The emergence of ‘hypervirulence’ in Clostridium difficile

The impact of Clostridium difficile-associated disease (CDAD) in healthcare settings throughout the developed world is considerable in terms of mortality, morbidity, and disease management. The incidence of CDAD has risen dramatically since the turn of this century, concomitant with the emergence of so-called hypervirulent strains which are thought to cause a more severe disease, higher relapse rates, and increased mortality. Pre-eminent amongst hypervirulent strains are those belonging to ribotype 027, which were first reported in Canada in 2003 and shortly thereafter in the UK. Since its arrival in Europe, it has spread rapidly and has now been reported in 16 member states and Switzerland. The physiological factors responsible for the rapid emergence of hypervirulent C. difficile strains remain unclear. It is known that they produce a binary toxin (CDT) in addition to toxins A and B, that they are resistant to fluoroquinolones due to mutations in gyrA, and that they are resistant to erythromycin. Representative strains have been suggested to produce more toxin A and B in the ‘laboratory flask’ (most likely due to a frameshift mutation in the repressor gene tcdC), to be more prolific in terms of spore formation, and also exhibit increased adherence to human intestinal epithelial cells due to altered surface proteins. However, the contribution of these and other as yet unidentified factors to the rapid spread of certain C. difficile variants (e.g., ribotypes 027 and 078) remains unclear at present. The advent of ClosTron technology means that it is now possible to construct genetically stable isogenic mutants of C. difficile and carry out reverse genetic studies to elucidate the role of specific gene loci in causing disease. The identification of virulence factors using this approach should help lead to the rational development of therapeutic countermeasures against CDAD.