Escherichia

GENERAL CONCEPTS:

Members of the genus Escherichia are common bacteria that colonize the human large intestine. Most are opportunistic normal flora but some are potent pathogens.
Transmission of diarrheal disease is generally person to person, usually related to hygiene, food processing and sanitation.
Four general categories of pathogenic E. coli are recognized:
1. Enterotoxigenic (ETEC)
2. Enteroinvasive or "Shigella-like" (EIEC)
3. Enteropathogenic (EPEC)
4. Enterohemorrhagic (EHEC)
Different groups are most often delineated by serology, in particular, by the immunogenic character of the O (somatic, LPS) and H (flagellar) antigens.

DISTINCTIVE PROPERTIES:

Escherichia are Gram-negative bacilli that ferment lactose. Most are motile by peritrichious flagella.
Escherichia possess a typical Gram-negative cell wall containing LPS.
Approximately 170 different O antigens have been delineated and some of these are cross-reactive with Shigella, Salmonella and Klebsiella.
Motile strains possess H (flagellar) antigens that can be used for epidemiologic purposes.
Escherichia also possess K (capsular) antigens similar to the Vi antigen of Salmonella.
Enterotoxigenic strains may also display colonization factor antigens (CFA/I, CFA/II).

PATHOGENESIS:

E. coli diarrhea is generally acquired via ingestion of water or food that has been contaminated by an infected person. The following table outlines the four major classifications:

Classification (Strain)	Site of Infection	Disease(s)	Pathogenic Mechanism(s)
Enterotoxigenic (ETEC)	Small intestine	Traveler's diarrhea Watery stool, cramps, nausea, low fever	Enterotoxins ST and LT (click here to view the mechanisms.)
Enteroinvasive (EIEC)	Large intestine	Shigella-like diarrhea Fever, cramps, watery diarrhea followed by scant, bloody stool	Tissue invasion and destruction of epithelial cells (plasmid-mediated)
Enteropathogenic (EPEC)	Small intestine	Infantile diarrhea Salmonella-like with fever, nausea, vomiting	Adherence and destruction of epithelial cells (plasmid-mediated)
Enterohemorrhagic (EHEC, O157:H7)	Large intestine	Hemorrhagic colitis Severe abdominal pain, watery diarrhea followed by grossly bloody stool	SLT-I, SLT-II cytotoxins ("verotoxins")

HOST DEFENSES:

A number of factors help prevent infection by E. coli. These include gastric acidity, intestinal motility and the normal intestinal flora.
Some evidence suggests a possible genetic component because the bacterial fimbriae are used to attach to specific cellular receptors.
In addition, breast milk contains neutralizing (non-immunoglobulin) factors that help to prevent disease in nursing infants.

EPIDEMIOLOGY:

E. coli is a human organism and, as a consequence, is transmitted from person to person via contaminated food or water.
E. coli is also a very commonly acquired nosocomial infection (this is described on the next page).

DIAGNOSIS:

Clinical: Generally, clinical diagnosis of E. coli infection is equivocal. The enterotoxigenic and enteropathogenic forms cause a watery diarrhea and nausea while the enteroinvasive and enterohemorrhagic forms subsequently produce bloody stools. These presentations alone, however, are not sufficient for confirmation.
Laboratory: E. coli infection might be generally suspected in absence of isolation of Salmonella or Shigella or intestinal parasites. The organisms themselves are easily isolated and identified by routine procedures. Specialized serotyping may be necessary for epidemiologic studies.

CONTROL:

Sanitary: As with other fecal-oral diseases, proper food handling and personal hygiene are the best means for preventing infection.
Immunological: New vaccines against fimbrial antigens are possible.
Chemotherapeutic: Antibiotic therapy is not generally recommended unless disease becomes life-threatening. Oral rehydration is the best treatment.