Type IV pili (T4P) are dynamic filamentous appendages found in many bacteria, playing key roles in motility, adhesion, DNA uptake, and host interaction. Based on sequence and structural differences, T4P have been traditionally classified into two major groups, including the type IVa pili (T4aP) and the type IVb pili (T4bP), each with distinct components and functional specializations. Furthermore, the Tad locus was first identified in Aggregatibacter actinomycetemcomitans as essential for biofilm formation and strong surface attachment. Although historically considered a variant of type IV pili, phylogenomic analyses have recently reclassified Tad systems as a distinct subgroup, now referred to as Type IVc pili (T4cP). This classification reflects their unique evolutionary origin and structural features, setting them apart from other type IV pili systems.

To fully explore the T4P genetic biodiversity within the genus Bifidobacterium, 117 reference strains representing the full (sub)species diversity of the taxon were included in the screening. To date, the bifidobacterial T4P database comprises 1074 genes belonging to a highly conserved T4P architecture present in all subspecies, with minor structural variations, as well as a second, less common T4P locus identified in 13 species.