Theodore E. Woodward Award. How bacterial enterotoxins work: insights from in vivo studies

Clostridium difficile is a spore forming, gram-positive anaerobic bacillus first described in 1935 by Hall and O'Toole as a commensal organism in the fecal flora of healthy newborn infants (1). The organism was given its unusual name because it grew slowly and was difficult to isolate in pure culture. Its presence in the stool of healthy neonates suggested that C. difficile was a nonpathogen, even though it produced toxins in broth culture. Following its original description, C. difficile passed quickly into relative obscurity in the 1960's and 1970's when antibiotic-associated pseudomembranous colitis became prevalent following the introduction into clinical practice of broad spectrum antibiotics. The frequent association of clindamycin and lincomycin therapy with pseudomembranous colitis led to the term "clindamycin colitis" (2). A breakthrough occurred in 1978 when C. difficile was identified as the source of a cytotoxin in the stool of patients with pseudomembranous colitis (3). During the two decades since its rediscovery, a great deal has been learned about the pathophysiology, epidemiology and management of C. difficile infection, yet many challenges remain. Currently this organism infects over 30% of individuals admitted to United States hospitals, making C. difficile colitis one of the most common nosocomial infections (4). It is estimated that approximately 10-12 million adults are infected with this organism each year in the United States, about a third of whom become symptomatic. The disease burden in the elderly is particularly severe as they are hospitalized more frequently and for longer duration. The pathophysiology of C. difficile diarrhea requires alteration of the colonic microflora by antibiotics, colonization by C. difficile, and release of two potent enterotoxins designated A and B (5). The toxins of Clostridium difficile are required virulence factors in both animals and humans since non-toxigenic strains do not cause disease. Recent cloning and sequencing of the toxin genes reveals extensive amino acid homology between them that is reflected in common molecular and cellular mechanisms. Both toxins damage cells by modifying the rho family of proteins, key regulators of cellular actin. C. difficile infection causes a florid acute inflammatory response seen in patients with pseudomembranous colitis. It is now realized that neurons and immune cells of the lamina propria are major determinants of toxin-induced diarrhea and mucosal damage. Early critical events following toxin exposure are release of the neuropeptides substance P and calcitonin gene related peptide (CGRP) from sensory afferent neurons and activation of lamina propria macrophages and intestinal mast cells. These peptides in turn release a complex cascade of other inflammatory mediators from lamina propria cells (5). The importance of the host immune response, specifically serum IgG directed against toxin A, is now recognized as a critical determinant of disease expression in man.