Clostridium difficile: (re)emergence of zoonotic potential.

Transmission routes of pathogenic bacteria can be very complex and can include direct transmission from human to human, contact with environment, contact with animals, aerosol transmission, vectors, and food and water intake. Clostridium difficile was historically regarded as a typically nosocomial pathogen, and hospitalization was one of the main risk factors for development of C. difficile infection. Within this environment, hand contamination with spores was the most important transmission mode. If hospital environment is the main reservoir for C. difficile, one would expect a very limited number of genotypes to prevail in a single hospital for a prolonged time. However, in nonoutbreak situations with good hospital hygiene, the number of genotypes found in a single hospital is rather high. A low number of genotypes (1 or 2) is constantly present, but many others are changing during time. This indicates that external sources play an important role in overall epidemiology and are constantly imported to the hospitals from the community. Outside hospitals,