The worldwide rise in multidrug resistant pathogens and the lack of new antibiotics for clinical use with novel targets or mechanisms constitutes a major medical crisis. trans-Translation, the main ribosome rescue mechanism in all bacteria, is a promising target (Keiler, 2008). Inhibitors of this process include synthetic compounds, that show a broad spectrum of activity against gram-positive and gram-negative bacteria, including Bacillus anthracis or Escherichia coli ∆tolC (Ramadoss et al., 2013). We found that complexation of these trans-translation inhibitors with Cu(II) can lead to potentiation of the antibacterial effects in Bacillus subtilis and Escherichia coli ∆tolC. Here, we report the antibacterial activity of KKL-35 and KKL-588 against E. coli ∆tolC as determined by minimal inhibitory concentration (MIC) assays and growth experiments. Complex formation of KKL-35 and KKL-588 with Cu(II) was characterized using absorption spectra, Jobs plots, hydrophobicity and binding affinity assays. Both compounds have high binding affinities for Cu(II) of 3.01⋅10-14(KKL-588) and 7.17⋅10-25 (KKL-35), with KKL:Cu(II) stoichiometries of 4:1 for KKL-35 and 1:1 for KKL-588. Furthermore, for both compounds hydrophobicity increases when complexed with Cu(II). Treatment of E. coli∆tolC with KKL-588 resulted in a significant inhibition of growth. This effect was abolished when simultaneously treating the cells with Cu(II). The mechanism underlying this antagonistic effect of Cu(II) on the anti-E.coli activity of KKL-588 remains to be investigated.

Keiler, K.C., 2008. Biology of trans-Translation. Annu. Rev. Microbiol. 62, 133–151.

Ramadoss, N.S., Alumasa, J.N., Cheng, L., Wang, Y., Li, S., Chambers, B.S., Chang, H., Chatterjee, A.K., Brinker, A., Engels, I.H., Keiler, K.C., 2013. Small molecule inhibitors of trans-translation have broad-spectrum antibiotic activity. Proc. Natl. Acad. Sci. U.S.A. 110, 10282–10287. https://doi.org/10.1073/pnas.1302816110