Identification of a siderophore receptor required for ferric ornibactin uptake in Burkholderia cepacia.

Ornibactins are linear hydroxamate siderophores produced by Burkholderia cepacia with peptide structures similar to that of pyoverdines produced by the fluorescent pseudomonads. The gene encoding the outer membrane receptor (orbA) was identified, sequenced, and demonstrated to have significant homology with hydroxamate receptors produced by other organisms. The orbA precursor was predicted to be a protein with a molecular mass of 81 kDa. An orbA mutant was constructed and demonstrated to be unable to take up (59)Fe-ornibactins or to grow in medium supplemented with ornibactins. Outer membrane protein profiles from the parent strain, K56-2, revealed an iron-regulated outer membrane protein of 78 kDa that was not detectable in the K56orbA::tp mutant. When this mutant harbored a plasmid containing the orbA gene, the 78-kDa protein was present in the outer membrane protein profiles and the mutant was able to utilize ornibactin to acquire iron. The orbA mutant was less virulent in a chronic respiratory infection model than the parent strain, indicating that ornibactin uptake and utilization are important in the pathogenesis of B. cepacia respiratory infections.