RESEARCH ARTICLE Detoxification of Indole by an Indole- Induced Flavoprotein Oxygenase from

Indole, a derivative of the amino acid tryptophan, is a toxic signaling molecule, which can inhibit bacterial growth. To overcome indole-induced toxicity, many bacteria have devel-oped enzymatic defense systems to convert indole to non-toxic, water-insoluble indigo. We previously demonstrated that, like other aromatic compound-degrading bacteria, Acineto-bacter baumannii can also convert indole to indigo. However, no work has been published investigating this mechanism. Here, we have shown that the growth of wild-type A. bauman-nii is severely inhibited in the presence of 3.5 mM indole. However, at lower concentrations, growth is stable, implying that the bacteria may be utilizing a survival mechanism to oxidize indole. To this end, we have identified a flavoprotein oxygenase encoded by the iifC gene of A. baumannii. Further, our results suggest that expressing this recombinant oxygenase pro-tein in Escherichia coli can drive indole oxidation to indigo in vitro. Genome analysis shows that the iif operon is exclusively present in the genomes of A. baumannii and Pseudomonas syringae pv. actinidiae. Quantitative PCR andWestern blot analysis also indicate that the iif operon is activated by indole through the AraC-like transcriptional regulator IifR. Taken together, these data suggest that this species of bacteria utilizes a novel indole-detoxifica-tion mechanism that is modulated by IifC, a protein that appears to be, at least to some extent, regulated by IifR