Porphyromonas gingivalis, a gram-negative anaerobic bacterium, is associated with the development of periodontal disease. The genetic diversity in virulence factors, such as adhesive fimbriae, among its strains affects the bacterial pathogenicity. P. gingivalis generally expresses two distinct types of fimbriae, FimA and Mfa1. Although the genetic diversity of fimA, encoding the major FimA fimbrilin protein, has been characterized, the genes encoding the Mfa1 fimbrial components, including the Mfa1 to Mfa5 proteins, have not been fully studied. We, therefore, analyzed their genotypes in 12 uncharacterized and 62 known strains of P. gingivalis (74 strains in total). The mfa1 genotype was primarily classified into two genotypes, 53 and 70. Additionally, we found that genotype 70 could be further divided into two subtypes (70A and 70B). The diversity of mfa2 to mfa4 was consistent with the mfa1 genotype, although no subtype in genotype 70 was observed. Protein structure modeling showed high homology between the genotypes in Mfa1 to Mfa4. The mfa5 gene was classified into five genotypes (A to E) independent of other genotypes. Moreover, genotype A was further divided into two subtypes (A1 and A2). Surprisingly, some strains had two mfa5 genes, and the 2nd mfa5 exclusively occurred in genotype E. The Mfa5 protein in all genotypes showed a homologous C-terminal half, including the conserved C-terminal domain recognized by the type IX secretion system. Furthermore, the von Willebrand factor domain at the N-terminal was detected only in genotypes A to C. The mfa1 genotypes partially correlated with the ragA and ragB genotypes (located immediately downstream of the mfa gene cluster) but not with the fimA genotypes.

Background Mfa1 fimbriae of the periodontal pathogen Porphyromonas gingivalis are responsible for biofilm formation and comprise five proteins: Mfa1–5. Two major genotypes, mfa1 70 and mfa1 53 , encode major fimbrillin. The mfa1 70 genotype is further divided into the mfa1 70A and mfa1 70B subtypes. The properties of the novel mfa1 70B remain unclear. Methods Fimbriae were purified from P. gingivalis strains JI-1 (mfa1 70A ), 1439 (mfa1 70B ), and Ando (mfa1 53 ), and their components and their structures were analyzed. Protein expression and variability in the antigenic specificity of fimbrillins were compared using Coomassie staining and western blotting using polyclonal antibodies against Mfa1 70A , Mfa1 70B , and Mfa1 53 proteins. Cell surface expression levels of fimbriae were analyzed by filtration enzyme-linked immunosorbent assays. Results The composition and structures of the purified Mfa1 fimbriae of 1439 was similar to that of JI-1. However, each Mfa1 protein of differential subtype/genotype was specifically detected by western blotting. Mfa1 70B fimbriae were expressed in several strains such as 1439, JKG9, B42, 1436, and Kyudai-3. Differential protein expression and antigenic heterogeneities were detected in Mfa2–5 between strains. Conclusion Mfa1 fimbriae from the mfa170A and mfa170B genotypes indicated an antigenic difference suggesting the mfa170B, is to be utilized for the novel classification of P. gingivalis.