Shigella flexneri: Introduction, Identification Features, Keynotes, and Shigella Footages

Introduction of Shigella flexneri

Shigella are Gram-negative bacilli measuring about 1-3 µm X 0.5 µm. They are non-motile, non-encapsulated, non-sporing, and non-acid-fast bacteria belonging to the family Enterobacteriaceae. They cause an infectious disease called Shigellosis. There are four species or serogroups based on biochemical and serological properties-A. Shigella dysenterae (12 Serotypes), B. S. flexneri(6 serotypes), C. Shigella boydii (18 serotypes) and D. Shigella sonnei (1 serotype).

Biochemical Reactions

Basic FeaturesProperties
1. Gram StainingGram-Negative Rods (GNRs)
2. SporeNon-Sporing
3. CapsuleNegative
4. MotilityNegative
5. PigmentNegative
6. Growth in potassium cyanide (KCN) mediumNegative
7. Catalase testPositive
8. Oxidase testNegative
9. Nitrate reduction testPositive
10. MR (Methyl Red) testPositive
11. VP (Voges- Proskauer) assayNegative
12. OF (Oxidative-Fermentative) testFermentative/Facultative anaerobe
13. Gas from GlucosePositive
14. H2S productionNegative
15. Indole formationVariable
16. Urease/ urea hydrolysis testNegative
17. Citrate/ citrate utilizationNegative
18. DNase testNegative
19. Glucose fermentationPositive 
20. Maltose fermentationVariable
21. Lactose fermentationNegative
22. Sucrose fermentationNegative
23. Xylose fermentationNegative
24. Mannitol fermentationPositive
25. Acetate UtilizationNegative
26. ONPG (β-galactosidase)Negative
27. Phenylalanine Deaminase (PDA)/PPA TestNegative
28. Lipase testNegative
29. Esculin Hydrolysis testNegative
30. Lysine Decarboxylase TestNegative
31. Ornithine Decarboxylase TestNegative
32. Arginine Dihydrolase TestNegative
Table: Properties of Shigella flexneri

Keynotes on Shigella

  • Colonies of Shigella species on MacConkey medium are colorless (non-lactose fermenting) except in the case of S. sonnei which forms pink colonies due to late lactose fermentation.
  • Shigella antigens are somatic (O), Capsular(K)-( some strains only), and fimbrial antigens.
  • Mannitol Fermenting Shigella species are Shigella flexneri,  Shigella boydii, and Shigella sonnei while nonfermenting is Shigella dysenteriae.
  •  Mannitol-fermenters:  Shigella flexneri, Shigella boydii and Shigella sonnei
  • Toxins of Shigella are endotoxin, exotoxin, and verotoxin.
  • Shigella suspected specimens should be transported immediately otherwise, use a transport medium like Sach’s buffered glycerol saline. Alkaline transport medium is not allowed since it is inhibitory for shigellae (but useful for Vibrios).
  • Colicin typing is preferred for subgroup D (S. sonnei) strains.
Related Picture

Shigella boydii growth on XLD agar

Shigella boydii colony morphology on XLD agar
Fig. Shigella boydii colony morphology on XLD agar

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