Necrotic Enteritis

Thus far only two pathogenic properties have been implicated in the pathogenesis of necrotic enteritis: the adhesion of C. perfringens type C isolates to the intestinal mucosa and the production of

P-toxin by those adherent isolates (5).

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.

Unfortunately, little is currently known about the genetic or physiological basis by which type C isolates adhere to the intestines.

With respect to P-toxin, early studies (62) had strongly suggested that the cpb gene encoding P-toxin is present on a large plasmid present in type C and type D isolates of C. perfringens. More recent studies (63,64) using molecular approaches have confirmed that finding and also indicated that the plasmid-borne cpb gene is associated with an insertion sequence. While it seems clear that maximal P-toxin expression occurs during the exponential phase of vegetative cell growth (5), the molecular regulation of P-toxin synthesis has not yet been intensively studied.

The cloning and sequencing of the cpb gene in the 1990s (63,64) provided some important insights into both P-toxin's action and structure/function relationship. Results from those cloning studies indicate that P-toxin is first expressed as a single polypeptide consisting of 336 amino acids. An N-terminal signal peptide of 27 amino acids is then removed during secretion, yielding a mature toxin of 309 amino acids (34,861 Mr). As mentioned earlier (Sec. VI), recent evidence (38) suggests that this P-toxin monomer can oligomerize, with that oligomerization process possibly contributing to pathogenesis.

Sequencing studies of the cloned cpb gene (63) also revealed that this toxin shares ~28% homology with another spore-forming toxin, Staphylococcus aureus a-toxin. Based upon this shared sequence homology and results from previous studies mapping S. aureus a-toxin structure/function relationships, site-directed mutagenesis was performed on cpb (64). Those mutagenesis studies produced results supporting the hypothesis that C. perfringens P-toxin and S. aureus a-toxin share partial, although not complete, similarity in their structure and action. The previously mentioned findings (38) indicating that, like S. aureus a-toxin, P-toxin can oligomerize and affect membrane permeability properties in mammalian cells, provide some additional evidence that these two toxins share some similarities in their action and structure.

Additional mutagenesis experiments (64) demonstrated that the single cysteine residue located at residue 265 of mature P-toxin is not important for action. However, those same mutagenesis studies did indicate that the region surrounding that cysteine 265 residue may be important for maintaining a proper secondary structure for P-toxin activity.

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