Updated publication reference for PubMed record(s): 27712078. Porphyrmonas gingivalis is an oral pathogen associated with the inflammatory disease periodontitis. The colonization of oral epithelial surfaces by P. gingivalis may also lead to the autoimmune disease rheumatoid arthritis. One of the hallmarks of rheumatoid arthritis is the loss of tolerance against citrullinated proteins. Citrullination is a post-translational modification of arginine residues, leading to a change in structure and function of the respective protein. This modification is catalysed by peptidylarginine deiminases (PAD). Interestingly, P. gingivalis secretes a citrullinating enzyme, known as P. gingivalis PAD (PPAD), which targets bacterial and human proteins. Since the extent of P. gingivalis protein citrullination by PPAD was not yet known, the present study was aimed at identifying the extracellular proteome and citrullinome of P. gingivalis. To this end, extracellular proteins of two reference strains, two PPAD-deficient mutants and three clinical isolates of P. gingivalis were analysed by mass spectrometry. The results uncovered substantial heterogeneity in the extracellular proteome and citrullinome of P. gingivalis, especially in relation to the extracellular detection of typical cytoplasmic proteins. In contrast, major virulence factors were identified in all investigated isolates although their citrullination was shown to vary. This may be related to post-translational processing of the PPAD enzyme.