Exploring the genetic and functional diversity of Porphyromonas gingivalis long fimbriae

Porphyromonas gingivalis is a key pathobiont in periodontitis. Its long fimbriae consist of a single anchor (FimB), a varying number of stalk (FimA), and three accessory (tip-related) proteins (FimC, FimD, and FimE). Based on 133 strains/genomes available, it was our aim to investigate the diversity within FimA and FimB and explain the variety of long fimbriae (super-)structures. Combining the new forward primer fimAnewF with the established fimAunivR, we were able to amplify and sequence fimA including its leader region covering all genotypes and serotypes for phylogenetic analysis. We designed two primer pairs sensing the presence of an internal stop codon in fimB with an impact on fimbrial length. Finally, we examined fimbrial secondary structures by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The phylogeny of fimA/FimA revealed two new subtypes (IIa and IIb) with specific changes in functional domains and thus adding to the current classification scheme (I, Ib, and II-V). Regarding evolution, we confirm that Porphyromonas gulae fimA-type A is closely related to human P. gingivalis strains of cluster Ib and might be its ancestor genotype. A fimB internal stop codon is rare and was found in ATCC 33277 only. Comparing P. gingivalis TEM/SEM pictures of type I ATCC 33277 with type V OMI622 revealed a broad spectrum of fimbrial structures including bundling, cell-cell knotting, and brick-wall formation. In conclusion, FimA forms more distinct subtypes than previously known. The bundling of long fimbriae, a mechanism known from EPEC/EHEC and Salmonella, is proposed and supported by TEM/SEM pictures for the first time here. The role and variations of terminal accessory FimC-E in superstructure formation and/or (co-) adhesion should be investigated more closely next.