Studies on the pathogenesis of enteric infections caused by invasive bacteria

Salmonellae, shigellae and some Escherichia coli must invade the intestinal epithelial cell and multiply within the mucosa to cause disease. Although the bacterial cell most likely possesses several properties essential to this invasive ability, the nature of the cell envelope complex is at present the only characteristic which has been implicated in this process. While a number of pathophysiological events result from invasion, some of our recent efforts have concerned the site and mechanism of intestinal fluid loss in salmonellosis and shigellosis. In both these disorders, bacterial invasion of the colonic mucosa, associated with an acute inflammatory reaction and mucosal damage, is regularly seen and colonic salt and water transport is abnormal. These defects may account for mild diarrhoea in salmonellosis and the dysenteric stools of shigellosis. However, in salmonella-infected animals with severe watery diarrhoea and in shigella-infected animals with diarrhoea alone or in combination with dysentery, the jejunum is in a net secretory state. This secretion occurs in the absence of bacterial invasion or morphological abnormalities. Thus, the diarrhoea caused by invasive bacteria may result from the inability of the colon to reabsorb the increased volume of fluid entering it from the small intestine. Although colonic mucosal damage is a feature of invasive-type diarrhoeas, the permeability of both the colon and small intestine to small molecules, mannitol and erythritol, is not altered. Thus intestinal fluid loss cannot be ascribed to transudation. In addition, the results of our Ussing chamber experiments, employing salmonella-infected rabbit ileum, reveal that salt and water secretion is an active process. Since secretion occurs in the jejunum in the absence of bacterial invasion, this might suggest the participation of an enterotoxin. Shigella dysenteriae I is the best-studied invasive organism in which an enterotoxin has been found, yet mutant strains which do not invade but retain the ability to elaborate enterotoxin fail to cause disease in either monkeys or man. Thus, the physiological relevance of Shiga enterotoxin and the mechanism of jejunal secretion in these disorders remain unclear. Recent data suggest that invasive enteropathogens, like the enterotoxin-producing bacteria, activate the mucosal adenylate cyclase-cyclic AMP system and that this activation may play a role in intestinal fluid secretion.