Effects of iron depletion and sub-inhibitory concentrations of antibodies on siderophore production by Staphylococcus aureus.

The effect of various sub-MICs of oxacillin and amikacin, alone or in combination, on siderophore production by Staphylococcus aureus growing under iron-depleted conditions was evaluated. Siderophore production varied greatly from strain to strain, ranging from 3.0 to 383.4 microM of Desferal equivalents. Siderophore production by some S. aureus strains was reduced significantly (P less than 0.001) under some sets of conditions. However, siderophore excretion in cultures with amikacin was sometimes equal to or greater than control cultures. Thus, at some sub-MICs, the bacteria tested were able to increase siderophore production under metabolic stress. This phenomenon may be a secondary effect of the antibiotics upon intermediary metabolism as a consequence of inhibition of peptidoglycan or protein synthesis. Alternatively the antibiotics may decrease the cellular association of siderophore molecules.