Bacteriology-Shigella | Quick Revision Notes 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-associated with high mortality)
        • Also causes the hemolytic uraemic syndrome
        • Mode of infection-faeco-oral route
        • Infective dose-10-100 organisms
        • Diarrhea followed by dysentery
        • Pathogenesis
          • The organisms invade the basolateral aspect of colonic epithelium with the help of VMA or 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
          • The 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 into3 biotypes-Manchester, Newcastle and Boyd 88 based on gas production from sugar.


      • Annual Cases worldwide- 164.7 million
      • 99% of cases of all cases occur in developing countries like Indian sub-continent and sub-Saharan Africa
      • Shigella flexneri- account for max. No. Of cases in developing areas as in India(60%)
      • Shigella sonnei -accounts for maximum cases in developed/west and industrialized areas(77%)
      • Children below five accounts for maximum cases(61%)




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