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Shigella spp.The genus Shigella is named after the Japanese bacteriologist Kiyoshi Shiga (1870-1957), who first identified the dysentery bacillus in 1897. They are Gram-negative non-spore forming rod-shaped bacteria, closely related to Escherichia and now generally considered to be another strain of E. coli. However, because Shigella is lactose (-), anaerogenic (does not produce gas from carbohydrates), and non-motile, it is usually discernable from E. coli. Unlike other pathogenic E. coli strains, Shigella is principally a human-specific pathogen, rarely occurring in animals except for other primates such as monkeys and chimpanzees. Found worldwide in water polluted with human feces, Shigella is also involved in outbreaks of foodborne illness. Because the bacteria are not easily killed by stomach acid, an infectious dose can be as small as 10 organisms. Shigellosis (bacillary dysentery) is a severe gastroenteritis marked by bloody diarrhea, fever, and vomiting; less common symptoms include headache, stiff neck, convulsions, and joint pain. In healthy adults, Shigellosis is a self-limiting disease, but it can be fatal to infants and young children, causing over 1 million deaths per year. Shigella is an invasive pathogen which can colonize the host's tissues as opposed to growing on tissue surfaces. The first step in infection is phagocytosis of the bacteria by M cells, which transport them beneath the intestinal epithelium. They are released by exocytosis, and then induce endocytosis by epithelial cells. The resultant endosomes (phagosomes) are lysed, leaving the bacteria free in the cytoplasm where they multiply at a high rate. In addition, Shigella is able to move between cells by coopting the host cell's actin filaments to push the nonmotile bacteria from cell to cell. Infected cells die and slough off, leading to intense inflammation, bleeding, and abcess formation. All virulent strains of Shigella (and Enteroinvasive E. coli) contain a large plasmid that is essential for the invasion of the host cells. The plasmid was acquired independently by different lineages of E. coli, resulting in four 'species' of Shigella which are sometimes referred to by a letter designation based on serological characteristics:
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