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Enterobacteriaceae (Gram Negative Bacilli)
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>Citrobacter species:
1] Citrobacter koseri
2] Citrobacter freundii
>Enterobacter species:
1] Enterobacter cloacae,
2] Enterobacter aerogenes)
>Escherichia coli
>Klebsiella species:
1] Klebsiella ozaenae
2] Klebsiella pneumoniae
3] Klebsiella rhinoscleromatis
>Morganella species
1] (Morganella morganii)
>Proteus species:
1] Proteus mirabilis
2] Proteus vulgaris
>Providencia species:
1] Providencia_rettgeri
2] Providencia stuartii
>Salmonella species:
1] Salmonella enteritidis
2] Salmonella typhi
3] Other
>Serratia marcescens
>Shigella species: 
1] Shigella dysenteriae (serogroup A)
2] Shigella flexneri
3] Shigella boydii
4] Shigella sonnei
5] Other
Shigella:
- Gram-negative, facultative anaerobic, nonspore forming, non-motile, rod-shaped bacteria closely related to Salmonella.
- Causative agent of human shigellosis. During infection, it typically causes dysentery.
- Shigella species are classified by four serogroups:
- Serogroup A: S. dysenteriae (12 serotypes)
- Serogroup B: S. flexneri (6 serotypes)
- Serogroup C: S. boydii (18 serotypes)
- Serogroup D: S. sonnei (1 serotype)
- Groups A- C are physiologically similar; S. sonnei (group D) can be differentiated on the basis of biochemical metabolism assays.
- Three Shigella groups are the major disease-causing species:
- S. flexneri is the most frequently isolated species worldwide, and accounts for 60% of cases in the developing world;
- S. sonnei causes 77% of cases in the developed world, compared to only 15% of cases in the developing world; and
- S. dysenteriae is usually the cause of epidemics of dysentery, particularly in confined populations such as refugee camps.
- Shigella infection is typically via ingestion (fecal-oral contamination); depending on age and condition of the host, fewer than 100 bacterial cells can be enough to cause an infection.
- Shigella causes dysentery that results in the destruction of the epithelial cells of the intestinal mucosa in the cecum and rectum.
- Some strains produce the enterotoxin shiga toxin, which is similar to the verotoxin of E. coli O157:H7 and other verotoxin-producing Escherichia coli. Both shiga toxin and verotoxin are associated with causing hemolytic uremic syndrome.
- The most common symptoms are diarrhea, fever, nausea, vomiting, stomach cramps and flatulence. The stool may contain blood, mucus or pus.
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