Escherichia coli: Morphology, Lab diagnosis-NotesMed

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, 37^OC 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.

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.

Toxins:

• 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).

Transmission:

• 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.

Microscopy:

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.

Agglutination:

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

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