Macrophages are responsible for the breakdown of erythrocytes, thereby making heme abundantly available within the cell. Recent findings have identified that, in addition to the siderophore pathway, M. tb uses free heme and heme from hemoglobin as a source of iron. The heme carrier protein Rv0203 scavenges heme from host proteins and transports it to the bacterium, while two other proteins Mmp3 and Mmp11 transport heme across the bacterial membrane (Tullius et al. 2011).
Siderophore-Mediated Iron Acquisition
Along with other species such as Nocardia and Rhodococci, Mycobacteria are unique in the fact that they synthesize both a membrane-bound and secreted siderophores. M. tb synthesizes mycobactin T, a membrane-bound sidero- phore, and carboxymycobactin, the secreted molecule. These siderophores belong to the hydroxamate and mixed ligand types. Mycobactins were first identified in the 1950s as a growth factor for M. paratuberculosis (Francis et al. 1953). Structurally, mycobactin T possesses short methyl and ethyl side-chains on the core and a long alkyl chain that renders it extremely insoluble in aqueous media. In addition to iron acquisition, it is also proposed to function as a temporary store for iron.
Carboxymycobactin is a variant of mycobactin, where the long alkyl chain is replaced by a shorter chain terminating in a carboxylic group (Ratledge 2004). Knockout studies have revealed that siderophores are essential for M. tb virulence and replication (De Voss et al. 2000). Enzymes involved in the biosynthesis of these siderophores are tightly regulated by the iron-dependent regulator (IdeR) (Gold et al. 2001).
As with other bacterial siderophores, carboxymycobactin withdraws iron from host proteins such as transferrin and lactoferrin and traffics it back to the bacterial surface (Olakanmi et al. 2004). It is proposed that iron may then be taken into the bacterium by transfer to membrane-bound mycobactin (Gobin and Horwitz 1996). It is known that carboxymycobactin is also directly internalized by a specific transporter known as IrtAB (Farhana et al. 2008; Ryndak et al. 2010). Intracellular ferric reductases then convert the bound Fe3+ to Fe2+ within the cell. Recent evidence also suggests that mycobactin-loaded membrane vesicles are secreted from bacilli during iron starvation. These vesicles allow the capture and delivery of iron from the environment into the bacterium (Prados- Rosales et al. 2014).
Transferrin-Mediated Iron Acquisition
M. tb is capable of obtaining iron from exogenous and endogenous sources within the host cell (Olakanmi et al. 2002). Several studies have demonstrated that both transferrin and lactoferrin are recruited to the phagosome (Clemens and Horwitz 1996). It is known that exogenously added transferrin and lactofer- rin can deliver iron to bacilli within the phagosome, and acquisition from lacto- ferrin is reported to be 30-fold greater as compared to transferrin (Olakanmi et al. 2004). While it has been demonstrated that siderophores withdraw iron from host proteins (Clemens and Horwitz 1996), little is known about the detailed events within the phagosome related to the transport of iron carrier proteins, iron release and acquisition, or the involvement of bacterial components in this process (Schaible and Kaufmann 2004).