Cyclotide stability against Pseudomonas aeruginosa and Staphylococcus aureus secreted proteases

Production and release of proteases is among the strategies by which pathogenic bacteria increase their virulence. Moreover, these bacteria are also known to produce a cocktail of enzymes to degrade and inhibit antimicrobial peptides. Cyclotides, on the other hand, are circular plant mini-proteins having a unique cyclic topology (termed as the cyclic cystine knot, CCK) which endows them with exceptional stability. Their ultra-stable framework has attracted interest in drug design as a scaffold to otherwise susceptible drugs including antibacterial peptides. We have determined the stability of three peptides with the CCK motif, i.e., the cyclotides cycloviolacin O2, kalata B1 and MCoTI-II in elastase and aureolysin and also in the supernatant cultures of P. aeruginosa and S. aureus. The cyclotides have resisted enzymatic degradation both from the specific proteases and the bacterial cultures, with MCoTI-II being the least stable of the three. The cyclotides find application in drug engineering as their stable framework can be used as a scaffold or as a template for unstable drugs. The findings of the current study further support this idea; the cyclotide structure can be utilized as a novel strategy in enhancing the stability of antibacterial peptides.