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Escherichia coli: Morphology, Lab diagnosis-NotesMed

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    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.

    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.

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