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Escherichia coli
This bacteria predominate among the aerobic commensal flora in the gut of humans and animals.

Morphology:
It is Gram –Ve bacilli, 2.4µ x 0.7µ, non spore forming, usually motile and some specially those from extra – intestinal infections, produces a polysaccharide capsule.

Cultural characters:
They are aerobic and facultative anaerobic growing on simple media.
They grow over a wide range temperature (10– 45)˚c , Optimum temp is 37˚c.
• Liquid media: It shows uniform turbidity after 8 to 24hr. incubation.
• Nutrient agar: Colonies appear after 18hr. of incubation. They are circular 2-3 mm in diameter, smooth, colorless and entire edage.
• MacConkey agar:Colonies are large, pink due to lactose fermentation.
• Blood agar: Some strains show zone of beta hemolysis.







Biochemical reactions:-
It ferments lactose, glucose, sucrose, maltose and mannitol with acid and gas. Indole and methyl-red is positive, Voges- proskauer and citrate is negative. Urease is negative H2s is not produced.
Resistance:-
It can survive for months in soil and water. Some strains are heat- resistant and will survive at 60˚c for 15min.

Antigenic structure:-
The bacteria possess four kinds of antigens:-
1) Somatic antigen(O-antigen): Over (160) different somatic antigens have been described (173) different O-antigen. They are lipopolysaccharide, heat stable. Numerous cross- reactions occur between individual E.coli O-antigens and between these and the O-antigens of the genera salmonella, shigella, Yersinia.
2) Capsular antigen(K-antigen): They are heat labile, polysaccharide in nature, cover the O-antigen, so they interfere with (O) agglutination unless destroyed by heating at 100˚c or autoclaved at 121˚c.
3) Flagellar antigen(H-antigen): More than (50) flagellar antigens are known. They are heat labile. There are only a few significant cross- reactions between them and with the H-antigens of other members of Entero-bacteria.
4) Fimbrial antigen(F-antigen):- These antigens are able to mediate adhesion that might play a role in pathogenecity. It's thermolabile proteins.
Toxin:-
It produces endotoxin. Beside it, it also produces two types of exotoxin:-
a) Enterotoxin:- which causes fluid accumulation in rabbit ileal loop. There are two types of enterotoxin:-
• Heat labile enterotoxin(L.T):- It is protein in nature, closely related to cholera toxin. The mode of action is by activation of adenylate cyclase, there by raising level of cyclic adenosine 5- monophosphate (C-AMP) in cell and causing excretion of fluid and electrolytes in the lumen of intestine.
• Heat stable enterotoxin(S.T):- This toxin activates guanylate cyclase resulting in an increase in the level of cyclic guanosine monophosphate (C-GMP), causing fluid secretion.
b) Hemolysin which may be:-
• Heat labile, filterable and lethal for animals.
• Associated with bacterial cell and is not filterable.

Pathogenesis:-
Strains of E.coli possess a battery of virulence determinants. The polysaccharides of the O and K antigens protect the organism from the bactericidal effect of complement and phagocytes.
Haemolytic strains are more virulent than non–haemolytic, they obtain iron from lysed erythrocytes of the host.
1. Diarrhea: Strains that cause diarrhea divided into five groups with different pathogenic mechanisms.
• Enteropathogenic E.coli (EPEC):- They cause infantile enteritis outbreaks often occur in hospitals and may have a high mortality.
• Entrotoxigenic E.coli (ETEC):- These strains produce enterotoxin S.T or L.T or both ETEC causes " travellers' diarrhea".
• Enteroinvasive E.coli (EIEC):- Cause an illness identical to shigella dysentery in patients of all ages.
• Enterohaemorrhagic E.coli (EHEC):- cause Haemorrhagic colitis and haemolytic uremic syndron.
• Enteroaggregative E.coli (EAEC):- also called Enteroadherent, because they are able to adhere the epithelial cells in the intestine. They cause watery diarrhea may be converted to chronic diarrhea.

2. Urinary tract infection (UTI):- E.coli that cause UTI often originate from the gut of the patient, the ability of E.coli to infect the urinary tract is associated with fimbriae that mediate adherence to the uroepithelial cells.
3. Pyogenic infection:-
Wound infection, abscess, neonatal meningitis. It may cause septicemia.

Laboratory diagnosis:-
1. Collection of specimen:- In urinary tract infection mid stream urine is collected under aseptic condition. In acute diarrhea a sample of faeces or a rectal swab is collected. Pus may be collected on sterile cotton swab or sterile container.
2. Microscopic examination:- Centrifuged urine is examined for pus cell, RBC and bacteria. Gram stained smear shows moderate to large number of pus cell and Gram negative bacilli. Examination of wet and stained smear of faecal matter is of little use.
3. Culture:- Material is inoculated on blood agar plate and MacConkey plate. Lactose fermenting. The api system uses in the laboratory, consist of several biochemical tests. When indole and Methyl- red is positive, Voges- proskauer and citrate is negative is suggestive of E.coli.
In UTI , the organism is generally present in pure culture at a count of 105 or more per milliliter of urine.
4. Diagnostic sera:- by using polyvalent antisera contain combinations of antisera to the enteropathogenis serotypes for examining feacal samples.

Treatment:-
E.coli is sensitive to ampicillin and cephalosporins, tetracycline, chloramphemicol, aminoglycosides, trimethoprim and sulphonamides. Many strains have acquired plasmids conferring resistance to one or more of these drugs.

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