Three different siderophores mycobactins (Francis et al., 1949), ferrichrome (Neilands, 1952) and coprogen (Hesseltine et al., 1952) were isolated as growth factors during 1949 to 1952.
Stability constants for some iron siderophore complexes (Boukhalfa and Crumbliss, 2002) Siderophores Log[([beta] Fe(III))] Log[([beta] Fe(II))] Enterobactin 49.0000 23.9100 Pyoverdin 30.8000 9.7800 Ferrichrome A 32.0000 9.9100 Ferrichrome E 32.5000 11.1600 Ferrichrome B 30.6000 10.2900 Aerobactin 22.5000 4.8600 Acetohydroxamic acid 28.2900 11.2000 Table 2.
Five different strains of Phialocephala fortinii, a dark septate fungal, were studied and all of them excreted three siderophores namely, ferricrocin, ferrirubin and ferrichrome
C, whose production was dependent on pH and iron(III) concentration of the culture medium .
Ferrichrome and ferriox- amine each have a separate substrate binding protein (FhuD and YxeB, respectively), but require the same membrane permease using an ATPase (FhuBGC) [29, 32].
 reported the requirement for two lipoproteins for the hydroxamates ferrichrome and ferrioxamine B in B.
Most notable among the heterologous siderophores utilized by these rhizobia is ferrichrome, a prototypical hydroxamate type siderophore secreted by several fungi, including Ustilago sphaerogena  and Ustilago maydis .
Bradyrhizobium japonicum USDA110 and 61A152 can utilize ferrichrome (made by numerous fungi), rhodotorulate (made by yeast), pseudobactin (made by pseudomonads), and so forth , and these strains are successfully used as commercial bioinoculant for soybeans.
Because ferrichrome is synthesized by a variety of soil fungi, it is likely iron source in the rhizosphere where hydroxamate concentrations have been estimated to be as high as 10 [micro]M [ 57] and ferrichrome is found in nanomolar concentrations, as estimated by physicochemical  as well as bioassay methods [ 25].
Thus, cloning of the ferrichrome receptor genein rhizobial bioinoculant strains and understanding the effect of ferrichrome utilization on rhizobial growth and survivability under conditions, wherein ferrichrome was made available by other producer species, were achieved by heterologously expressing the E.
japonicum to utilize ferrichrome. These observations reinstate the belief that among all the ligand-protein interactions of members of the bacterial iron-acquisition system, the binding of ferrisiderophores to the outer membrane receptor proteins is the most specific.
Rhizobium meliloti DM4 can utilize the hydroxamate type siderophores, ferrichrome (made by numerous fungi), and ferrioxamine B (made by actinomyces) in addition to its own siderophore rhizobactin .