<i>Viola</i> '<i>inconspicua</i>' No More: An Analysis of Antibacterial Cyclotides.
Parsley NC, Sadecki PW, Hartmann CJ, Hicks LM.
The emergence of rapidly evolving multidrug-resistant pathogens and a deficit of new compounds entering the clinical pipeline necessitate the exploration of alternative sources of antimicrobial therapeutics. Cyclotides revealed in <i>Viola spp</i>. are a class of highly stable, cyclic, and disulfide-bound peptides with diverse intrinsic bioactivities. Herein we have identified a novel complement of 42 putative cyclotide masses in the plant species <i>Viola inconspicua</i>. Cyclotide-containing fractions of a <i>V. inconspicua</i> peptide library revealed potent bioactivities against the Gram-negative bacteria <i>Escherichia coli</i> ATCC 25922 and the highly virulent and multidrug-resistant <i>Klebsiella pneumoniae</i> VK148. As such, six previously uncharacterized cyclotides, cycloviolacins I1-6 (cyI1-cyI6), were prioritized for molecular characterization. Cyclotides cyI3-cyI6 contain a novel 'TLNGNPGA' motif in the highly variable loop six region, expanding the already substantial sequence diversity of this peptide class. Library fractions comprised of cyclotides cyI3-cyI6 exhibited MIC values of 18 and 35 μM against <i>E. coli</i> and <i>K. pneumoniae</i>, respectively, whereas isolated cyI3 killed ∼50% of <i>E. coli</i> at 60 μM and isolated cyI4 demonstrated no killing at concentrations >60 μM against both pathogens. This work expands the repertoire of bioactive cyclotides found in <i>Viola spp</i>. and highlights the potential of these antibacterial cyclic peptides.