Escherichia coli (E. coli) is a bacterium commonly found in the gut of humans and warm-blooded animals. Most E. coli are normal flora or commensals found in the intestinal tract. It is a gram-negative short rod, a member of the family Enterobacteriaceae.
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Pathogenic strains of E. coli are distinguished from normal flora by possessing virulence factors such as exotoxins. Diarrheagenic strains of Escherichia coli are Shiga toxin-producing E. coli (STEC), enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), and enteroaggregative E. coli (EAEC). The role of STEC and ETEC in disease has been firmly established. However, the roles of the other E. coli pathotypes, EIEC, EPEC, and EAEC, are less clear.
Enterotoxigenic E. coli (ETEC)
Diarrhea caused by ETEC is usually watery, self-limited, and of short duration (1-3 days) which is frequently associated with travel and is also known as traveler’s diarrhea. It is transmitted by contaminated food and water.
|Strains of E. coli||Virulence Factors||Disease|
|Enterotoxigenic E. coli (ETEC)||Pili helps in colonization in GI tract. Heat-labile (LT) and heat-stable (ST) enterotoxins mediate the secretion of water and electrolytes into the bowel lumen.||Travelers and childhood diarrhea characterized by profuse, watery stools.|
|Enteroinvasive E. coli (EIEC)||Virulence factors are uncertain, but the organism invades enterocytes lining the large intestine in a manner nearly identical to Shigella spp.||Dysentery; is usually seen in young children living in areas of poor sanitation.|
|Enteropathogenic E. coli (EPEC)||Bundle-forming pilus, intimin, and other factors mediate the organism’s attachment to mucosal cells of the small bowel, resulting in changes in cell surface (i.e., loss of microvilli)||Diarrhea in infants in developing/underdeveloped countries; can cause chronic diarrhea.|
|Enterohemorrhagic E. coli (STEC)||Toxin similar to Shiga toxin produced by Shigella dysenteriae. Most frequently associated with certain serotypes such as E. coli O157: H7||Hemorrhagic colitis can also lead to hemolytic uremic syndrome (HUS).|
|Enteroaggregative E. coli (EAEC)||Probably involves binding by pili, ST-like, and hemolysis-like toxins; actual pathogenesis unknown.||Watery diarrhea|
Enteropathogenic E. coli (EPEC)
Enteropathogenic Escherichia coli (EPEC) is one of the diarrheagenic E. coli pathotypes. It is one of the leading causes of persistent diarrhea in children. The ability to produce attaching and effacing (A/E) lesions is a hallmark phenotype of EPEC. EPECs are identified based on the presence of specific virulence genes.
Enteroinvasive E. coli (EIEC)
Enteroinvasive E. coli (EIEC), one of the significant pathotypes of E. coli, are a group of intracellular pathogens able to enter epithelial cells of the colon, multiply within them, and move between adjacent cells with a mechanism similar to Shigella. EIEC are etiological agents of bacillary dysentery in humans, particularly in low-income countries.
Enteroaggregative E. coli (EAEC)
Enteroaggregative E. coli (EAEC) is a heterogeneous group of E. coli strains that can cause acute and persistent diarrhea in children, especially <2 yr old and malnourished children. EAEC infections are caused by exposure to food or water contaminated with human or animal waste. EAEC causes diarrhea in children worldwide and adults who travel to developing countries.
Enterohemorrhagic E. coli (EHEC)
Enterohemorrhagic E. coli (EHEC) are also referred to as verocytotoxin-producing E. coli (VTEC) or Shiga-like toxin-producing E. coli (STEC). EHEC are verotoxigenic E. coli (as they produce one or both verocytoxins vt1 and vt2.
Infection with EHEC results in a dysentery-like syndrome characterized by bloody diarrhea, abdominal cramping, and fever similar to that caused by Shigella.
Carriers/Reservoirs of infections:
The reservoirs for EHEC O157:H7 are ruminants, particularly cattle and sheep, which are infected asymptomatically and shed the organism in feces. Other animals such as rabbits and pigs can also carry this organism.
Mode of transmission
Humans acquire EHEC O157:H7 by direct contact with animal carriers, their feces, and contaminated soil or water, or by ingesting contaminated food (such as raw or undercooked meat products, unpasteurized milk & dairy products) and contaminated raw vegetables). The infectious dose is very low, which increases the risk of disease.
EHEC can cause severe foodborne disease, diarrhea, or hemorrhagic colitis (bloody diarrhea without pus cells) in humans. Hemorrhagic colitis occasionally progresses to hemolytic uremic syndrome (HUS), a significant cause of acute renal failure in children and morbidity and mortality in adults.
When infection with EHEC O157:H7 is suspected (hemorrhagic colitis), a presumptive diagnosis can be made by isolating sorbitol non-fermenting E. coli on sorbitol MacConkey agar. The colonies can be identified as E. coli O157 by testing with specific O157 antiserum, available as a latex agglutination test.