Sequence Diversity of Actinomyces naeslundii Fimbriae Subunit Proteins

Objectives: fimA and fimP genes code for the major subunit proteins of actinomyces type-2 and type-1 fimbriae respectively. A. naeslundii genospecies 2 possesses both types of fimbriae while genospecies 1 possesses only type-2. The fimbriae exhibit different binding affinities for salivary proteins and β-linked N-acetylgalactosamine. The complete nucleotide sequences of a small number of fimA genes have been reported but there is no comprehensive sequence analysis of the fimP gene. We have therefore undertaken an analysis of major fragments of both genes in a collection of A. naeslundii reference strains and clinical isolates (n=116).

Methods: A 690bp fragment of the fimA and a 680bp fragment of the fimP gene were amplified, sequenced and aligned in-frame. Phylogenetic trees were constructed for each gene and the sets of sequences were tested for evidence of recombination.

Results: The analysis of the fimA sequences indicated that the majority of genospecies 1 and 2 were well separated but a distinct branch was present which included 9 genospecies 1 and 8 genospecies 2 strains. The fimP sequences were obtained primarily from genospecies 2 isolates and these were present in a single cluster but other discrete clusters were observed and fimP gene sequence was identified in a small number of genospecies 1 strains. There was statistical evidence of recombination within the fimA and fimP gene sequences.

Conclusions: The sequence diversity of both fimA and fimP genes of A. naeslundii was greater than previously appreciated but the functional consequence of this diversity requires investigation. The sequence of fimA is not genospecies-specific.

This work was funded by Dental Institute King's College London.