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 Features | Properties |
1. Gram Staining | Gram-Negative Rods (GNRs) |
2. Spore | Non-Sporing |
3. Capsule | Negative |
4. Motility | Negative |
5. Pigment | Negative |
6. Growth in potassium cyanide (KCN) medium | Negative |
7. Catalase test | Positive |
8. Oxidase test | Negative |
9. Nitrate reduction test | Positive |
10. MR (Methyl Red) test | Positive |
11. VP (Voges- Proskauer) assay | Negative |
12. OF (Oxidative-Fermentative) test | Fermentative/Facultative anaerobe |
13. Gas from Glucose | Positive |
14. H2S production | Negative |
15. Indole formation | Variable |
16. Urease/ urea hydrolysis test | Negative |
17. Citrate/ citrate utilization | Negative |
18. DNase test | Negative |
19. Glucose fermentation | Positive |
20. Maltose fermentation | Variable |
21. Lactose fermentation | Negative |
22. Sucrose fermentation | Negative |
23. Xylose fermentation | Negative |
24. Mannitol fermentation | Positive |
25. Acetate Utilization | Negative |
26. ONPG (β-galactosidase) | Negative |
27. Phenylalanine Deaminase (PDA)/PPA Test | Negative |
28. Lipase test | Negative |
29. Esculin Hydrolysis test | Negative |
30. Lysine Decarboxylase Test | Negative |
31. Ornithine Decarboxylase Test | Negative |
32. Arginine Dihydrolase Test | Negative |
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

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