A History of Iron Metabolism in the Mycobacteria - Iron Acquisition by the Genus Mycobacterium - page 25

Fig. 2.7 a (magnification ~140,000-fold), and b (magnification ~700,000-fold). Cellular loca-
tion of mycobactin. Electron micrographs of iron-deficiently grown Mycobacterium smegma-
tis incubated with 0.1 % ammonium metavanadate for 10 min at 4 °C (The vanadyl ions react
very quickly and specifically with desferrimyobactin but are not metabolically removed from the
mycobactin). The large black circular areas in the cytoplasm are polyphosphate granules which
are also seen in iron-sufficiently grown cells [ 74 ]
the iron from transferrin but this did not explain how the two molecules might
legitimately come into contact. Kochan et al. [ 76 ] and later Golden et al. [ 77 ] then
showed that mycobactin could be solubilized and transferred into the medium if
a detergent, such as Tween 80, Triton or lecithin, was added to the medium. But
again, to my group (at Hull University), that now included Leo Macham as post-
doctoral research assistant and who made some significant contributions in this
area, this did not seem a likely condition for pathogenic mycobacteria to experi-
ence when growing in vivo. To us, the experimental conditions used by Kochan
seemed contrived and unlikely to be realistic. His proposals certainly could not
explain how iron was mobilized by mycobacteria growing in simple laboratory
culture medium devoid of any detergent. We, therefore, were of the opinion that
another iron binding component, that was water-soluble and which would be
released by the cells into their surrounding environment, was necessary to explain
how iron was solubilized in the medium before being transferred into the cells.
The first and obvious candidate for this was salicylic acid—why else did it occur
in increased quantities in the culture medium during iron-deficient growth (Fig. 2.6 )?