"Clostridium difficile is a Gram-positive, spore-forming anaerobic bacillus that often resides in the gut of mammals and is the causative agent of the illness C. difficile infection (CDI). The hospital environment and patients undergoing antibiotic treatment provide a discrete ecosystem where C. difficile persists and select virulent clones thrive. CDI is the most frequent cause of infectious diarrhea in hospitals worldwide, causing in the last decade outbreaks of increased severity in Europe and North America. The continued rise of CDI worldwide has been accompanied by the rapid emergence and transcontinental spread of at least two highly virulent clones that are commonly referred to as 027 and A-B+. The overall aim of this project is to determine the genetic and phenotypic attributes that might explain the emergence of these hypervirulent 027 strains. To this end we will obtain mutants in genes we suspect to be important for 027 strains virulence. Those mutants will be characterized in detail by genetic and phenotypical studies. This project provides a unique opportunity to study the recent emergence of virulence and will contribute more generally to our understanding of how bacterial pathogens evolve."