Evolutionary History of Clostridium difficile Pathogenicity Locus

Clostridium difficile is an important cause of healthcare-associated and community acquired diarrhoea, with potentially fatal complications. A region of the genome called the PaLoc encodes two toxins which cause the symptoms of infection. In this study, we show that the ancestors of today’s C. difficile lacked the PaLoc and were therefore harmless. Some of their modern day descendants have remained so, but others have gained the PaLoc and become pathogens. This has happened on multiple separate occasions. Some of these bacteria subsequently lost the PaLoc, becoming harmless once more. We dated the most recent loss to around 30 years ago. A novel genetic variant of the PaLoc was identified which contained a stably inserted sequence of DNA. This insertion helped us to find a novel family of mobile DNA sequences known as transposons. These were independent of the PaLoc, but they often carried antibiotic resistance genes, and are therefore also important in human infection. In conclusion, we demonstrated the importance of two genetic mechanisms in mobilising DNA sequences which contribute to C. difficile pathogenicity, indicating their potential to generate new pathogenic strains in future.