Shigella species are closely related to E. coli. Most fail to produce gas when fermentingglucose and do not ferment lactose. Their antigenic makeup has been characterized in a manner similar to E. coli with the exception that they lack flagella and thus H antigens.
All Shigella species are nonmotile. The genus is divided into four species which are de-fined by biochemical reactions and specific O antigens organized into serogroups. The species are Shigella dysenteriae (serogroup A), Shigella flexneri (serogroup B), Shigellaboydii (serogroup C), and Shigella sonnei (serogroup D). All but S. sonnei are furthersubdivided into a total of 38 individual O antigen serotypes specified by numbers. Shigella is the prototype invasive bacterial pathogen. All species are able to invade andmultiply inside a wide variety of epithelial cells, including their natural target, the entero-cyte. S. dysenteriae type A1 (Shiga bacillus), the species that was first discovered, is the most potent producer of Shiga toxin. Other Shigella species produce various molecular forms of Shiga toxin.