Inability to detect mycobactin in mycobacteria-infected tissues suggests an alternative iron acquisition mechanism by mycobacteria in vivo.
Although most species of mycobacterium are capable of producing mycobactin, it is not known if conditions within the host allow for mycobactin synthesis or whether it even plays a role in iron acquisition in vivo. We employed the mycobactin-auxotroph, Mycobacterium paratuberculosis, in a bioassay to examine tissues from animals infected with either Mycobacterium tuberculosis, Mycobacterium avium or M. paratuberculosis for the presence of mycobactin or compounds which demonstrate mycobactin-like activity. Other iron-binding compounds, including purified siderophores from unrelated organisms and host iron-binding proteins were also evaluated in the bioassay for growth induction of M. paratuberculosis in the absence of mycobactin. Although mycobactin could be easily demonstrated in tissues artificially seeded with mycobacteria, no mycobactin could be detected in heavily infected tissues. None of the purified siderophores from unrelated microorganisms were found to support growth of M. paratuberculosis in the absence of mycobactin. Host iron-binding proteins (transferrin, lactoferrin, ferritin, hemin) also failed to induce growth in the bioassay at pH 6.8, however, when the pH was adjusted between 5-6.2, transferrin and lactoferrin promoted growth of M. paratuberculosis without mycobactin, probably as a result of the dissociation of iron rather than a specific interaction. We confirm that mycobacteria are incapable of iron uptake when iron is chelated to siderophores from unrelated organisms and conclude that mycobactin-mediated mechanisms of iron-acquisition by mycobacteria do not appear to have as significant a role in vivo as in vitro. In addition, evidence is presented that suggests iron-containing transferrin and lactoferrin at low pH may circumvent the need for mycobactin by M. paratuberculosis.