Clostridium difficile infection: toxins and non-toxin virulence factors, and their contributions to disease establishment and host response

Abstract

Clostridium difficile infection is the leading cause of antibiotic- and healthcare-associated diarrhea, and its containment and treatment imposes a significant financial burden, estimated to be over $3 billion in the USA alone. Since the year 2000, CDI epidemics/outbreaks have occurred in North America, Europe and Asia. These outbreaks have been variously associated with, or attributed to, the emergence of Clostridium difficile strains with increased virulence, an increase in resistance to commonly used antimicrobials such as the fluoroquinolones, or host susceptibilities, including the use of gastric acid suppressants, to name a few. Efforts to elucidate C. difficile pathogenic mechanisms have been hampered by a lack of molecular tools, manipulatable animal models, and genetic intractability of clinical C. difficile isolates. However, in the past 5 y, painstaking efforts have resulted in the unraveling of multiple C. difficile virulence-associated pathways and mechanisms. We have recently reviewed the disease, its associated risk factors, transmission and interventions (Viswanathan, Gut Microbes 2010). This article summarizes genetics, non-toxin virulence factors, and host-cell biology associated with C. difficile pathogenesis as of 2011, and highlights those findings/factors that may be of interest as future intervention targets.

Figure 1. Schematic of human C. difficile infection. Spores, vegetative cells, bacterial factors and host factors that impact/modulate disease are depicted.

Figure 2. Spore surface of C. difficile clinical isolates. High-resolution, scanning electron micrograph overlayed with transmission-electron micrograph data of spore morphology of C. difficile strain VPI10463 immobilized on a solid surface. The orderly arrangement of spore protein complexes is clearly visible, as is an exosporial structure (diffusely-shaped and surrounding the spore. Micrograph collection, Vedantam laboratory.

Figure 3.C. difficile spores and vegetative cells adhere to host intestinal epithelium. False-color, high resolution, scanning electron micrograph of C. difficile spore (blue) and vegetative cell (red) adhering to human intestinal epithelial cells (green) grown in culture. Microvilli are clearly visible (feather-like green protrusions). Micrograph collection, Vedantam laboratory.

Figure 4.C. difficile clinical isolates produce flagella. False-color, high-resolution, scanning electron micrograph of a C. difficile ribotype 001 strain. Flagella (red) are clearly visible on vegetative cells (purple) after growth to exponential phase (OD₆₀₀ = 0.6) in Brain-Heart infusion medium supplemented with yeast and cysteine. Biophysical measurements indicate that flagella have a diameter of 17–22 nm, consistent with those described on other enteric bacteria. Micrograph collection, Vedantam laboratory.