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Enterobacteriaceae Enterobacteriaceae Presentation Transcript

  • Enterobacteriaceae Dr Sabrina Moyo Department of Mircobiology and Immunology
  • Introduction
    • The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals.
    • The family includes many genera ( Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, and others).
    • Some enteric organisms, eg, Escherichia coli, are part of the normal flora and incidentally cause disease, while others, the salmonellae and shigellae, are regularly pathogenic for humans.
  • Introduction
    • The Enterobacteriaceae are facultative anaerobes or aerobes, ferment a wide range of carbohydrates, possess a complex antigenic structure, and produce a variety of toxins and other virulence factors.
    • Thay are called Enterobacteriaceae, enteric gram-negative rods, and enteric bacteria but these bacteria may also be called coliforms.
  • Enterobacteriaceae
    • Classification – more than15 different genera
      • Escherichia
      • Shigella
      • Edwardsiella
      • Salmonella
      • Citrobacter
      • Klebsiella
      • Enterobacter
      • Hafnia
      • Serratia
  • Enterobacteriaceae
      • Proteus
      • Providencia
      • Morganella
      • Yersinia
      • Erwinia
      • Pectinobacterium
  • Morphology and General Characteristics
      • Gram-negative, non-sporing, rod shaped bacteria
      • Oxidase –
      • Ferment glucose and may or may not produce gas in the process (aerogenic vs anaerogenic)
      • Reduce nitrate to nitrite (there are a few exceptions)
  • Morphology and General Characteristics
      • Are facultative anaerobes
      • If motile, motility is by peritrichous flagella
      • Many are normal inhabitants of the intestinal tract of man and other animals
      • Some are enteric pathogens and others are urinary or respiratory tract pathogens
      • Differentiation is based on biochemical reactions and and differences in antigenic structure
  • Antigenic structure
    • O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide.
    • Some O-specific polysaccharides contain unique sugars.
    • O antigens are resistant to heat and alcohol and usually are detected by bacterial agglutination.
    • Antibodies to O antigens are predominantly IgM.
    • While each genus of Enterobacteriaceae is associated with specific O groups, a single organism may carry several O antigens.
    • Thus, most shigellae share one or more O antigens with E coli.
    • E coli may cross-react with some Providencia, Klebsiella, and Salmonella species.
    • Occasionally, O antigens may be associated with specific human diseases, eg, specific O types of E coli are found in diarrhea and in urinary tract infections.
  • Antigenic structure
    • K antigens are external to O antigens on some but not all Enterobacteriaceae.
    • Some are polysaccharides, including the K antigens of E coli; others are proteins.
    • K antigens may interfere with agglutination by O antisera, and they may be associated with virulence
    • eg, E coli strains producing K1 antigen are prominent in neonatal meningitis
    • K antigens of E coli cause attachment of the bacteria to epithelial cells prior to gastrointestinal or urinary tract invasion.
    • Klebsiellae form large capsules consisting of polysaccharides (K antigens) covering the somatic (O or H) antigens and can be identified by capsular swelling tests with specific antisera.
    • Human infections of the respiratory tract are caused particularly by capsular types 1 and 2; those of the urinary tract, by types 8, 9, 10, and 24.
    • Boiling for 15 minutes will destroy the K antigen and unmask O antigens.
    • K antigen is called the Vi (virulence) antigen in Salmonella
  • Antigenic structure
    • H antigens are located on flagella and are denatured or removed by heat or alcohol.
    • They are preserved by treating motile bacterial variants with formalin. Such H antigens agglutinate with anti-H antibodies, mainly IgG.
    • The determinants in H antigens are a function of the amino acid sequence in flagellar protein (flagellin).
    • Within a single serotype, flagellar antigens may be present in either or both of two forms,
    • phase 1 (conventionally designated by lower-case letters) and
    • phase 2 (conventionally designated by Arabic numerals).
    • The organism tends to change from one phase to the other; this is called phase variation.
    • H antigens on the bacterial surface may interfere with agglutination by anti-O antibody.
  • Antigenic Structure of Enterobacteriaceae
  • Growth Characteristics
      • Carbohydrate fermentation patterns and the activity of amino acid decarboxylases and other enzymes are used in biochemical differentiation .
      • Some tests, eg, the production of indole from tryptophan, are commonly used in rapid identification systems, while others, eg, the Voges-Proskauer reaction (production of acetylmethylcarbinol from dextrose), are used less often.
      • Culture on "differential" media that contain special dyes and carbohydrates (eg, eosin-methylene blue [EMB], MacConkey's, or deoxycholate medium) distinguishes lactose-fermenting (colored) from non-lactose-fermenting colonies (nonpigmented) and may allow rapid presumptive identification of enteric bacteria.
  • Growth Characteristics
      • Most grow well on a variety of lab media including a lot of selective and differential media originally developed for the the selective isolation of enteric pathogens.
        • Most of this media is selective by incorporation of dyes and bile salts that inhibit G+ organisms and may suppress the growth of nonpathogenic species of Enterobacteriaceae.
        • Many are differential on the basis of whether or not the organisms ferment lactose and/or produce H 2 S.
  • Growth Characteristics
      • They may or may not be hemolytic.
      • The three most useful media for screening stool cultures for potential pathogens are TSI, and urea or phenylalanine agar.
      • The antigenic structure is used to differentiate organisms within a genus or species. Three major classes of antigens are found:
  • Culture
      • E coli and most of the other enteric bacteria form circular, convex, smooth colonies with distinct edges.
      • Enterobacter colonies are similar but somewhat more mucoid. Klebsiella colonies are large and very mucoid and tend to coalesce with prolonged incubation.
      • Salmonellae and Shigellae produce colonies similar to E coli but do not ferment lactose.
      • Some strains of E coli produce hemolysis on blood agar.