E.coli

The factors important in pathogenicity are pili (for initial adhesion to cell), enterotoxins (heat stable and heat labile toxins), endotoxin (as in all Gram negative bacteria) and capsule (seen in invasive strains causing systemic infections and septicaemia).

Heat Labile Toxin: Just like Cholera toxin and Enterotoxin of Bacillus cereus, heat labile toxin of E.coli causes ADP ribosylation and activation of Gs, thus stimulating adenylate cyclase causing an increase in cAMP. Eventually this causes loss of Na, Cl and water from the cells presenting as a watery diarrhea. The toxin has two subunits A and B of which B binds to Gm ganglioside receptors on the small intestinal epithelium while the subunit A is the active unit.

Heat Stable Toxin: It is a polypeptide not inactivated by boiling for 30 minutes. It stimulates guanylate cyclase to increase cGMP, inhibiting the reabsorption of sodium and causing diarrhoea.

Verotoxin or Shiga-like Toxin: It is produced by EHEC (enterohemorrhagic E.coli) or STEC, especially famous for strains of E coli O 157:H7. It inactivates protein synthesis by removing adenine from 28 S rRNA. It binds to receptors on the kidney and endothelium of small vessels causing Hemolytic Uremic Syndrome and bloody diarrhea.

Human Pathology:

E.coli are common causes of UTI and Diarrhea.

UTI: It is the leading cause of community acquired UTI. It presents as dysuria (pain and burning sensation during micturition), increased frequency and rarely fever. Pili help the bacteria to adhere to the urothelium and cause UTI. Some strains carrying “K” antigen are also prone to cause UTI.

Diarrhea: Five types of E.coli cause diarrhea and or dysentery (blood in stools). These are as follows

EPEC (Enteropathogenic E.coli) - Seen in cases of diarrhea in children. Causes disruption of brush border in small intestines.

ETEC (Enterotoxigenic E.coli)- Cause of “traveller’s diarrhea”. Produces heat labile and stable toxins.

EIEC (Enteroinvasive E.coli) - Causes bloody stools due to invasion of intestinal mucosa. Are peculiar in being non lactose fermenters and non motile. Keep in mind for differentials of bloody diarrhea aka dysentery.

EHEC (Enterohemorrhagic E.coli) - New terminology is **STEC **or shiga toxin producing E.coli. It may cause mild diarrhea to severe HUS, especially in small children, older adults and immunodeficient individuals. Common foods that are implicated include raw (or undercooked) meat, lettuce, alfalfa sprouts, salami, or raw (unpasteurized) milk, juice, or cider. E.coli O157 diarrhea should NOT be treated with antibiotics especially beta lactams and fluoroquinolones as they have been shown to be associated with the development of HUS.

EAEC (Enteroaggregative E.coli) - Cause diarrhea due to production of heat stable enterotoxin.

Other infections caused by E.coli are meningitis and sepsis in infants and older adults, peritonitis and wound infections.

Laboratory diagnosis of E.coli infections

Samples are urine, stool, blood, csf, wound discharge etc, depending on the clinical findings. Isolation by culture is not essential as empiric treatment is typically started on the basis of clinical features and history alone. Culture is important only in cases showing resistance to antibiotics, septicaemia, meningitis and in suspected E.coli O 157 diarrhea.

Gram stain will show Gram negative bacilli. MacConkey’s agar will show pink lactose fermenting colonies except in EIEC. EMB agar will show colonies with a green metallic sheen. E.coli O 157 H7 can be identified by inability to ferment sorbitol which is characteristic.