Identification of a transposon variant of Porphyromonas gingivalis expressing long Mfa1 fimbriae due to mfa2 inactivation

Abstract

Objectives

Fimbriae expressed by Porphyromonas gingivalis, a periodontal pathogen, play a pivotal role in biofilm formation. In the type strain ATCC 33277, Mfa1 fimbriae form short structures anchored to the cell surface by Mfa2, an outer membrane protein involved in regulation of fimbrial length. Mfa1, the major fimbrilin, is classified into three genotypic types—mfa1^70A, mfa1^70B, and mfa1⁵³—based on gene sequence. Although strain D83T3 is classified as mfa1^70A, like ATCC 33277, it expresses a 73 kDa Mfa1. This study aimed to investigate D83T3's unique functional properties to improve mfa1 genotyping.

Methods

The fimA-deficient ATCC 33277 strain (JI-1) was used as a reference. Mfa1 polymerization and localization were analyzed using immunoblotting. N-terminal processing was evaluated by Edman degradation. Fimbrial morphology was examined using transmission electron microscopy. The region downstream of mfa1 was sequenced. Mfa2 expression and the presence of Mfa3–5, putative tip proteins in the fimbriae, were confirmed by immunoblotting.

Results

The 73 kDa Mfa1 polymer was predominantly detected in D83T3's culture supernatant. Cleavage was confirmed at the gingipain recognition site. Mfa1 fimbriae in D83T3 were longer than those in JI-1. An IS5 transposase insertion was observed between mfa1 and mfa2 at D83T3. Mfa2 expression was reduced in D83T3 cells, and Mfa3–5 was absent from the fimbriae.

Conclusions

D83T3 is a transposon insertion variant that releases abnormally long Mfa1 fimbriae extracellularly due to mfa2 inactivation. Our study's findings offer new insights, and analysis of the mfa1-mfa2 gene structure can complement mfa1 genotyping for the classification of P. gingivalis.