Clostridium difficile colitis: pathogenesis and host defence

Abstract

Clostridium difficile is a major cause of intestinal infection and diarrhoea in individuals following antibiotic treatment. Recent studies have begun to elucidate the mechanisms that induce spore formation and germination and have determined the roles of C. difficile toxins in disease pathogenesis. Exciting progress has also been made in defining the role of the microbiome, specific commensal bacterial species and host immunity in defence against infection with C. difficile. This Review will summarize the recent discoveries and developments in our understanding of C. difficile infection and pathogenesis.

Figure 1. Sporulation and germination of C. difficile

A limited nutrient environment induces sporulation. The transcription factor stage 0 sporulation protein A (Spo0A) is phosphorylated by histidine kinases, activating a cascade of signalling and morphological events that create a forespore within the mother cell of the bacterium. After lysis, the spore is released into the environment. The core of the spore, which contains the condensed chromosome, is encapsulated by three protective layers: the cortex, coat and exosporium. Germination of the spore can be initiated by bile acids, such as taurocholate, which signal through the CspC receptor. Activation of the SleC enzyme by CspB leads to the degradation of the cortex of the spore and eventually leads to outgrowth of a new vegetative cell. P, phosphate.

Figure 2. Mechanism of action of C. difficile toxin in epithelial cells

a | The combined repetitive oligopeptide repeat (CROP) domain of TcdA binds to carbohydrates on the apical surface of epithelial cells, whereas TcdB binds to poliovirus receptor-like 3 (PVRL3) expressed on colonic epithelial cells. Toxin is internalized and acidification of the endosome enables the CROP domain to embed into the endosomal membrane and the subsequent transport of the cysteine protease domain (CPD) and the glucosyl transferase domain (GTD) into the cytosol. Inositol hexakisphosphate (IP₆) activates the cysteine protease to cleave and release the toxin glycotransferase. Glycosylation and thereby inactivation of RHO or RAC GTPases ultimately results in the breakdown of tight junctions and epithelial integrity. b | Binary toxin (also known as Clostridium difficile transferase (CDT)) binds to the lipolysis-stimulated lipoprotein receptor (LSR) and is internalized. The CdtB subunit creates pores in the acidified endosome that enable the release of the CdtA subunit into the cytosol. The ADP-ribosyl transferase activity of CdtA inhibits actin polymerization near the cell membrane, enabling fibronectin microtubules to elongate and protrude through microvilli, which increases C. difficile adherence to the epithelium through type IV pili.

Figure 3. Microbiota-mediated defences against C. difficile

a | The intact microbiota converts primary bile acids into secondary bile acids, several derivatives of which inhibit the growth of Clostridium difficile through detergent-induced toxicity to vegetative bacilli. Commensal bacteria that express sialidases cleave sugars that are attached to epithelial cells and release sialic acid into the intestinal lumen. Fermenting commensal bacterial species convert carbohydrates into short-chain fatty acids (SCFAs), such as succinate. Bystander commensal bacterial populations can consume these metabolites as energy sources. b | Antibiotic-mediated disruption of the microbiota depletes primary bile acid converters, which enables C. difficile sporulation and growth. Antibiotics can also deplete competing sialic acid and succinate consumers, liberating an energy source for C. difficile.

Figure 4. Innate immune-mediated defences against C. difficile

The acute host response to Clostridium difficile is initiated by toxin-mediated damage, loss of epithelial integrity and the detection of translocating bacteria. Intestinal epithelial cells and resident innate immune cells secrete pro-inflammatory chemokines (such as chemokine C-X-C motif ligand 1 (CXCL1), CXCL2, and interleukin-8 (IL-8)) and pro-inflammatory cytokines (such as IL-1β, IL-12 and IL-23), which leads to the recruitment of neutrophils and the activation of innate lymphoid cells (ILCs). IL-12 signalling drives the expression of interferon-γ (IFNγ), whereas IL-1β and IL-23 signalling induces the production of IL-22. The effector cytokines IFNγ and IL-22 induce defence mechanisms such as increased phagocytic activity of macrophages and neutrophils and the production of antimicrobial peptides and enzymes that synthesize reactive oxygen species (ROS) and reactive nitrogen species (RNS). These defence mechanisms limit bacterial dissemination, attenuate toxin activity and repair epithelial damage. DCs, dendritic cells.