Iron transport in Mycobacterium smegmatis: a restricted role for salicylic acid in the extracellular environment.

Abstract

Abstract Salicylic acid is found extracellularly before growth of Mycobacterium smegmatis begins: at the earliest times of sampling (28 h) and under iron-deficient conditions as much as 250 μg salicyclic acid/mg cell dry weight is formed. The influence of salicylate on the status of iron (precipitated, colloidal or soluble) depends principally upon the pH and the prevailing concentrations of phosphate and Mg 2+ . In growth medium or 0.1 M phosphate buffer, pH 7.0, salicylate cannot hold iron in solution although in the absence of phosphate the formation of a 3:1 complex with Fe(III) occurs even at pH 9.0. Thus, during growth of mycobacteria in the presence of inorganic phosphate solubilization of iron must be by another, and as yet undiscovered, compound and, furthermore, the function of salicylate under such conditions must be different from that previously supposed. In the absence of inorganic phosphate, however, salicylate is probably still able to function as a solubilizer of iron as uptake of iron occurred rapidly from salicylate both into iron-deficient cells or into mycobactin dissolved in n -octanol used as a model system. Other aromatic acids could substitute for salicylate in this role but citrate, oxalate, acetate and EDTA were ineffective. Cells grown with a sufficiency of iron were similar to iron-deficient cells in requiring iron to be in suitably chelated form, such as with salicylate before uptake could occur. Citrate was again ineffective in this capacity.