Invasive enteric pathogens.

Invasive enteric pathogens of the Salmonella or Shigella genera initiate infections by invading the intestinal epithelium. Depending on the species, salmonellae either translocate across the mucosa of the small intestine and cause a systemic febrile disease or they evoke a localized inflammatory response in discreet areas of the infected mucosa. The latter type of infection is characterized by gastroenteritis, and a choleragen-like enterotoxin may contribute to the symptomology. Shigellae can also evoke diarrheal episodes; however, classic shigellosis is characterized by localized invasion of the colonic epithelium, with inflammation and ulceration of the mucosa. Derangement of the colonic mucosa is manifested in the bloody, mucoid stool characteristic of bacillary dysentery. Genetic analysis of invasive enteric pathogens has shown that extrachromosomal elements (plasmids) are required for full expression of virulence in Salmonella typhimurium, Yersinia enterocolitica, Shigella sonnei, and Shigella flexneri. In the latter species, at least three chromosomal regions are also necessary for virulence.