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  1. 1. Enterobacteriaceae Dr Sabrina Moyo Department of Mircobiology and Immunology
  2. 2. Introduction <ul><li>The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. </li></ul><ul><li>The family includes many genera ( Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, and others). </li></ul><ul><li>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. </li></ul>
  3. 3. Introduction <ul><li>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. </li></ul><ul><li>Thay are called Enterobacteriaceae, enteric gram-negative rods, and enteric bacteria but these bacteria may also be called coliforms. </li></ul>
  4. 4. Enterobacteriaceae <ul><li>Classification – more than15 different genera </li></ul><ul><ul><li>Escherichia </li></ul></ul><ul><ul><li>Shigella </li></ul></ul><ul><ul><li>Edwardsiella </li></ul></ul><ul><ul><li>Salmonella </li></ul></ul><ul><ul><li>Citrobacter </li></ul></ul><ul><ul><li>Klebsiella </li></ul></ul><ul><ul><li>Enterobacter </li></ul></ul><ul><ul><li>Hafnia </li></ul></ul><ul><ul><li>Serratia </li></ul></ul>
  5. 5. Enterobacteriaceae <ul><ul><li>Proteus </li></ul></ul><ul><ul><li>Providencia </li></ul></ul><ul><ul><li>Morganella </li></ul></ul><ul><ul><li>Yersinia </li></ul></ul><ul><ul><li>Erwinia </li></ul></ul><ul><ul><li>Pectinobacterium </li></ul></ul>
  6. 6. Morphology and General Characteristics <ul><ul><li>Gram-negative, non-sporing, rod shaped bacteria </li></ul></ul><ul><ul><li>Oxidase – </li></ul></ul><ul><ul><li>Ferment glucose and may or may not produce gas in the process (aerogenic vs anaerogenic) </li></ul></ul><ul><ul><li>Reduce nitrate to nitrite (there are a few exceptions) </li></ul></ul>
  7. 7. Morphology and General Characteristics <ul><ul><li>Are facultative anaerobes </li></ul></ul><ul><ul><li>If motile, motility is by peritrichous flagella </li></ul></ul><ul><ul><li>Many are normal inhabitants of the intestinal tract of man and other animals </li></ul></ul><ul><ul><li>Some are enteric pathogens and others are urinary or respiratory tract pathogens </li></ul></ul><ul><ul><li>Differentiation is based on biochemical reactions and and differences in antigenic structure </li></ul></ul>
  8. 8. Antigenic structure <ul><li>O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide. </li></ul><ul><li>Some O-specific polysaccharides contain unique sugars. </li></ul><ul><li>O antigens are resistant to heat and alcohol and usually are detected by bacterial agglutination. </li></ul><ul><li>Antibodies to O antigens are predominantly IgM. </li></ul><ul><li>While each genus of Enterobacteriaceae is associated with specific O groups, a single organism may carry several O antigens. </li></ul><ul><li>Thus, most shigellae share one or more O antigens with E coli. </li></ul><ul><li>E coli may cross-react with some Providencia, Klebsiella, and Salmonella species. </li></ul><ul><li>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. </li></ul>
  9. 9. Antigenic structure <ul><li>K antigens are external to O antigens on some but not all Enterobacteriaceae. </li></ul><ul><li>Some are polysaccharides, including the K antigens of E coli; others are proteins. </li></ul><ul><li>K antigens may interfere with agglutination by O antisera, and they may be associated with virulence </li></ul><ul><li>eg, E coli strains producing K1 antigen are prominent in neonatal meningitis </li></ul><ul><li>K antigens of E coli cause attachment of the bacteria to epithelial cells prior to gastrointestinal or urinary tract invasion. </li></ul><ul><li>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. </li></ul><ul><li>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. </li></ul><ul><li>Boiling for 15 minutes will destroy the K antigen and unmask O antigens. </li></ul><ul><li>K antigen is called the Vi (virulence) antigen in Salmonella </li></ul>
  10. 10. Antigenic structure <ul><li>H antigens are located on flagella and are denatured or removed by heat or alcohol. </li></ul><ul><li>They are preserved by treating motile bacterial variants with formalin. Such H antigens agglutinate with anti-H antibodies, mainly IgG. </li></ul><ul><li>The determinants in H antigens are a function of the amino acid sequence in flagellar protein (flagellin). </li></ul><ul><li>Within a single serotype, flagellar antigens may be present in either or both of two forms, </li></ul><ul><li>phase 1 (conventionally designated by lower-case letters) and </li></ul><ul><li>phase 2 (conventionally designated by Arabic numerals). </li></ul><ul><li>The organism tends to change from one phase to the other; this is called phase variation. </li></ul><ul><li>H antigens on the bacterial surface may interfere with agglutination by anti-O antibody. </li></ul>
  11. 11. Antigenic Structure of Enterobacteriaceae
  12. 12. Growth Characteristics <ul><ul><li>Carbohydrate fermentation patterns and the activity of amino acid decarboxylases and other enzymes are used in biochemical differentiation . </li></ul></ul><ul><ul><li>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. </li></ul></ul><ul><ul><li>Culture on &quot;differential&quot; 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. </li></ul></ul>
  13. 13. Growth Characteristics <ul><ul><li>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. </li></ul></ul><ul><ul><ul><li>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. </li></ul></ul></ul><ul><ul><ul><li>Many are differential on the basis of whether or not the organisms ferment lactose and/or produce H 2 S. </li></ul></ul></ul>
  14. 14. Growth Characteristics <ul><ul><li>They may or may not be hemolytic. </li></ul></ul><ul><ul><li>The three most useful media for screening stool cultures for potential pathogens are TSI, and urea or phenylalanine agar. </li></ul></ul><ul><ul><li>The antigenic structure is used to differentiate organisms within a genus or species. Three major classes of antigens are found: </li></ul></ul>
  15. 15. Culture <ul><ul><li>E coli and most of the other enteric bacteria form circular, convex, smooth colonies with distinct edges. </li></ul></ul><ul><ul><li>Enterobacter colonies are similar but somewhat more mucoid. Klebsiella colonies are large and very mucoid and tend to coalesce with prolonged incubation. </li></ul></ul><ul><ul><li>Salmonellae and Shigellae produce colonies similar to E coli but do not ferment lactose. </li></ul></ul><ul><ul><li>Some strains of E coli produce hemolysis on blood agar. </li></ul></ul>
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