Enterobacteria

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Enterobacteria

Enterobacteria are bacteria from the family Enterobacteriaceae. They live in the intestinal tract. In this natural setting, they are innocuous. However, if they contaminate food, drinking water, wounds, or environments such as the lungs, they can be the cause of illness.

Examples of human illnesses include urinary tract infections, wound infections, gastroenteritis, meningitis, septicemia, and pneumonia. Some are true intestinal pathogens, whereas others are merely opportunistic pests that attack weakened victims.

Because of their intestinal origin, enterobacterial-related illness is often the result of contamination with feces (this is commonly described as fecal-oral transmission). 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 (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 that 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 biochemical tests exist for the 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 pinpoint specific intestinal pathogens including Escherichia coli (E. coli ), Shigella, Salmonella, and several Yersinia. As well, tests that detect target sequences of genetic material or specific proteins also can quickly detect enterobacteria, even one with only a few cells present in a sample.

E. coli is indigenous to the gastorintestinal tract and is generally harmless. However, the bacterium 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 are.

Shigella can produce serious disease when the toxins it can produce (called Shiga 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 hand washing. 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 which uses raw eggs such as mayonnaise, homemade ice cream, or Caesar salad dressing can carry these bacteria. The best prevention when serving these dishes is to keep them refrigerated as much as possible, since enterobacteria do not grow at refrigeration temperatures.

Typhoid is caused by a type of Salmonella. An infamous example of the carnage inflicted by the disease is Typhoid Mary, who worked as 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 the most notorious and fatal infections known to humans. Yersinia pestis is the agent of bubonic plague and is highly fatal. 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.

KEY TERMS

Dysentery— An enterobacterial disease characterized by severe diarrhea with bloody stools.

Facultatively anaerobic— Bacteria that grow best in the absence of oxygen, but which can survive and grow in its presence.

Gastroenteritis— Infection of the bowel following ingestion of enterobacteria, which is characterized by diarrhea, fever, and abdominal pain.

Septicemia— Prolonged fever, chills, anorexia, and anemia in conjunction with tissue lesions.

Strain— A bacterial subclass of a particular tribe and genus.

The last widespread epidemic of Y. pestis beganinHong 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 mainlyfound (in the United States) 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 with bloody stools.