Haemophilus ducreyi (chancroids) Aggregatibacter actinomycetemcomitans (periodontitis) Escherichia coli (various diseases) Shigella dysenteriae (dysentery) Salmonella enterica serotype Typhi (typhoid fever) Campylobacter upsaliensis (enterocolitis) Campylobacter jejuni (enterocolitis) CDT-producing bacteria are often associated with mucosal linings, such as those in the stomach and intestines, and with persistent infections. The toxins are either secreted freely or associated with the membrane of the producing bacteria. Nomenclature Individual cytolethal distending toxins are named for the bacterial species that they are isolated from. As of 2011, most scientists have adopted the practice of placing the first letter of both the genus and species in front of the toxin name to reflect its source (i.e., the CDT from Haemaphilus ducreyi is referred to as HdCDT). If several subspecies produce different toxins, as in the case of E. coli, Roman numerals may be added after the second letter. Both complete toxins and individual subunits are labeled using this convention. In response to the continued discovery of additional cytolethal distending toxins, a 2011 review has proposed that the toxin names be expanded to include the first three letters of the species (i.e., HducCDT for Haemaphilus ducreyi CDT). Cellular effects CDT toxins are genotoxins capable of directly damaging DNA in target cells. They are the only AB-type toxins discovered that display DNase activity, allowing them to introduce breaks into the target cell's DNA. In many cell lines including human fibroblasts, epithelial cells, endothelial cells, and keratinocytes, CDTs cause G2/M cell cycle arrest, cytoplasmic distension, and eventual cell death via apoptosis. Most publications attribute the G2/M cycle arrest to the buildup of irreversible DNA damage from the toxin's DNase activity as the trigger for the G2/M cell cycle arrest, but other research suggests that this model is incomplete. The cytoplasmic distension is a direct result of the G2/M cell cycle arrest. The cell enlarges in preparation for mitosis, but cannot divide to restore its normal size. Aside from classical apoptosis, signs of cellular senescence has also been observed in normal and cancer cell lines (fibroblasts, HeLa and U2-OS) after CDT intoxication In lymphocytes, cell death occurs quickly and is not preceded by significant cytoplasmic distension. The ability of these toxins to effect lymphocytes differently may be advantageous to the bacteria that utilize these toxins, but the mechanism behind this phenomenon is not yet well understood. Toxin structure The active, assembled toxin is a tripartite structure with three distinct subunits- CdtA, CdtB, and