UNIQUE FIMBRIAE-LIKE STRUCTURES ENCODED BY SEFD OF THE SEF14 FIMBRIALGENE-CLUSTER OF SALMONELLA-ENTERITIDIS

The SEF14 gene cluster of Salmonella enteritidis was recently shown to contain three genes, sefABC, encoding a unique fimbrin, and proteins homologous to fimbrial chaperones and outer membrane proteins (ushers) , respectively. A fourth open reading frame, designated sefD, was foun d immediately downstream of sefABC and overlapping sefC. The translate d protein sequence of sefD was unique, but the composition was similar to that of other bacterial fimbriae. SefD was produced in abundance b y wildtype S. enteritidis as shown by Western blot analysis using anti bodies raised to affinity-purified, recombinant SefD. Furthermore, unu sually long, thin, fimbriae-like structures were evident on S. enterit idis and Escherichia coli by immunoelectron microscopy, but in other b acterial species SefD was expressed as amorphous material. Therefore, in S. enteritidis and E. coli, SefD is the predominant structural subu nit of SEF18. The SEF18 fimbriae-like structures were shown to be sero logically distinct from the three known S. enteritidis fimbriae SEF14, SEF17 and SEF21. Furthermore, SEF18 was still produced in sefA insert ion mutants, indicating that SEF14 and SEF18 were structurally distinc t. Thus, the SEF14 gene cluster is the first example in the Enterobact eriaceae of a gene cluster that encodes two fimbrin-like proteins, whi ch are assembled into two distinct cell-surface structures, SEF14 and SEF18. DNA hybridization and Western blot analyses showed that SefD wa s widely distributed among the Enterobacteriaceae and was present in E . coli, Shigella, Enterobacter, Citrobacter, Erwinia, Hafnia, Klebsiel la, Providencia, and Proteus but not in the non-Enterobacteriaceae Gra m-negative bacteria Pseudomonas and Aeromonas, or in Gram-positive bac teria Bacillus or Staphylococcus. Immunoelectron microscopy revealed t hat SefD was also present on the surface of Providencia and Klebsiella but did not appear filamentous. This is the first instance of highly conserved, thin fimbriae-like structures which are ubiquitous among th e Enterobacteriaceae.