Short fimbriae of Porphyromonas gingivalis and their role in coadhesion with Streptococcus gordonii

Abstract

Porphyromonas gingivalis, one of the causative agents of adult periodontitis, attaches and forms biofilms on substrata of Streptococcus gordonii. Coadhesion and biofilm development between these organisms requires the interaction of the short fimbriae of P. gingivalis with the SspB streptococcal surface polypeptide. In this study we investigated the structure and binding activities of the short fimbriae of P. gingivalis. Electron microscopy showed that isolated short fimbriae have an average length of 103 nm and exhibit a helical structure with a pitch of ca. 27 nm. Mfa1, the major protein subunit of the short fimbriae, bound to SspB protein, and this reaction was inhibited by purified recombinant Mfa1 and monospecifc anti-Mfa1 serum in a dose-dependent manner. Complementation of a polar Mfa1 mutant with the mfa1 gene restored the coadhesion phenotype of P. gingivalis. Hence, the Mfa1 structural fimbrial subunit does not require accessory proteins for binding to SspB. Furthermore, the interaction of Mfa1 with SspB is necessary for optimal coadhesion between P. gingivalis and S. gordonii.

FIG. 1.

Electron micrograph of purified short fimbriae. The short fimbrial preparations were purified from P. gingivalis KDP98 and negatively stained with 2% phosphotungstic acid. Bar, 0.2 μm.

FIG. 2.

Blot analysis of P. gingivalis extract coprecipitated with SspB. Lane 1, biotinylated P. gingivalis extract only developed with avidin-peroxidase; lane 2, control without SspB antibodies in the coprecipitation reaction; lane 3, P. gingivalis extract coprecipitated with SspB, boiled in sample buffer for 5 min and developed with Mfa antibodies; lane 4, P. gingivalis extract coprecipitated with SspB, boiled in sample buffer for 10 min and developed with Mfa antibodies. Molecular mass markers are at right.

FIG. 3.

Recombinant Mfa1 protein or rMfa antibodies inhibit P. gingivalis-S. gordonii coadhesion. (A) Tritiated P. gingivalis cells (10⁸) were reacted with rMfa antiserum or with preimmune serum (control) prior to incubation with streptococcal cells adsorbed to a nitrocellulose membrane. (B) Adsorbed S. gordonii cells were reacted with rMfa or control (SerB) protein prior to exposure to tritiated P. gingivalis cells (10⁸). Error bars indicate standard deviations (n = 3). Asterisk, statistically significant in comparison to control (P < 0.01; t test).

FIG. 4.

P. gingivalis strain cSMF1, complemented in trans with mfa1, expresses Mfa1 protein which is located on the cell surface and confers a coadhesion phenotype on the organism. (A) Immunoblot of whole-cell lysates. Lanes 1, strain 33277; lanes 2, SMF1 (Mfa1−); lanes 3, cSMF1 probed with antiserum to rMfa or to 33277 whole cells (1:10,000). Size standards (kDa) are shown in lane M. (B) ELISA of fixed cells of P. gingivalis strains 33277, SMF1, or cSMF1 probed with preimmune serum, antiserum to rMfa, or antiserum to 33277 whole cells. (C) Coadhesion of tritiated cells of P. gingivalis stains 33277, SMF1, or cSMF1 with S. gordonii cells adsorbed to a nitrocellulose membrane. Plot is the result of a curve-fitting algorithm using SigmaPlot V8. Error bars represent standard deviation (n = 3).

FIG. 5.

rMfa binds to synthetic BAR peptide in an ELISA format. Immobilized BAR was reacted with rMfa (20 μg/ml) and binding was detected with antiserum to rMfa (1:5,000). Plot is the result of a curve-fitting algorithm using SigmaPlot v8. Error bars represent standard deviations (n = 3). OD_405nm, optical density at 405 nm.