Mycobacterial Iron Acquisition Mechanisms - Iron Acquisition by the Genus Mycobacterium - page 48

steady-state growth of M. smegmatis required an iron addition of 0.4-0.8 μ M.
Higher levels of iron did not significantly increase steady-state growth but did
cause changes in protein expression, indicating a response to the iron level in the
chemostat. Although Tween 80 addition was not required for batch cultivation of
M. smegmatis , Tween 80 was required for establishment of steady-state growth.
Flow cytometry with fluorocein diacetate and propidium iodide staining revealed
that most of the mycobacterial cells in the chemostat were viable and numbers
obtained by plate counting were in good correlation. Using organisms like M.
avium and M. tuberculosis , steady-state growth studies at different iron levels will
allow examination of various parameters, including siderophore gene expression,
protein expression, iron uptake kinetics, and the capacity to infect host cells.
Many mycobacteria form biofilms at liquid-air interfaces and on solid surfaces
[ 55 ]. Development of M. smegmatis biofilms required an iron supplement and was
accompanied by induction of siderophore genes. While it is difficult to assess the
effects of iron on biofilm growth because different concentrations of iron probably
exist within a biofilm leading to differences in siderophore biosynthesis, iron lev-
els influence mycobacterial biofilm formation.
3.8 The Complexity and Perplexity of Mycobacterial Iron
Acquisition: Conclusions
The complexity of mycobacterial iron uptake is illustrated by the several routes
for iron procurement found in these microorganisms. Multiple ways may exist
to pursue the single purpose of iron capture, an apparent redundancy that causes
confusion in identification of which of the iron uptake tracks looms critical
under the experimental conditions being tested. A perplexing question is whether
there is an interaction between mycobactin and carboxymycobactin of patho-
gens and between the mycobactin siderophores and exochelin of the terrestrial
saprophytes. The answer to this question may be both yes and no and will vary
with test conditions. Adventitious iron in the culture system, prior growth condi-
tions, inoculum size, and the amount of iron supplied to the microorganism as
well as the species tested and other undefined conditions will influence results
and must be considered. Iron acquisition paths may overlap, intersect, or be a sin-
gle conduit for the flow of iron into metabolism. If excreted carboxymycobac-
tin can replace exochelin in mutants unable to produce exochelin, then why is
exochelin necessary? Exochelin may represent the most direct path for iron to
reach metabolism in a saprophyte. While iron uptake through the carboxymyco-
bactin/mycobactin system of pathogenic mycobacteria may appear to be a com-
plex route, the mycobactin siderophores may confer special capacities needed
for growth of mycobacteria in host tissue and macrophages. If possible, it might
be helpful to ascertain the iron acquisition capacities of mutants altered in pro-
duction of either mycobactin or carboxymycobactin [ 24 ]. This probably would
require alteration of the side chains attached to the mycobactin core structure. A