fimbrie

Fimbrie, singular fimbria, are slender, hair-like appendages on the surface of many bacteria. In English, the plural form fimbriae is commonly used. These structures mediate adhesion to surfaces, including host tissues and abiotic substrates, and thereby promote colonization, persistence, and the formation of biofilms.

Fimbriae are protein polymers that project from the bacterial envelope. In many Gram-negative bacteria, they are assembled by the chaperone-usher pathway. Subunits are secreted into the periplasm, where a chaperone stabilizes them before an outer membrane usher polymerizes them into a rigid, rod-like filament that extends from the cell surface. Some bacteria also possess fimbriae assembled by alternative routes; in Gram-positive organisms, fimbriae are often anchored to the cell wall by sortase enzymes. Each fimbrial fiber typically bears one or more adhesive tip structures that recognize specific receptors on target surfaces.

Functional diversity among fimbriae is linked to receptor specificity. Type 1 fimbriae of Escherichia coli, for example, often bind mannose-containing receptors, while P fimbriae (Pap) recognize Gal(alpha1-4)Gal-containing receptors, contributing to urinary tract infections. Other fimbriae recognize different carbohydrate or glycan motifs and participate in tissue tropism, colonization, and biofilm initiation. Expression of fimbriae is frequently regulated and can be phase-variable, allowing bacteria to adapt to changing environments, including host-associated niches.

Fimbriae are distinct from pili in several ways. They are typically shorter, more numerous, and specialized for adhesion, whereas pili (longer, fewer) can mediate processes such as conjugation and certain forms of surface motility. Nonetheless, both are filamentous surface structures critical to bacterial interactions with their surroundings.