Diversity and assembly mechanisms of Porphyromonas gingivalis fimbriae: Conservation and structural heterogeneity of type V fimbriae

Porphyromonas gingivalis is a periodontal pathogen that primarily mediates its interactions with host tissues and other bacteria through fimbrial structures. The diversity and assembly mechanisms of its fimbriae are fundamental to understanding its pathogenicity, and its roles in periodontal and systemic diseases. This bacterium mainly expresses two types of fimbriae, FimA and Mfa1. Recent structural and genetic studies have demonstrated that these fimbriae conform to the conserved assembly principles of Type V fimbriae shared by Bacteroidia-class bacteria. This review focuses on Mfa1 fimbriae, and it summarizes the current knowledge on their assembly mechanisms, genetic diversity, components, and strain-dependent structural variations. In particular, the balance between conserved structural features and strain-specific variations is discussed, focusing on the molecular basis of fimbrial formation and incorporation of accessory proteins. The discussion centers on the major fimbrillin Mfa1 anchor protein, Mfa2, accessory proteins, Mfa3 and Mfa4, and the variable Type IX secretion system-dependent accessory protein, Mfa5. By integrating recent findings, this review emphasizes how a conserved Type V assembly framework, combined with variations in accessory components, enables Mfa1 fimbriae to maintain structural stability, while accommodating strain-dependent diversity. This synthesis provides an updated perspective on the balance between stability and strain-dependent structural adaptability in Mfa1 fimbrial systems, and their potential implications for fimbrial assembly, bacterial adhesion, and host–microbe interactions. • Mfa1 fimbriae are shaped through the V-type core framework, with the N -mature region also potentially contributing, and through T9SS-dependent transport. • Genetic variation in the mfa1–4 genes contribute to fimbrial diversity among strains, while generally being accommodated within the conserved structural framework. • Mfa5, which belongs to a separate genetic and structural class from the mfa1-4 , may generate strain-dependent diversity in fimbrial composition that is unique to P. gingivalis.