Growth and siderophore production in Bradyrhizobium (lupin) strains under iron limitation

Six Bradyrhizobium (lupin) strains were evaluated for their ability to produce siderophores using four chemical assays. Two strains gave positive reactions with chrome azurol S assay (CAS) and produced hydroxamate-type siderophores. The other four strains gave negative results for siderophore production using the four assays. Generation time, growth yield and hydroxamate production of one strain (WPBS 3201 D) were affected by the iron concentration of the culture medium and the previous culture history of the cells. Resuspension of washed cells grown previously in media supplemented with 0 and 20 mu mol/L Fe into differing iron regimes (0, 05, 1, 2, 4, 8, 10, 15 and 20 mu mol/L Fe) suggest that the extent of hydroxamate production depended on the growth history of the cells. Cells pregrown in 20 mu mol/L Fe produced a high amount of hydroxamates compared with cells pregrown in iron-free medium when resuspended in medium containing up to 4 mu mol/L Fe. Cells pregrown in 20 mu mol/L Fe were more sensitive to iron repression than those pregrown in 05 mu mol/L Fe. Mannitol was the best carbon source for siderophore production. Siderophore synthesis was inhibited by 4-chloromercuribenzenesulfonic acid, 2,4-dinitrophenol, sodium azide and MgCl2 suggesting that an energized membrane and a mercapto group are essential and required for hydroxamate synthesis in strain WPBS 3201 D.