Influence of metal ions on the formation of mycobactin and salicylic acid in Mycobacterium smegmatis grown in static culture

Abstract

Mycobacterium smegmatis was grown on trace-metal-free medium in static culture. Throughout the growth phase, the concentration of mycobactin increased continuously, reaching a maximum of about 30 to 40 mug of mycobactin/mg of cell dry weight after 6 days; the concentration of salicylic acid remained approximately constant at 1 to 2 mug of salicylic acid/mug of cell dry weight. Fe(2+) (or Fe(3+)), Zn(2+), Mn(2+), and Mg(2+) were all essential to a maximum formation of mycobactin. Optimum concentrations required were: Fe(2+), about 1.8 mum; Mn(2+) and Zn(2+), about 0.5 mum; and Mg(2+), at least 0.17 mm. Higher levels of Fe(2+) (9 to 90 mum) and Zn(2+) (2 to 7 mum) repressed mycobactin to about half the maximum value. No other cation or anion apparently is required for mycobactin biosynthesis. Salicylic acid concentration increased about fourfold when iron was omitted from the medium, but this is not as great as the increase reported previously for this strain of M. smegmatis. Mycobactin formation in another strain of M. smegmatis, NCIB 8548, showed similar dependencies on Fe(2+), Zn(2+), and Mn(2+). Maximum accumulation of mycobactin with this strain was 85 mug of mycobactin/mg of dry cell weight, under iron-deficient (1.8 mum Fe(2+)) conditions.