Clostridium perfringens beta-toxin forms potential-dependent, cation-selective channels in lipid bilayers.

Beta-toxin is produced by Clostridium perfringens type B and C strains and is the primary lethal factor in the type C strains. No molecular mechanism has been elucidated for beta-toxin which could be used as a basis for investigating its role in the pathogenesis of these clostridial pathogens. It has been suggested that beta-toxin may be a pore-forming toxin on the basis of weak similarities (10% identity) between the primary structure of beta-toxin and those of the pore-forming alpha-hemolysin and gamma-hemolysin and the leukocidin from Staphylococcus aureus (9). Whether or not beta-toxin is cytotoxic remains unclear; only a single report has suggested that beta-toxin is weakly cytotoxic on intestinal 407 cells (6). However, a previous study suggested that the cytotoxicity associated with beta-toxin preparations was not linked to the beta-toxin itself, but to minor contaminants in the toxin preparation from C. perfringens (11). Recently, Steinthorsdottir et al. demonstrated that beta-toxin could induce the release of arachidonic acid and inositol from human umbilical vein endothelial cells (HUVECs) (32). No cytolytic effects were reported, suggesting that beta-toxin may not be necessarily lethal to these cells. Several other cell types were also tested by these investigators, but they were unresponsive to beta-toxin. C. perfringens type C strains cause necrotic enteritis primarily in pigs, chickens, cattle, sheep, and goats. Although adult animals can contract this disease, it most frequently occurs in the young of these species (34). Piglets are particularly susceptible to type C infections (5, 10, 18, 33), although a similar infection occurs in neonatal calves (7), lambs (8), and goats. During a type C infection, necrosis of the intestine can be extensive; death appears to be the result of toxemia with beta-toxin (reviewed in reference 29). Acute and peracute deaths frequently occur in these animals, suggesting that systemic effects of the toxin are important. In a C. perfringens type C disease of adult sheep, termed “struck,” the animals succumb to the infection so rapidly that they appear to have been struck by lightning. Prior to death, nervous signs such as tetani and opisthotonus have been observed in these animals (reviewed in reference 29), suggesting neurological involvement. Infection of humans by type C strains appears to be largely restricted to certain tribal populations in Papua New Guinea, although infrequent cases of type C infection have occurred in humans throughout the world. Type C infections result in necrotizing enterocolitis (“pigbel”) in these individuals after consumption of undercooked pork during certain ritualistic practices (13). Typically, type C necrotizing enterocolitis in humans resembles the disease in animals. The importance of beta-toxin in both animal and human disease has been demonstrated by immunization studies using a toxoid of beta-toxin. When immunized with the toxoid of beta-toxin, the Papua New Guinea tribespeople experienced a fivefold reduction in the incidence of necrotic enteritis (13), whereas a beta-toxin toxoid administered to infant pigs during an outbreak of necrotizing enterocolitis reduced mortality by approximately 30% (30). In the case of agriculturally important animals, vaccination against type C infections is universally advocated in order to avoid devastating losses. Therefore, beta-toxin plays a key role in the lethal outcome of type C infections, yet we know very little about its mechanism or the cell types it affects. The results presented below demonstrate that beta-toxin is an efficient pore-forming toxin which generates potential-dependent, cation-selective channels in membranes. The channels formed by beta-toxin exhibit characteristics that may provide some insight into the lethal activity of this toxin.