Abstract: Antigenic preparations active against species of bacteria naturally producing Type 4 fimbriae, such as Bacteroides nodosus which causes footrot in sheep, are produced by culturing a suitable genetically engineered aerobic or facultatively anaerobic host bacterial cell such as Pseudomonas aeruginosa. The host cell contains the gene encoding the fimbrial structural subunit antigens of at least one strain of the said species of bacteria and an endogenous compatible system for the morphogenetic assembly of Type 4 fimbriae and/or the genes for the morphogenetic assembly of such fimbriae derived from a Type 4 fimbriate species. The genetically engineered host cells are cultured such that nature fimbriae are produced as extracellular structures and the fimbriae are harvested, substantially free of the host cells, for use as the antigenic preparation. In preferred embodiments of the invention the natural fimbrial subunit gene is modified to enhance expression in the new host.