Porphyromonas gingivalis oral infection increased arthritis development in mice immunized with collagen II in incomplete Freund’s adjuvant. Mice infected with P. gingivalis (Pg) in the oral cavity for 15 days were immunized with incomplete Freund’s adjuvant (IFA)/collagen II (CII) 15 days later as described in Methods. (A) Arthritis development and severity was scored by a visual assessment score on a 0 to 4 scale. The total number of arthritic paws was significantly higher in mice with P. gingivalis oral infection and IFA/CII immunization compared with IFA/CII alone. (B) Paws collected at D73 demonstrated increased synovitis and greater bone involvement if mice had P. gingivalis oral infection and IFA/CII immunization compared with IFA/CII alone. (C) Paws collected at timeline D0, D30, D44, and D73 were scored for tartrate-resistant acid phosphatase (TRAP)-positive cells. Increased osteoclasts numbers were observed at D73 if mice had P. gingivalis infection and IFA/CII immunization. Error bars represent ± standard error of the mean of eight mice per group, P <0.05, Student’s t test and analysis of variance.