Bacteriology-Shigella| Quick Revision for NEET-PG

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Gram negative non-sporing ,non-motile & non-capsulated organism

      • Ferments glucose producing acid but no gas(amerogenic)
      • Non-lactose fermentors except S. soneei which is a late lactose fermentor
      • On the basis of mannitol fermentation & somatic O polysaccharide antigen it is divided into 4 groups
        1. Mannitol fermentors -S. dysenteriae(group A)
        2. Mannitol non-fermentors-S. flexneri(group B),S. boydii(group B),S. sonnei(group D)
      1. Dystentriae type 1/shiga bacillus
        • Catalse -ve
        • It causes bacillary dysentery(most severe type-associated with high mortality)
        • Also causes hemolytic uraemic syndrome
        • Mode of infection-faeco-oral route
        • Infective dose-10-100 organisms
        • Diarrhea followed by dysentery
        • Pathogenesis
          • The organisms invades the basolateral aspect of colonic epithelium with the help of VMA i.e Virulence Marker Antigen . Invasion is more important for disease development than toxins
          • Organisms produce shiga toxins which inhibit 28s subunit of 60s ribosomes leading to inhibition of protein synthesis.
        • Diagnosis
          • Sample to be collected-faeces
          • Use of enrichment media like Selenite broth/Gram-ve broth (NOT tetrathionate broth)
          • Then subculture on selective media like
            •  Mc conkey’s agar showing non-lactose fermenting colonies except S. soneei which is a late lactose fermentor.
            • DCA(deoxycholate citrate agar) showing colorless colonies w/o black center and no H2S production.
            • Shigella does no grow on wilson & blair medium
      • Shigella sonnei- cause least severe disease(may only cause dysentery).It has only 1 serotype
      • Shigella flexneri -it has 6 serotypes. Serotype 6 is indole negative and is further divided into 3 biotypes-Manchester,Newcastle and Boyd 88 based on gas production from sugar.

 

 

      • Epidemiology
        • Annual Cases worldwide- 164.7 million
        • 99% cases of all cases occur in developing countires like indian sub-continent and sub-saharan Africa.
        • Shigella flexneri- account for max. No. Of cases in developing areas(60%).
        • Shigell sonnei -accounts for maximum cases in developed and industrialized areas(77%).
        • Children below five accounts for maximum cases(61%).

 

 

 

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