Like Vibrio cholerae, most salmonellas are killed by acid, and so a large number must generally be ingested to survive passage through the stomach. Upon reaching the lower small and large intestine, an adhesin on the bacterial surface attaches the bacteria to specific receptors on the surface of the epithelial cells. Contact with intestinal epithelium activates a type III secretion system. In only a matter of minutes, transfer of bacterial substances into the epithelial cell tricks it into taking in the bacterium by endocytosis. The bacteria multiply within a phagosome and are discharged from the base of the cell by exocytosis. In most instances the bacteria are quickly taken up and killed by macrophages, but the inflammatory response to the infection increases fluid secretion, causing diarrhea. ■ endocytosis, exocytosis, p. 72 ■ type III secretion system, p. 467

Some strains, such as Salmonella Typhi, are not so easily eliminated by host defenses. They cross the mucous membrane via M cells, resist killing by macrophages, multiply within them, and are carried by the bloodstream throughout the body. Released by death of the macrophages, the bacteria can invade tissues, cause prolonged fever, abscesses, septicemia, and shock, often with little or no diarrhea. Salmonella Typhi can also cause destruction of Peyer's patches, leading to intestinal rupture and hemorrhage. ■ septicemia, p. 719 ■ Peyer's patches, p. 397

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