Iron-binding compounds of Mycobacterium avium, M. intracellulare, M. scrofulaceum, and mycobactin-dependent M. paratuberculosis and M. avium

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Abstract
Strains (53) of M. avium and related species all produced 1 or more exochelins, the extracellular Fe-binding compounds of the mycobacteria, when grown Fe deficiently. Only those strains which could grow without the addition of mycobactin (i.e., mycobactin independent) produced mycobactin, the intracellular Fe-binding compound of the mycobacteria. Exochelins varied from 20-2000 .mu.g/g of cell dry wt; mycobactins were between 1-10 mg/g of cell dry wt. M. paratuberculosis (13 strains) and 13 strains of M. avium, both species dependent on mycobactin for growth, failed to produce spectrophotometrically detectable amounts of mycobactin (< 0.2 .mu.g/g of cell dry wt), although mycobactin could be recognized in 1 strain of M. avium grown with an additional supply of salicylate and examined by a radiolabeling technique. On repeated subculture 3 of the mycobactin-dependent strains of M. avium, but none of those of M. paratuberculosis, lost their mycobactin dependence and on reexamination were found to produce their own mycobactin at 0.3 mg/g of cell dry wt. Mycobactin biosynthesis is probably strongly repressed in the mycobactin-dependent strains rather than being a genetic deletion. The exochelins, when examined by high-pressure TLC were revealed as being multiples of similar compounds, with up to 20 individual Fe-binding compounds being recognizable with some strains. It is argued that the exochelins represent the single most important means of Fe acquisition in mycobacteria growing in vitro and in vivo, and their elaboration by the fastidious M. paratuberculosis and related species explains how these organisms are able to grow in vivo in the absence of an external supply of mycobactin.