Tubercle bacilli exposed to an iron-poor medium multiplied at a slower rate but released more of the serum-tuberculostasis neutralizing factor (TNF) than the bacilli in an iron-rich medium. This growth-promoting factor found in spent medium exhibited characteristics which suggested its relationship to or identity with mycobactin. The identity of these two bacillary products was established by showing that both iron-free mycobactin and TNF promoted bacillary multiplication in tuberculostatic serum. This study resolved a long-standing controversy as to whether mycobactin serves as a growth factor or as a carrier of iron for tubercle bacilli. It was found that the tuberculostasis in mycobactin-neutralized serum was reconstituted by the addition of iron-free transferrin (Tr). The investigation of the interplay between mycobactin and Tr revealed that mycobactin does not serve as a growth factor but as a carrier of growth-essential iron which mycobactin (as contrasted to Tr) provides to tubercle bacilli in a utilizable form.
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