The Enterobacteriaceae are a large heterogeneous group of gram negative rods whose natural habitat is the intestinal tract of humans and animals. The family includes many genera (Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus and others).
Some enteric organisms, e.g. Escherichia coli are part of the normal flora and incidentally cause disease, while others, the Salmonellae and Shigellae are regularly pathogenic for humans. The Enterobacteriaceae are facultative anaerobes or aerobes, ferment glucose and a wide range of carbohydrates, oxidase negative and reduce nitrate to nitrite and produce a variety of toxics and other virulence factors.
Enterobacteriaceae, enteric gram-negative rods may also be called coliforms. They cause a variety of diseases with different pathogenic mechanisms. The clinical manifestations of infections with E. coli and other enteric bacteria depend on the site of infection and cannot be differentiated by symptoms or signs from processes caused by other bacteria.
Urinary tract infection: E. coli is the most common cause of urinary tract infection and account for 90% of first urinary tract infections in young women. The symptoms and signs include urinary frequency, dysuria, hematuria, and pyuria. Frank pain is associated with upper tract infection. U.T.I can result in bacteremia with clinical signs and sepsis.
E. coli-associated diarrheal diseases: E. coli that cause diarrhea are extremely common worldwide. Enteropathogenic E. coli (EPEC) is an important cause of diarrhea in infants especially in developing countries. The result of EPEC infection is watery diarrhea. Enterotoxigenic E. coli (ETEC) is a common cause of “traveler’s” diarrhea and a very important cause of diarrhea in infants in developing countries.
Care in the selection and consumption of foods potentially contaminated with ETEC is highly recommended to help prevent traveler’s diarrhea. Antimicrobial prophylaxis can be effective but may result in increased antibiotic resistance in the bacteria and probably should not be uniformly recommended.
Once diarrhea develops, antibiotic treatment effectively shortens the duration of disease. Enterohemorrhagic E. coli (EHEC) has been associated with hemorrhagic colitis, a severe form of diarrhea, and with hemolytic uremic syndrome, a disease resulting in acute renal failure, hemolytic anemia and thrombocytopenia.
Enteroinvasive E. coli (EIEC) strains are nonlactose or late lactose fermenters and are nonmotile. EIEC produce disease by invading intestinal mucosal epithelial cells. The disease occurs most commonly in children in developing countries and in travelers to these countries.
Enteroaggregative E.coli (EAEC) causes acute and chronic diarrhea (> 14 days in duration) in persons in developing countries. These organisms are also the cause of food-born illnesses in industrialized countries. They are characterized by their characteristic pattern of adherence to human cells.
Sepsis: when normal host defenses are inadequate, E. coli may reach the blood stream and cause sepsis. New borns may be highly susceptible to E. coli sepsis because they lack IgM antibodies. Sepsis may occur secondary to urinary tract infection. Meningitis: E. coli is one of the leading causes of meningitis in infants.
Specimens suspected of containing E. coli are grown on blood agar plate and on differential media such as Mac-Conkey’s agar or Ethylene methylene blue agar (EMB). E. coli which ferments lactose forms pink colonies. On EMB agar E. coli colonies have a characteristic green sheen.
E. coli can be distinguished from other lactose fermenting gram negative rods by: a. Its production of indole from Tryptophan. b. It is motile. c. It uses acetate as the only source of carbon. d. It decarboxylates lysine.
Study Questions Complete the following questions: The Enterobacteriaceae are a large heterogeneous group of gram negative rods whose natural habitat is ………….of humans and animals. The family includes many genera (………, ……., ………,…………, Klebsiella,…………)
E. coli can be distinguished from other lactose fermenting gram negative rods by: a) b) c) d)
Recommended Textbooks Manual of Clinical Microbiology, Vols. 1 and 2: Eighth Edition Patrick R. Murray