The Siderophore Product Ornibactin Is Required for the Bactericidal Activity of Burkholderia contaminans MS14

Burkholderia contaminans MS14 was isolated from soil in Mississippi. When cultivated on nutrient broth—yeast extract (NBY) agar medium, the colonies exhibit bactericidal activity against a wide range of plant pathogenic bacteria. Only a bacteriostatic compound with siderophore activity was successfully purified, which was determined by nuclear magnetic resonance (NMR) to be ornibactin. The isolation of the bactericidal compound has not yet been achieved, thus, the exact nature of the bactericidal compound is still unknown. In an attempt to isolate bactericidal compound, an interesting relationship between the production of ornibactin and the bactericidal activity of MS14 was characterized. Transposon mutagenesis resulted in two strains that lost bactericidal activity; an insertional mutation in a nonribosomal peptide synthetase (NRPS) gene for ornibactin biosynthesis and a luxR family transcriptional regulatory gene. Co-culture of these two mutant strains resulted in the restoration of the bactericidal activity. Furthermore, the addition of ornibactin to the NRPS mutant restored the bactericidal phenotype. In MS14, it has been demonstrated that ornibactin has an alternative function aside from iron sequestration. Comparison of the ornibactin biosynthetic genes in Burkholderia species show diversity among the regulatory elements, while the gene products for ornibactin synthesis are conserved. This is an interesting observation given that ornibactin is supposed to have the same defined function within Burkholderia species. Ornibactin is produced by most Burkholderia species and further consideration for its role in regulating secondary metabolites production should be investigated. Importance Identification of the antibacterial product from strain MS14 is not the key feature to this study. We display a series of experiments that demonstrate that ornibactin is directly involved in the bactericidal phenotype of MS14. This observation provides evidence for an alternative function for ornibactin aside from iron sequestration. Ornibactin should be further evaluated for its role in regulating secondary metabolite biosynthesis in other Burkholderia species.