Mobilisable Genetic Elements from the Clostridia

Mobilisable elements are genetic entities such as genomic islands, plasmids and transposons that are capable of intercellular movement, but are not in themselves conjugative. These elements rely on co-resident conjugative elements to provide in trans factors that facilitate their transfer to a recipient cell. A number of mobilisable transposons have been identified in the genus Clostridium and these fall into one of three broad categories. The first category consists of elements that encode enzymes responsible for their own transposition and mobilisation, exemplified by the Tn4451/3 family from Clostridium perjringens and Clostridium difficile. The second category defines elements that encode enzymes for their own transposition but which do not encode any mobilisation proteins, such as tlS Cpe8, an IS1595-like element from C.perjringens. The third and last category consists of elements that do not contain any discernable transposase, recombinase or mobilisation genes, as found with Tn5398 from C. difficile. Although each of these elements is unique, they have common features that include an origin of transfer site (oriT), which is an essential feature of mobilisable elements, and the presence of antibiotic resistance genes. These diverse elements are all capable of movement between bacterial cells when co-resident with conjugative elements and demonstrate that a plethora of diverse genetic elements are capable of disseminating antibiotic resistance determinants in this group of medically important pathogens. An understanding of how these elements evolve and spread is therefore important in a world where antibiotic resistance and a lack of treatment options represents an ever increasing threat to public health.