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Background  Fimbriae are bacterial cell surface organelles involved in the pathogenesis of many bacterial species, including  Gallibacterium anatis,  in which a F17-like fimbriae of the chaperone-usher (CU) family was recently shown to be an important virulence factor and vaccine candidate. To reveal the distribution and variability of CU fimbriae 22 genomes of the avian host-restricted bacteria  Gallibacterium  spp. were investigated. Fimbrial clusters were classified using phylogeny-based and conserved domain (CD) distribution-based approaches. To characterize the fimbriae in depth evolutionary analysis and  in vitro  expression of the most prevalent fimbrial clusters was performed.    Results  Overall 48 CU fimbriae were identified in the genomes of the examined  Gallibacterium  isolates. All fimbriae were assigned to γ4 clade of the CU fimbriae of Gram-negative bacteria and were organized in four-gene clusters encoding a putative major fimbrial subunit, a chaperone, an usher and a fimbrial adhesin. Five fimbrial clusters (Flf-Flf4) and eight conserved domain groups were defined to accommodate the identified fimbriae. Although, the number of different fimbrial clusters in individual  Gallibacterium  genomes was low, there was substantial amino acid sequence variability in the major fimbrial subunit and the adhesin proteins. The distribution of CDs among fimbrial clusters, analysis of their flanking regions, and evolutionary comparison of the strains revealed that  Gallibacterium  fimbrial clusters likely underwent evolutionary divergence resulting in highly host adapted and antigenically variable fimbriae.  In vitro , only the fimbrial subunit FlfA was expressed in most  Gallibacterium  strains encoding this protein. The absence or scarce expression of the two other common fimbrial subunits (Flf1A and Flf3A) indicates that their expression may require other  in vitro  or  in vivo  conditions.    Conclusions  This is the first approach establishing a systematic fimbria classification system within  Gallibacterium spp. , which indicates a species-wide distribution of γ 4  CU fimbriae among a diverse collection of  Gallibacterium  isolates. The expression of only one out of up to three fimbriae present in the individual genomes  in vitro  suggests that fimbriae expression in  Gallibacterium  is highly regulated. This information is important for future attempts to understand the role of  Gallibacterium  fimbriae in pathogenesis, and may prove useful for improved control of  Gallibacterium  infections in chickens.    Electronic supplementary material  The online version of this article (doi:10.1186/1471-2164-15-1093) contains supplementary material, which is available to authorized users.

Chaperone-usher fimbriae in a diverse selection of Gallibacterium genomes