Gram-negative rods four main groups

Gram-negative rods four main groups: Sh. dysenteriae, flexneri, boydii and sonnei. Sh. Dysenteriae- most severe disease Culture- non-lactose fermenters (MacConkey & EMB), non-motile, non-H2S producers(TSI slant agar) Epidemiology Fecal- oral spread (5Fs)  Although shigellae are found world-wide, transmission is strongly associated with poor hygiene.  More prevalent in the developing world, where the main burden falls on children( under 5)  Only small numbers need to be ingested to cause illness < 200 (i.e. very acid stable)  Pathogenesis 1. 2. Invasive: Invade M cells, get into the cytoplasm,replicate & then polymerize actin jet trails & go laterally into adjacent cells without the need to go back into the ECF. Thus RBCs & PMNs in stool Endotoxin: (Shiga toxin). Produced by Sh. Dysenteriae type 1. Has 3 acivities : Neurotoxic, cytotoxic & enterotoxic. A-B toxin. Inhibits protein synthesis by clipping the 60S ribosomal subunit. Recall which other toxin has a similar mode of action? Clinical features   • • • • • 1-4 day incubation preiod Disease severity varies: 1. Sh. sonnei infections- mild & may escape detection 2. Sh. flexneri infections- more severe 3. Sh. Dysenteriae-most severe form of dysentery may be fulminating and cause death within 48 hours Diarrhoea-mucus,bloody with PMNs Colicky abdominal pain Tenesmus Fever