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What is The only Species of Shigella that produces exotoxin? and explain in detail?
Ans. d. Shigella dysenteriae. (Ref Ananthanarayan 8th/pg.277)Shigela dysenteriae (Subgroup A)# Mannitol nonfermenting bacilli consisting of 10 serotypes.# Indole negative.# Only member of the shigella family that is always catalase negative.# Sh. Dysentariae type 1 forms a toxin (shigela toxin), the earliest example of an exotoxin produced by a gram negative bacillus.# Three types of toxic ativities noted:Neurotoxicity- Enterotoxicity- CytotoxicitySubgroupABCDSpeciesSh. DysenteriaeSh. FlexineriSh. BoydiiSh. SonnelMannitol-AcidAcidAcidLactose---A LateSucrose---A LateDulcitol--Variable-IndoleVariableVariableVariable-Orithine decarboxylase---+Serotypes106 + Variants15Only 1SHIGELLA# S. dysentriae does not produce catalase & does not ferment mannitel# S. Sonnie is late lactose fermentor# S. Flexinerri produces gas.# S. dysenteriae type 1 forms a toxin, the earliest example of a exotoxin produced by a gram-negative bacillus.# All except Shigella dysenteria ferment mannitol.# All except S. dysenteriae type 1 produces catalase.# Colonies on MacConkey agars are colorless due to absence of lactose fermentation except S. sonnie, which ferments lactose late and forms pink colonies.# DCA and Wilson and Blair selective media are the selective media.# Infective dose of Shigella is 10-100 bacteria, while that of Salmonella is 102-105.Additional Educational points:# Shigella Shiga toxin (also produced by E. coli 0157:H7) cleaves host cell rRNA; also enhances cytokine release, causing HUS.# Verotoxin has many properties that are similar to the Shiga toxin produced by some strains of Shigella dysenteriae type 1; however, the two toxins are antigenically and genetically distinct. Of the E coli serotypes that produce verotoxin, O157:H7 is the most common and is the one that can be identified in clinical specimens. EHEC O157:H7 does not use sorbitol, unlike most other E coli, and is negative on sorbitol MacConkey agar (sorbitol is used instead of lactose); O157:H7 strains also are negative on MUG tests.# The infective dose of S. typhi V. cholerae is 102 - 105 bacilli, while that of Shigella is 102.