The Fluoroquinolone Levofloxacin Triggers the Transcriptional Activation of Iron Transport Genes That Contribute to Cell Death in Streptococcus pneumoniae

We studied the transcriptomic response ofStreptococcus pneumoniae to levofloxacin (LVX) under conditions inhibiting topoisomerase IV but not gyrase. Although a complex transcriptomic response was observed, the most outstanding result was the upregulation of the genes of thefatDCEB operon, involved in iron (Fe2+ and Fe3+ ) uptake, which were the only genes varying under every condition tested. Although the inhibition of topoisomerase IV by levofloxacin did not have a detectable effect in the level of global supercoiling, increases in general supercoiling andfatD transcription were observed after topoisomerase I inhibition, while the opposite was observed after gyrase inhibition with novobiocin. SincefatDCEB is located in a topological chromosomal domain downregulated by DNA relaxation, we studied the transcription of a copy of the 422-bp (including the Pfat promoter) region located upstream offatDCEB fused to thecat reporter inserted into the chromosome 106 kb away from its native position: Pfat fatD was upregulated in the presence of LVX in its native location, whereas no change was observed in the Pfat cat construction. Results suggest that topological changes are indeed involved in Pfat fatDCE transcription. Upregulation offatDCEB would lead to an increase of intracellular iron and, in turn, to the activation of the Fenton reaction and the increase of reactive oxygen species. In accordance, we observed an attenuation of levofloxacin lethality in iron-deficient media and in a strain lacking the gene coding for SpxB, the main source of hydrogen peroxide. In addition, we observed an increase of reactive oxygen species that contributed to levofloxacin lethality.