You are currently viewing Escherichia coli: Morphology, Lab diagnosis-NotesMed
Spread the love

Overview of Escherichia

Escherichia coli is first isolated by Escherich in 1885 and it is the most important species encountered as a human pathogen. Most commonly found in human and animal intestines. There is various or numerous form of Escherichia coli present in nature.

Morphology of Escherichia coli

Escherichia coli is a gram-negative, non-sporing, bacillus. Size is 1-3 X 0.4-0.7 µm. Most strains of E.coli are motile via. peritrichous flagella. Capsule and fimbriae present in some strains. Growth occurs in a wide range of temperatures.

Cultural characteristics of Escherichia coli

Aerobic and facultative anaerobic bacteria and grow on ordinary culture media, 37OC in 8-24 hours.

  • Nutrient agar: Colonies are large (2-3mm), thick, colorless, moist, easily emulsifiable.
  • MacConkey’s agar: Colonies are flat and pink (due to lactose fermentation), circular, convex, smooth non-viscous with a clear-cut margin.
  • Blood agar:  Colonies are big, circular, grey, moist, and some strain beta hemolysis (mainly UPEC strain).
  • The liquid medium, such as peptone water broth: produce uniform turbidity.

Escherichia coli

Biochemical reactions:

  • Break down most of the sugars (lactose, glucose, mannitol, maltose) with the production of acid and gas.
  • Typical strains: do not ferment sucrose.
  • Some strains: late lactose fermenters.
  • Indole production and methyl red reaction: positive.
  • V.P. (Voges-Proskauer) and citrate utilization tests: negative.
  • Catalase: Positive.
  • Oxidase: Negative.
  • Nitrate is reduced to nitrite.

Antigenic structure:

Various forms of antigenic structure present, they are; Somatic, flagellar, capsular, fimbrial antigen.

Somatic antigen (O antigen):

  • Lipopolysaccharide ( LPS) antigen.
  • Heat stable, over 170 types, designated as 1,2,3, and so on.
  • Cross reaction with other genera of enterobacteria.
  • Normal colon strains: early O groups (1,2,3,4,5 etc).
  • Enteropathogenic strains: latter O groups (26,86, 111, 112, etc.).
  • O antigen is the most important virulence factor, responsible for endotoxicity activity; it protects the bacteria from phagocytosis and the bactericidal effect of complement.

Flagellar (H antigen):

  • H from Hauch, meaning film of breath.
  • It is thermolabile.
  • More than 50.
  • To be grown on semisolid agar for H antigen determination.
  • The presence of H antigen (flagella) makes the bacteria motile, hence contributing to their virulence.

Surface (K antigen):

  • Acidic polysaccharide surface (capsule) antigens. two groups present are Group I & Group II –Role:
  • O inagglutinability and inhibit phagocytosis.

Fimbrial antigen:

  • Common pili: chromosomally determined, present in large numbers, cause bacterial cell attachment to the host cell.
  • Sex pili: which are determined via. conjugative plasmids and appear to be organs of conjugation.
  • Type I fimbriae: adhesion.


  • Enterotoxins:
    • Gastrointestinal disorders.
    • Enterotoxigenic strains: Heat labile, Heat stable toxins, Shiga like a toxin.
  • Haemolysins:
    • Uropathogenic strain.
  • Antiphagocytic capsule:
    • K1 strain: Major cause of meningitis.
  • Cytotoxic necrotizing factor 1 (CN F1) and secreted autotransporter toxin (SAT):
    • Cytotoxic to bladder and kidney cells.

Clinical syndromes:

  • Urinary tract infections.
  • Septic infection of the wound.
  • Diarrhoeas, dysentery.
  • Septicemia.
  • Pneumonia.
  • Neonatal meningitis.
  • Abscesses in a variety of organs.

Urinary tract infections:

About 60-80% of Urinary tract infections. Most strains but mostly by O1, O2, O4, O6, O7, O18, O75 called nephritogenic strain. Adhesins- P pili, AAF-I, AAF-III, Dr bind to the cells lining the urinary bladder and upper urinary tract.

Gastrointestinal disease:

  • Enterotoxigenic E.coli(ETEC).
  • Enteropathogenic E.coli (EPEC).
  • Enterohemorrhagic E.coli (EHEC).
  • Enteroaggregative E.coli (EAEC).
  • Enteroinvasive E.coli (EIEC).
  • Diffusely adherent E.coli (DAEC).


  • Fecal-oral transmission.
  • Food-borne transmission.
  • Endogenous transmission:
    • UTIs:
      • From perineum.
      • During sexual activity (honeymoon cystitis).

Laboratory diagnosis

Specimen collection:

  • Clean catched mid-stream urine.
  • Feces or rectal swab or wound swab.
  • Pus.
  • CSF.
  • Cerebrospinal fluid (CSF).
  • Peritoneal exudate.
  • Sputum.
  • Tracheal aspirate.
  • Blood.


Centrifuged deposit of urine, CSF, pleural fluid, and other body fluids: Pus cells and bacteria.

Gram staining: Gram-negative bacilli with pus cells.

Fluorescent-labeled O group antisera in acute gastroenteritis.


Preliminary identification: Smear examination, motility preparation, and biochemical tests.

Confirmed: Agglutination tests and group-specific and type-specific antisera.


Hi, I am Notesmed, CEO of NotesMed. I am a man whose road to success is under construction. A medical personal by fate & a Medicopreneur by passion. You can add me to the social profiles below.

Leave a Reply