UPTAKE OF IRON BY BIFIDOBACTERIUM-THERMOPHILUM DEPENDS ON THE METAL CONTENT OF ITS GROWTH-MEDIUM

Bifidobacterium thermophilum was grown in the trypticase-protease-yeast extract medium containing different concentrations of Mg2+, Ca2+, and Fe3+, and Fe2+ uptake was then measured in cells grown in the presence of metal ions. Cells that grew in metal-depleted media took up more iron than was the case with cells grown in metal-replete media. Such differences were not due to a necessity to transport iron against a concentration gradient, because soluble iron content was the same in cells grown in all types of media. Moreover, iron uptake could be diminished in cells grown only in the presence of increased amounts of Ca2+ or of Mg2+. Iron uptake in cells grown in metal-depleted media was dependent on iron concentration, and the uptake was virtually abolished at 0-degrees-C and after heating the cells at 65-degrees-C for 20 min. A possible explanation for this behavior is that the metals suppress the expression of a metal transporter.