The cyclotide cycloviolacin O2 from Viola odorata has potent bactericidal activity against Gram-negative bacteria

To determine the antibacterial activity of small cyclic plant proteins, i.e. cyclotides, and the importance of the surface exposed charged residues for activity.

Prototypic cyclotides, including the Möbius kalata B1 and the bracelet cycloviolacin O2 (cyO2), were isolated using reversed-phase HPLC. Initial activity screenings were conducted using radial diffusion assays (RDAs) and MIC assays with Salmonella enterica serovar Typhimurium LT2, Escherichia coli and Staphylococcus aureus as test strains. For the most active peptide, cyO2, time-kill kinetics was determined in sodium phosphate buffer (containing 0.03% trypticase soy broth) against several Gram-negative and Gram-positive bacterial species. Charged residues in cyO2 were chemically modified and activity was determined in time-kill assays.

CyO2 was the most active cyclotide and efficiently inhibited the growth of S. enterica serovar Typhimurium LT2 and E. coli in RDAs and MIC assays, while the other peptides were less active. In time-kill assays, cyO2 also had bactericidal activity against the Gram-negative species Klebsiella pneumoniae and Pseudomonas aeruginosa. In contrast, none of the cyclotides had high activity against S. aureus. Chemical masking of the charged Glu and Lys residues in cyO2 caused a near total loss of activity against Salmonella, while masking Arg caused a less pronounced activity reduction.

CyO2 is a cyclotide with potent activity against Gram-negative bacteria. The charged residues in cyO2 are all required for optimum antibacterial activity. In combination with its previously demonstrated cytotoxic activity against cancer cells and the general stability of cyclotides, cyO2 provides a promising scaffold for future drug design.