Antimicrob Agents Chemother. 2015 Nov 2. pii: AAC.02299-15. [Epub ahead of print]
We studied the transcriptomic response of Streptococcus pneumoniae to the fluoroquinolone moxifloxacin at a concentration that inhibits DNA gyrase. Treatment of the wild-type strain R6, at a concentration of 10× MIC, triggered a response involving 132 genes after 30 minutes of treatment. Genes from several metabolic pathways involved in the production of pyruvate were up-regulated. These included 3 glycolytic enzymes, which ultimately convert fructose-6P to pyruvate, and 2 enzymes that funnel P-sugars into the glycolytic pathway. In addition, acetyl-CoA carboxylase was down-regulated, likely leading to an increase in acetyl-CoA. When coupled with an up-regulation in formate acetyltransferase, an increase in acetyl-CoA would raise the production of pyruvate. Since pyruvate is converted by pyruvate oxidase (SpxB) into hydrogen peroxide (H2O2), an increase in pyruvate would augment intracellular H2O2. Here, we confirm a 21-fold increase in the production of H2O2 and a 55-fold increase in the amount of hydroxyl radical in cultures treated during 4 h with moxifloxacin. This increase in hydroxyl radical through the Fenton reaction, would damage DNA, lipids, and proteins. These reactive oxygen species contributed to the lethality of the drug, a conclusion supported by the observed protective effects of a SpxB deletion. These results support the model whereby fluoroquinolones cause redox alterations. The transcriptional response of S. pneumoniae to moxifloxacin is compared with the response to levofloxacin, an inhibitor of topoisomerase IV. Levofloxacin triggers the transcriptional activation of iron transport genes and also enhances the Fenton reaction.
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