Uptake of iron in Mtb, especially when the bacterium is in the host, strongly depends on siderophores. Humans, through secretion of lactoferrin, maintain an iron concentration of 10^(-18) M within macrophages, and the bacterium has evolved the siderophores mycobactin T and exomycobactin T (formerly exochelin) to cope with this shortage. While nonpolar mycobactin T stays in the cell wall and only moves around in liquid droplets, polar exochelin is abundantly secreted. As it can bind iron with higher affinity than lactoferrin, it frequently scavenges iron ions from this molecule (Miethke & Marahiel 2007).