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 mu M ( FeSO4)-Fe-59 and 300 mu M 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 Samples were withdrawn after 5, 30, 60, and 120 min .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 Fe-59(III) measured in the supernatant fracti on (cell walls) and protoplasts (cell membranes). These experiments were al so repeated in the presence of 400 mu M Al(III), which, in whole cells, cau sed an increase in Fe(III) binding. The amount of iron in the cell wall fra ction was constant regardless of time of incubation, was unaffected by Al(I II), and was reduced by similar to 20% by glucose. On the other hand, the a mount of iron on the protoplasts increased with time and was affected by bo th 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 d ry cell weight at zero time, whereas that of cell membranes was 1/10 of tha t. 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 initiat ed by free radicals. Bifidobacteria can therefore function in the intestina l tract as probiotics by making Fe(OH)(3) unavailable to pathogens and by m oderating fi ee radical activity in the intestinal tract.