Porphyromonas gingivalis is one of the major anaerobic oral pathogens responsible for the extremely widespread inflammatory disorder periodontitis. Moreover, in recent years, P. gingivalis has appeared to be connected to the etiology of the autoimmune disease rheumatoid arthritis, whose causative agents are still not fully understood. It appears that the peptidylarginine deiminase enzyme of P. gingivalis (PPAD) could potentially citrullinate certain proteins in the host, leading to the production of anti- citrullinated proteins antibodies in genetically predisposed subjects. Considering that these antibodies have been found to be extremely specific for rheumatoid arthritis, the loss of tolerance for citrullinated proteins might be one of the causes of the disease and PPAD the link between this autoimmune disorder and P. gingivalis.The most astounding property of PPAD was considered to be its uniqueness among prokaryotes, with P. gingivalis being the only bacterium to possess it. The enzyme is in fact evolutionary unrelated to mammalian peptidylarginine deiminases, with which it shares only the function, and has never been previously reported in another species. Unprecedentedly, our analyses of 11 strains isolated from non-human hosts showed both the presence of P. gingivalis in different animals and, for the first time, the presence of PPAD in Porphyromonas gingivalis–related species.
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