Enterotoxic Clostridia: Clostridium perfringens Enteric Diseases

Abstract

In humans and livestock, Clostridium perfringens is an important cause of intestinal infections that manifest as enteritis, enterocolitis, or enterotoxemia. This virulence is largely related to the toxin-producing ability of C. perfringens. This article primarily focuses on the C. perfringens type F strains that cause a very common type of human food poisoning and many cases of nonfoodborne human gastrointestinal diseases. The enteric virulence of type F strains is dependent on their ability to produce C. perfringens enterotoxin (CPE). CPE has a unique amino acid sequence but belongs structurally to the aerolysin pore-forming toxin family. The action of CPE begins with binding of the toxin to claudin receptors, followed by oligomerization of the bound toxin into a prepore on the host membrane surface. Each CPE molecule in the prepore then extends a beta-hairpin to form, collectively, a beta-barrel membrane pore that kills cells by increasing calcium influx. The cpe gene is typically encoded on the chromosome of type F food poisoning strains but is encoded by conjugative plasmids in nonfoodborne human gastrointestinal disease type F strains. During disease, CPE is produced when C. perfringens sporulates in the intestines. Beyond type F strains, C. perfringens type C strains producing beta-toxin and type A strains producing a toxin named CPILE or BEC have been associated with human intestinal infections. C. perfringens is also an important cause of enteritis, enterocolitis, and enterotoxemia in livestock and poultry due to intestinal growth and toxin production.

FIGURE 1

Structure of CPE and the C-terminal CPE structure bound to a claudin-4 receptor. (A) Structure of the CPE monomer, colored from blue at the N-terminus to red at the C-terminus. Note the presence of two distinct domains, including the C-terminal domain (red/yellow) mediating receptor binding and the blue-green N-terminal domain mediating oligomerization, membrane insertion, and pore formation. The alpha helix labeled αA is located at the TM1 region that becomes a beta-hairpin when CPE is assembled in the prepore. Beta-hairpins from the seven CPE molecules in the prepore are thought to then form a beta-barrel that inserts into membranes to form the active pore. (B) Structure of the C-terminal CPE region (C-CPE) bound to the human claudin-4 receptor. The claudin receptor is rainbow-colored and the C-CPE is purple. The membrane orientation of the claudin receptor, including the transmembrane helices, is also shown. Panels A and B are reproduced with permission from references and , respectively.

FIGURE 2

Model of the CPE mechanism of action. At the top left, CPE binds to receptors forming a small complex that contains both receptor claudins and nonreceptor claudins, as well as CPE. At 37°C, several small complexes interact to form a prepore on the membrane surface. Portions of CPE in the prepore insert into the membrane to form a pore that allows Ca²⁺ entry into the cytoplasm. With high CPE doses, there is a massive Ca⁺² entry that causes strong calpain activation to trigger oncosis (a form of necrotic cell death). At low CPE doses, there is a more limited Ca²⁺ entry that causes a mild calpain activation; this results in mitochondrial membrane depolarization, cytochrome C release, and caspase-3 activation to cause death by apoptosis. Dying CPE-treated cells undergo morphologic damage that exposes their basolateral surface to CPE, resulting in formation of a second large complex containing occludin (as well as CPE and claudins), which induces internalization of these molecules. This effect may contribute to paracellular permeability changes, at least in cultured cells. Reproduced with permission from reference .

FIGURE 3

Organization of cpe loci in type C, D, E, and F strains of C. perfringens. (A) Organization of plasmid-borne cpe loci in type F, E, C, and D strains. (B) Organization of the type F chromosome cpe locus. Asterisks indicate a region with similarity to sequences present downstream of the cpe gene in F4969, except for the absence of an IS1470-like gene. Modified with permission from reference .

FIGURE 4

Model of C. perfringens sporulation and enterotoxin production. Via the Agr-like quorum sensing system, CcpA and CodY, and unidentified intermediates, a histidine kinase(s) affects spooA expression and/or possibly Spo0A phosphorylation to initiate sporulation. This triggers a sigma factor cascade, where SigF controls production of three other sporulation-associated sigma factors (SigE, SigG, and SigK). SigE and SigK then regulate CPE production during sporulation by enhancing cpe expression. All four sigma factors are needed for sporulation. Reproduced with permission from reference .