Iron metabolism in bifidobacteria

Abstract

Abstract Bifidobacteria are Gram-positive, anaerobic microaerophilic rods that are capable of internalizing ferrous iron at pH 5.0 and 6.5 when assayed in a post-logarithmic growth phase. Dependent upon iron concentration, iron uptake is most efficient in cells grown in a metal-depleted medium. There are two iron-uptake systems: one operating at low outside iron concentrations (1 to 20 μM); and one operating at higher concentrations (up to 400 μM). Iron is located largely on the surface and in the cell-soluble fraction at low iron concentrations. At high levels, iron is associated mostly with the insoluble (particulate) fraction. While the soluble iron is in the ferrous state, most of the insoluble iron is Fe(OH)3. It has been proposed that bifidobacteria contain an intracellular ferroxidase that oxidizes internalized Fe2+ to Fe(III) using oxygen as an electron acceptor. In B. thermophilum, the Km of this ferroxidase is 518 μM. Iron uptake by bifidobacteria depends on sugar metabolism, though minor amounts of iron are taken up in the absence of a carbon source. Iron uptake does not involve any siderophores or other carriers. The iron is apparently transported by a divalent metal permease, which requires a functioning ATPase and a proton gradient. There are many similarities between the mode of iron transport in bifidobacteria and other Gram-positive bacteria such as S. mutans and L. acidophilus, and between iron transport in bifidobacteria and manganese transport in L. plantarum. Future work on metal transport in bifidobacteria is likely to focus on the identification and isolation of proteins involved in iron transport and oxidation. Clinical implications of the iron uptake phenomenon in bifidobacteria are also promising areas of future research.