors are filamentous appendages, called fimbriae, which are shorter than flagella and bind tightly to the tissue under attack, thus keeping hold of its host. Endotoxins are the cell wall components, which trigger high fevers in infected individuals. Enterotoxins are bacterial toxins which act in the small intestines and lead to extreme water loss in vomiting and diarrhea.
A number of tests exist for rapid identification of enterobacteria. Most will ferment glucose to acid, reduce nitrate to nitrite, and test negative for cytochrome oxidase. These biochemical tests are used to pin-point specific intestinal pathogens. Escherichia coli (E. coli), Shigella species, Salmonella , and several Yersinia strains are some of these intestinal pathogens.
E. coli is indigenous to the gastrointestinal tract and generally benign. However, it is associated with most hospital-acquired infections as well as nursery and travelers diarrhea. E. coli pathogenicity is closely related to the presence or absence of fimbriae on individual strains. Although most E. coli infections are not treated with antibiotics , severe urinary tract infections usually are.
The Shigella genus of the Escherichieae tribe can produce serious disease when its toxins act in the small intestine. Shigella infections can be entirely asymptomatic, or lead to severe dysentery . Shigella bacteria cause about 15% of pediatric diarrheal cases in the United States. However, they are a leading cause of infant mortality in developing countries. Only a few organisms are need to cause this fecal-orally transmitted infection. Prevention of the disease is achieved by proper sewage disposal and water chlorination , as well as personal hygiene such as handwashing. Antibiotics are only used in more severe cases.
Salmonella infections are classified as nontyphoidal or typhoidal. Nontyphoidal infections can cause gastroenteritis, and are usually due to contaminated food or water and can be transmitted by animals or humans. These infections cause one of the largest communicable bacterial diseases in the United States. They are found in contaminated animal products such as beef, pork, poultry, and raw chicken eggs. As a result, any food product that uses raw eggs, such as mayonnaise, homemade ice cream, or Caesar salad, could carry these bacteria. The best prevention when serving these dishes is to adhere strictly to refrigeration guidelines.
Typhoid Salmonella infections are also found in contaminated food and water. Typhoid Mary was a cook in New York from 1868 to 1914. She was typhoid carrier who contaminated much of the food she handled and was responsible for hundreds of typhoid cases. Typhoid fever is characterized by septicemia (blood poisoning), accompanied by a very high fever and intestinal lesions. Typhoid fever is treated with the drugs Ampicillin and Chloramphenicol.
Certain Yersinia bacteria cause one of the most notorious and fatal infections known to man. Yersinia pestis is the agent of bubonic plague and is highly fatal without treatment. The bubonic plague is carried by a rat flea and is thought to have killed at least 100 million people in the sixth century as well as 25% of the fourteenth century European population. This plague was also known as the "black death," because it caused darkened hemorrhagic skin patches. The last widespread epidemic of Y. pestis began in Hong Kong in 1892 and spread to India and eventually San Francisco in 1900. The bacteria can reside in squirrels, prairie dogs, mice, and other rodents, and are mainly found (in the U.S.) in the Southwest. Since 1960, fewer than 400 cases have resulted in only a few deaths, due to rapid antibiotic treatment.
Two less severe Yersinia strains are Y. pseudotuberculosis and Y. enterocolotica. Y. pseudotuberculosis is transmitted to humans by wild or domestic animals and causes a non-fatal disease which resembles appendicitis. Y. enterocolotica can be transmitted from animals or humans via a fecal-oral route and causes severe diarrhea.
See also Colony and colony formation; Enterobacterial infections; Infection and resistance; Microbial flora of the stomach and gastrointestinal tract