Testing for Shigella
The genus Shigella consists of four species: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. These gram-negative, non-motile and non-sporulating bacilli belong to the Enterobacteriaceae family. Genetically, they are extremely closely related to E. coli.
Shigella spp. are predominantly connected to humans and are rarely isolated from other animals, with the exception of primates. Typical vehicles of foodborne Shigella are salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk and dairy products, as well as poultry. Fecally contaminated water and unsanitary treatment by food handlers are the most common causes of infection.
Merck’s unique dehydrated culture media for cultivating Shigella come in granules that dissolve in water easily. Granulation of the media ensures homogenous distribution, so that no components separate out or clump, even in warm and humid conditions. Merck’s dehydrated culture media are not only extremely easy to use and highly efficient but also meet the highest performance standards as described in ISO 11133.
The typical symptoms of Shigella infection (shigellosis) are abdominal pain, cramps, (bloody) diarrhea, fever and vomiting. Onset time from ingestion is typically 12–50 hours. The infective dose may be as low as 10 cells, depending on the age and condition of the host. The disease is caused when virulent Shigella organisms attach to and penetrate epithelial cells of the intestinal mucosa. After invasion, they multiply and spread to adjacent cells, resulting in tissue destruction. Some strains produce the enterotoxin shiga toxin, which is similar to the verotoxin of E. coli O157:H7. Although most patients recover within 5–7 days, fatality may be as high as 10–15% with some strains. Associated complications include reactive arthritis and hemolytic-uremic syndrome.
|Method||ISO 21567||FDA BAM Chapter 6||FDA BAM Chapter 6 – DNA Hybridization|
|Enrichment (MPN)||Shigella broth w. Novobiocin (0.5 ppm),
41.5 ± 1°C, 18 ± 2 h, anaerobic
|Shigella broth w. Novobiocin (3 ppm): 42°C (0.5 ppm Novobiocin; 44°C for S. sonnei), 20 h, anaerobic||Tryptic Soy broth with Yeast extract, 36 ± 1°C, 22 ±2 h|
|Isolation (MPN) and / or direct plate count||Tryptic Soy broth with Yeast extract, 36 ± 1°C, 22 ±2 h||MacConkey agar, 35°V, 20 h|
MPN = most probable number