Genetic and antigenic analyses of Porphyromonas gingivalis FimA fimbriae

The periodontal pathogen Porphyromonas gingivalis generally expresses two distinct fimbriae, FimA and Mfa1, which play a role in biofilm formation. The fimA gene that encodes FimA fimbrilin is polymorphic, and polymerase chain reaction analysis has identified six genotypes called types I–V and Ib. We found recently that fimbriae exhibit antigenic heterogeneity among the genotypes. In the present study, we analysed the fimA DNA sequences of 84 strains of P. gingivalis and characterized the antigenicity of FimA fimbriae. Strains analysed here comprised 10, 16, 29, 13, 10 and 6 strains of types I, Ib, II, III, IV and V, respectively. DNA sequencing revealed that type Ib does not represent a single cluster and that type II sequences are remarkably diverse. In contrast, the fimA sequences of the other types were relatively homogeneous. Antigenicity was investigated using antisera elicited by pure FimA fimbriae of types I–V. Antigenicity correlated generally with the respective genotype. Type Ib strains were recognized by type I antisera. However, some strains showed cross‐reactivity, especially, many type II strains reacted with type III antisera. The levels of fimbrial expression were highly variable, and expression was positively correlated with ability of biofilm formation on a saliva‐coated plate. Further, two strains without FimA and Mfa1 fimbriae expressed fimbrial structures, suggesting that the strains produce other types of fimbriae.