The salicylate-derived mycobactin siderophores of Mycobacterium tuberculosis are essential for growth in macrophages

Mycobacterium tuberculosis is an important pathogen of mammals that relies on 2-hydroxyphenyloxazoline-containing siderophore molecules called mycobac tins for the acquisition of iron in the restrictive environment of the mamm alian macrophage, These compounds have been proposed to be biosynthesized t hrough the action of a cluster of genes that include both nonribosomal pept ide synthase and polyketide synthase components. One of these genes encodes a protein, MbtB, that putatively couples activated salicylic acid with ser ine or threonine and then cyclizes this precursor to the phenyloxazoline ri ng system. We have used gene replacement through homologous recombination t o delete the mbtB gene and replace this with a hygromycin-resistance casset te in the virulent strain of M. tuberculosis H37Rv, The resulting mutant is restricted for growth in iron-limited media but grows normally in iron-rep lete media. Analysis of siderophore production by this organism revealed th at the biosynthesis of all salicylate-derived siderophores was interrupted. The mutant was found to be impaired for growth in macrophage-like THP-1 ce lls, suggesting that siderophore production is required for virulence of M. tuberculosis, These results provide conclusive evidence linking this genet ic locus to siderophore production.