Iron requirement in the bactericidal mechanism of streptonigrin

Mutants of Escherichia coli K-12 that are unable to make use of the enterochelin transport system were used to confirm that streptonigrin requires iron for its bactericidal action. Correlation of viability studies and 55Fe3+ uptake experiments showed that killing by streptonigrin increased with an increase in 55Fe3+ uptake by the cells. Streptonigrin did not kill iron-starved mutants that were unable to import iron. The level of iron uptake by these mutants was manipulated by agents such as (i) the enterochelin biosynthetic precursors 2,3-dihydroxybenzoic acid (2 x 10(-5) M) and shikimic acid (2 x 10(-4) M), (ii) citrate (10(-2) M), which promotes iron uptake by an independent pathway, and (iii) the chelating agents desferrioxamine (2 x 10(-4) M) and orthophenanthroline (10(-4) M). Addition of the precursors shikimate and dihydroxybenzoate to strain AB2847 (aroB) and dihydroxybenzoate to strain AN193 (entA), allowing these strains to make enterochelin, resulted in an increase in Fe3+ uptake and a corresponding sharp increase in killing by streptonigrin. Addition of enterochelin itself (10(-6) M) caused an even more pronounced effect. Studies on the effect of citrate in strain AN102 (fep) showed that this mutant was not killed by streptonigrin (4 x 10(-5) M), even in the presence of citrate; however, overnight growth in citrate induced Fe3+ uptake by means of the ferric citrate transport system and resulted in killing by streptonigrin. These studies showed a clear correlation between the change in levels of intracellular iron and the bactericidal effectiveness of streptonigrin.