A History of Iron Metabolism in the Mycobacteria - Iron Acquisition by the Genus Mycobacterium - page 16

The structure of mycobactin from M. phlei was published in January 1965 [ 35 ]
and given the name mycobactin P. This was to distinguish it from mycobactin T
which Snow [ 34 ] had briefly described earlier. Snow commented in letter dated
August 12th 1955, to Philip D'Arcy Hart with whom he had considerable corre-
spondence, that the team
“… have done quite a number of experiments on Myco. tuberculosis . However, we have
met considerable difficulties in this research. In the first place, we have grown our tuber-
cle bacillus on a medium similar to that used for the growth of phlei with beef infusion
present in the hope of stimulating production of growth factor (i.e. mycobactin). This
medium, however, was not a suitable one for growth of large quantities of the tubercle
bacillus, and it took us many months to accumulate even a small quantity of the dried
organism as starting material.”
Snow went on to say that the methods used to extract the mycobactin from
M. phlei “… were quite unsuitable for extraction of the growth factor from tuber-
cle. We have also been severely hampered in our facilities for testing the activity
of the concentrates.”
The initial results of the work being done in the 1950s had also produced some
problems. Snow commented in the same letter to Philip Hart that the growth fac-
tor for M. johnei that had been extracted from M. tuberculosis was similar but not
identical to the mycobactin from M. phlei . Thus, the magnitude of this project that
was facing Alan Snow cannot be over-stated.
The structure of mycobactin T itself was eventually solved and published in 1965
by Snow [ 36 ]. Hough and Rogers [ 37 ] were subsequently able to confirm in detail
Snow's structure and stereochemistry of mycobactin P by using X-ray crystallogra-
phy. The ferric ion was found to lie in a V-shaped cleft with a very strained octahe-
dral configuration involving five oxygens and one nitrogen. The exceptional stability
of mycobactin with ferric iron was then explained together with explaining how the
iron could be easily released from mycobactin by reduction of ferric to ferrous iron
[ 38 - 40 ] where the resultant ferrous ion had little or no affinity to mycobactin and
would thus be available for incorporation in apoenzymes and other proteins.
It was only in the two major papers of 1965 by Snow that dealt with the struc-
tures of mycobactins [ 35 , 36 ] was iron binding recognized as a major attribute of
them. Snow [ 35 ] now appreciated that mycobactin was, in fact, a microbial sidero-
phore—or what were then called 'sideramines'. He commented that the isola-
tion of the desferri-form was directly attributable to the cultivation medium being
used, that was beef infusion broth, having a low content of ionized iron. In other
words, and with hindsight, the iron-containing components of the medium would
be various haem compounds and also ferritin and transferrin that would with-
hold iron from bacteria and, therefore, the cells were accidentally being grown
iron deficiently. This was confirmed by Norman Morrison [ 41 , 42 ], at the Johns
Hopkins-Leonard Wood Memorial Laboratory, Baltimore, USA, who in a personal
communication to Alan Snow, reported that large amounts of mycobactin could be
produced by growing M. phlei in a synthetic medium with less than 0.2 μ g iron/
ml. This then indicated a much easier way to optimize the accumulation of myco-
bactins in mycobacteria.