A History of Iron Metabolism in the Mycobacteria - Iron Acquisition by the Genus Mycobacterium - page 31

of the related material that they had isolated from M. tuberculosis [ 102 , 103 ]. In
this case, however, the terminating group of the alkyl chain was not a carboxy
group but was the methyl ester of this group. Possibly the esterification reaction
may have occurred during the late growth phase of the cultures that had been used
whereas for the work of Lane et al. [ 101 , 104 ], culture filtrates had been prepared
from cells in their active phase of growth during which iron solubilization and
uptake should be at their maximum.
It is clear that all the siderophores obtained earlier from culture filtrates of path-
ogenic mycobacteria and had been termed 'chloroform-soluble exochelins' were,
in fact, carboxymycobactins. A list of species and strains that produce carboxymy-
cobactins is given in Table 2.3 . Of considerable importance and interest was the
finding that all (13 out of 13) strains of M. paratuberculosis that had been exam-
ined by Barclay and Ratledge [ 99 ] produced a carboxymycobactin but all of them
still required mycobactin for growth. However, it was ruled out that the added
mycobactin was being converted into the extracellular product as the amount of
carboxymycobactin recovered was much greater than the amount of mycobactin
that had been added as a growth factor into the culture medium. Also there was no
observable conversion of 14 C-labelled carboxymycobactin into mycobactin itself
when fed to cultures of M. bovis [ 100 ].
This discovery then provides a partial explanation as to how M. paratuberculo-
sis and other mycobactin-dependent species are able to grow in vivo. They simply
produce carboxymycobactin, an extracellular siderophore that then acquires the
iron from host tissues and transfers the iron to the bacterial cells. There must then
be a slow transfer of iron into the pathogenic bacteria without the participation of
mycobactin, though how this occurs is still not clear. Evidently, however, when the
bacteria are isolated from an infected animal, mycobactin becomes essential for
the uptake process to be complete and for the bacteria to grow in vitro. Homuth
et al. [ 105 ] took an alternative view: they suggested, on the basis of their finding
of an extracellular ferric reductase in cultures of M. paratuberculosis that could
reduce the iron in transferrin and lactoferrin, that this may be an alternative means
of iron acquisition in this bacterium in vivo. The possibility of the direct uptake of
iron from transferrin had been suggested earlier by Lambrecht and Collins [ 106 ].
But, if this were the case, one would then have expected extracts of animal tis-
sues to have supported growth of this bacterium in laboratory culture medium but
clearly this does not happen [ 22 - 24 ]. Further work on this aspect of iron metabo-
lism in M. paratuberculosis and other mycobactin-dependent strains is therefore
still needed to give some vital clues as to how it is accomplished.

Table 2.3 Occurrence of carboxymycobactins in mycobacteria (see also Fig. 2.9 ) a
Pathogens M. africanum, M. avium, M. bovis BCG , M. intracellulare, M. paratuber-

culosis, M. trivial, M. tuberculosis H37Ra and H37Rv, M. xenopi

M. smegmatis b M. neoaurum c

Non-pathogens

a Data from [ 60 , 99 ]
b [ 108 ]
c From Tan Eng Lee and Ratledge [unpublished]