A History of Iron Metabolism in the Mycobacteria - Iron Acquisition by the Genus Mycobacterium - page 24

salicylic acid and mycobactin was clear, it was far from evident how these two
molecules might interact, if at all, to achieve uptake and assimilation of iron into
the mycobacteria. However, for a continuation of this work, there was jump of sev-
eral years as, following my time in Dublin, I then spent the next 3 years working
in industry and it was only when I returned to academia to the University of Hull,
UK, in late 1967 that interest in the mycobacteria and iron metabolism was re-
awakened. The role of salicylic acid in mycobacteria then became one of the major
foci of our work and this is described in more detail in the following section.
2.5 Extracellular Siderophores of Mycobacteria
2.5.1 The Problem with Mycobactin
The essential puzzle with mycobactin was not whether or not it was involved with
iron metabolism, as clearly it was just judging from the very large increase in its
production during iron-deficient growth of any mycobacterium, but how did it actu-
ally acquire iron from the environment. Mycobactins all have a long alkyl chain
(see Fig. 2.4 ) that makes them almost completely insoluble in water but easily solu-
ble in organic solvents such as ethanol, methanol or chloroform. Thus, they were
cell-associated materials and were not released into the culture medium. It was later
shown that mycobactin forms a discrete but discontinuous layer abutting on to the
cytoplasmic membrane of the cells and some distance from the outer surface of the
cell (Fig. 2.7 ) [ 74 ]. For this work, staining of mycobactin was achieved by using
vanadyl ions as they reacted faster with mycobactin than did iron and was also
much more specific in what it was able to bind to. (Alan Snow pointed out to me an
interesting experiment that could be done by adding a mixed solution of ferric chlo-
ride and vanadyl ions in the form of ammonium metavanadate to desferrimycobac-
tin in ethanol. The solution would immediately turn deep blue due to formation of
the vanadate complex but then, on standing overnight, the solution became red due
to the formation of the ferric complex. This indicated a difference between the rate
of the reaction and the stability of the product. Ferric ions although reacting slower
than vanadyl ions nevertheless could replace it and form a complex with a much
higher affinity which was then the final stable form of the chelate.)
It was suggested in this paper of Ratledge et al. [ 74 ] that the mycobactin was
probably intercalated between the cytoplasmic membrane and the peptidogly-
can backbone of the cell wall with, perhaps a small amount of it possibly being
within the membrane itself. There did not appear to be any of it within the cyto-
plasm. Thus, what was the function of mycobactin and how was it able to acquire
iron from the extracellular medium, or the environment in which it grew within
an infected host cell, if it was a wholly intracellular and water-insoluble material?
Ivan Kochan [ 75 ] working in Miami of Ohio University, suggested that mycobac-
tin could indeed fulfill this role by demonstrating that mycobactin could remove