REVIEW ARTICLE The Clostridium perfringens a-toxin Richard W. Titball 1 * , Claire E. Naylor 2 and Ajit K. Basak 2 1 Defence Evaluation and Research Agency, CBD Porton Down, Salisbury, Wiltshire, SP4 OJQ, U.K.; and 2 Department of Crystallography, Birkbeck College, Malet St, London, WC1E 7HY, U.K. (Received 11 March 1999, accepted 19 March 1999) Key Words: C. perfringens, phospholipase C, gas gangrene, a-toxin; haemolysin The gene encoding the a-(cpa) is present in all strains of Clostridium perfringens, and the purified a-toxin has been shown to be a zinc-containing phospholipase C enzyme, which is preferentially active towards phospha- tidylcholine and sphingomyelin. The a-toxin is haemolytic as a result if its ability to hydrolyse cell membrane phospholipids and this activity distinguishes it from many other related zinc-metallophospholipases C. Recent studies have shown that the a-toxin is the major virulence determinant in cases of gas gangrene, and the toxin might play a role in several other diseases of animals and man as diverse as necrotic enteritis in chickens and Crohn's disease in man. In gas gangrene the toxin appears to have three major roles in the pathogenesis of disease. First, it is able to cause mistrafficking of neutrophils, such that they do not enter infected tissues. Second, the toxin is able to cause vasoconstriction and platelet aggregation which might reduce the blood supply to infected tissues. Finally, the toxin is able to detrimentally modulate host cell metabolism by activating the arachidonic acid cascade and protein kinase C. The molecular structure of the a-toxin reveals a two domain protein. The amino-terminal domain contains the phospholipase C active site which contains zinc ions. The carboxyterminal domain is a paralogue of lipid binding domains found in eukaryotes and appears to bind phospholipids in a calcium-dependent manner. Immunisation with the non-toxic carboxyterminal domain induces protection against the a-toxin and gas gangrene and this polypeptide might be exploited as a vaccine. Other workers have exploited the entire toxin as the basis of an anti-tumour system. # 1999 Academic Press Introduction Clostridium perfringens is a Gram-positive anaerobe which is ubiquitous in the environment, being found in the soil, in decaying organic matter and as a member of the normal gut flora of many animals [1]. The ability of the bacterium to form highly resistant endospores means that it is able to persist in these environments, and especially within soil. All strains of the bacterium possess the gene encoding the a- toxin (cpa), and the differential possession of other toxin-encoding genes is used to identify strains as biotypes A to E [1,2]. The a-toxin is the best studied of *Corresponding author. Tel.: Int +441980 613301; fax: Int +441980 613284; E-mail: rtiteall@dera.gov.uk 1075-9964/99/020051+14 $30:00/0 # 1999 Academic Press Anaerobe (1999) 5, 51± 64 Article No. anae.1999.0191