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16 Cards in this Set
- Front
- Back
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Morphology |
Gram negative bacillus 1 to 3 micron * 0.4 to 0.7 micron Motile with peritrichate flagella ing Non sporingNon capsulated |
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Culture |
Aerobe and facultative anaerobe Temp - 10 to 40 Time - 18 to 24 Some strains show beta haemolysis on blood agar. Pink in MacConkey's medium. Liquid medium shows uniform turbidity.
Colonies are circular, moist, smooth with entire margin and mucoid. |
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Biochemical Reaction |
Ferment sugars with acid and gas production. Do not ferment sucrose. Indole and methyl red reaction are positive. Voges-Proskauer and citrate utilisation tests are negetive. Urea does not split Gelatin is not liquified H2S is not formed Growth does not occur in KCN medium. |
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Virulence factors |
Surface antigens - O, K, H and F antigen. Toxins - enterotoxin, haemolysin, verocytotoxin |
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O antigen |
Somatic antigen Heat stable, lipopolysaccharide antigen of cell wall. 173 O antigen types For O agglutination, culture should be boiled to overcome inagglutinability caused by K antigen. Cross reaction between O antigen of E. coli and other E. species. Commensals belong to early O groups Pathogens belong to latter group - 26, 55, 86, 111 Endotoxic activity Protects the bacteria from phagocytosis and bactericidal action of complement. |
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K antigen |
Capsular antigen Cause O antigen inagglutinability Not demonstrable by light microscopy. Divided into L, A, B classes based on 1) effect of heat on agglutinability 2) antigenicity 3) antibody binding power of bacterial strains carrying them L antigen: thermolabile, capacity to combine with specific antibody is lost, group 2 A antigen: thermostable, associated with well marked capsule, group 1 B antigen: heat labile but antibody binding power remains unaffected. 103 types Protect bacilli from killing action of antibody, complement, impending phagocytosis. Most strains do not possess this antigen. If they do, it's of L type. |
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H antigen |
Flagellar antigen Thermolabile 75 types Monophasic For its determination, the organism has to be grown in semisolid agar. Very few cross reactions. |
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F antigen |
Thermolabile proteins Heating these organisms at 100°C leads to detachment of fimbriae. Type I fimbriae - adhesion of bacteria to cells that contain mannose residue - enhanced bacterial pathogenicity. Fibrin structures resembling fimbriae - cause mannose-resistant haemagglutination - pathogenesis of diarrhoea and UTI - K88, K99 and CFAs. |
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Resistance |
Killed by moist heat at 60°C within 30 mins. Killed by 0.5 to 1 ppm chlorine in water. |
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Heat labile toxin |
- Resembles enterotoxin produced by V. cholerae. - Composed of enzymatically active polypeptide A and 5 B subunits. - B subunit binds to Gm1 ganglioside receptor at brush border of epithelial cells of small intestine - facilitate the entry of subunit A. - Subunit A is activated to produce two fragments - A1 and A2. - A1 activates adenyl cyclase in enterocyte - converts ATP to cAMP - Increased cAMP results in intense and prolonged hypersecretion of water and chlorides and inhibits reabsorption of sodium - watery diarrhoea. - two types - LT-I and LT-II. - LT-I does not react with LT-I antiserum. |
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Heat stable toxin |
- Low molecular weight polypeptide - Poorly immunohenic - two types - STI/STa and STII/STb - STI stimulates fluid secretion in the gut through cGMP. - rapid acting than LT. - STII cannot be detected by ligated rabbit ileal loop test or infant mouse intragastric test. - due to poor immunogenic nature, development of immunological assays if ST was not possible but it is possible by preparing antiserum from toxin coupled to a hapten. - thus ELISA with ST monoclonal antibodies for ST are available. |
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Hemolysin |
Observed in extra intestinal lesions rather than in faeces. |
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Verocytotoxin |
- Shiga like toxin - SLT - of Sh. dysentriae type 1. - two types - VT1 - neutralised by antiserum to Shiga toxin and VT2 - resists neutralisation to this antiserum. - cytotoxic to Vero and HeLa cells. - enterotoxin in rabbit ileal loop - show paralytic-lethality in mouse. - VT has A and B subunits. - another toxin - VT2v - human and porcine variants. - VT2v is not cytotoxic to HeLa cells and is not phage encoded. |
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Cytotoxic Necrotising Factor-1 |
Present in uropathogenic E. coli Produce biofilm and adhesion. |
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Pathogenesis |
Four type of clinical syndromes, 1. UTI 2. Diarrhoea 3. Pyogenic Infections 4. Septicaemia Two categories: extraintestinal pathogenic E. coli and intestinal E. Coli. ExPEC - UPEC & MNEC Intestinal - EPEC, ETEC, EIEC, EHEC, EAEC and DAEC |
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Urinary Tract Infection |
- Most common organism responsible. - O serotypes- 1, 2, 4, 6, 7, 18, 75 - Special nephropathogenic potential is due to - a) polysaccharides of O & K antigens - prevent bactericidal effects of complement and phagocytes. Strains with K1 and K5 are more virulent. b) |
