Binding of Ferric Iron to the Cell Walls and Membranes of Bifidobacterium thermophilum:  Effect of Free Radicals

Bifidobacterium thermophilum (ATCC 25866) was incubated with 100−120 μM 59FeSO4 and 300 μM excess of H2O2 for up to 120 min in the presence and absence of glucose. Samples were withdrawn after 5, 30, 60, and 120 min. These were protoplasted and 59Fe(III) measured in the supernatant fraction (cell walls) and protoplasts (cell membranes). These experiments were also repeated in the presence of 400 μM Al(III), which, in whole cells, caused an increase in Fe(III) binding. The amount of iron in the cell wall fraction was constant regardless of time of incubation, was unaffected by Al(III), and was reduced by ∼20% by glucose. On the other hand, the amount of iron on the protoplasts increased with time and was affected by both Al(III) (upward) and glucose (downward). Scatchard plots indicated that the number of Fe(III) binding sites on the cell walls was 37.6 nmol/mg of dry cell weight at zero time, whereas that of cell membranes was 1/10 of that. It was concluded that Fe(III) binding by bifidobacterial cell walls was instantaneous and marginally dependent on free radical action. That of the cell membranes was time-dependent and was due to lipid peroxidation initiated by free radicals. Bifidobacteria can therefore function in the intestinal tract as probiotics by making Fe(OH)3 unavailable to pathogens and by moderating free radical activity in the intestinal tract. Keywords: Iron; probiotics; Bifidobacterium thermophilum; free radicals