The Genus Shigella
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The Genus Shigella YVES GERMANI AND PHILIPPE J. SANSONETTI
Introduction Shigella is a Gram-negative, nonmotile, nonsporeforming, non-lactose-fermenting, facultatively anaerobic bacillus belonging to the family Enterobacteriaceae. It causes bacillary dysentery or shigellosis, as do enteroinvasive Escherichia coli strains (EIEC). The disease naturally occurs only in humans and primates. It is characterized, in its classical form, by acute colonic and rectal mucosal inflammation that leads to the triad of the dysenteric syndrome: fever, intestinal cramps, and passage of blood and mucus in diarrheal stools. Shigella dysenteriae 1 was the first of the four Shigella species to be described in 1898 by the Japanese microbiologist K. Shiga, thus the name of Shiga bacillus. The four species share several major characteristics such as lack of motility, optimal growth at 37°C, oxidasenegative character, and lack of growth in synthetic media containing salts and a simple carbon source, unless glucose and nicotinic acid are added. Some isolates may require the addition of certain amino acids, purines or vitamins. The chromosomal DNA of Shigella has a G+C content of about 50 mol%. The Shigella genus is very closely related to the species Escherichia coli; because the degree of homology of their chromosomal DNAs is close to 100%, considering Shigella as a member of the genus Escherichia is regarded as justifiable (Brenner et al., 1973). However, whereas Escherichia coli isolates are usually prototrophic, motile, and able to ferment many sugars, Shigella isolates are auxotrophic, nonmotile, ferment few sugars and, with a few exceptions, produce no gas in the presence of glucose. Shigella spp. therefore appear to be unique among the Enterobacteriaceae, in that their species status is maintained for reasons that are historical and linked to their particular metabolic profile and capacity to cause dysentery.
Bacteriological Characters The genus Shigella consists of four species, S. dysenteriae (subgroup A), S. flexneri (subgroup
B), S. boydii (subgroup C) and S. sonnei (subgroup D), characterized by biochemical and serological properties. The subgroup A differs from the other subgroups by its inability to ferment mannitol. Shigella serotypes are differentiated on the basis of their somatic O antigens. Shigella spp. express numerous “negative” characters. Isolates are always nonmotile and nonflagellated. They do not grow on synthetic media with Simmons’ citrate, they do not express a deaminase for phenylalanine or tryptophan, and they are also negative for the following characters: urease, lysine and arginine decarboxylase, production of H2S (on triple-sugar-iron agar), growth in KCN medium, utilization of Christensen’s citrate or malonate, oxidation of gluconate, or liquefaction of gelatin. They give a positive methyl red and a negative Voges-Proskauer reaction. Like all Enterobacteria, Shigella spp. ferment glucose, but they (with the exception of S. flexneri 6, S. boydii 13 and 14 and S. dysenteria
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