Novel action of proline-rich antimicrobial peptides Api88, Api137, Onc72 and Onc112 against Pseudomonas aeruginosa and Escherichia coli in ion-rich environments.

The rise in antibiotic resistance has meant that there is a need for new strategies and one avenue is the use of proline-rich antimicrobial peptides (PrAMPs). Here we investigate how different metal ion environments (Na+, Mg2+, Ca2+) affect antimicrobial activity of PrAMPs derived from apidaecin 1b (Api88, Api137) and Oncopeltus antibacterial peptide 4 (Onc72, Onc112) against Pseudomonas aeruginosa and Escherichia coli. Initial antimicrobial testing in an ion-rich media (ion levels similar to mammalian body fluids) found that the PrAMPs were effective against E. coli but not P. aeruginosa. Both Api88 and Api137 were bactericidal, while Onc72 and Onc112 were bacteriostatic against E. coli. In a lower ion-media the activity of the PrAMPs significantly improved against both bacteria and Onc72 and Onc112 altered the mode of action to bactericidal. In low Na+, Ca2+ and Mg2+ ion conditions all of the peptides were able to penetrate the outer membrane of P. aeruginosa, however at higher ion concentrations none of the peptides were able to penetrate the outer membrane. PrAMPs were found to cause E. coli cells to swell and have a hyperpolarised membrane indicating a new mechanism of action for PrAMPs. Our data indicates that bacteria reduce susceptibility to AMPs by stabilising their LPS layer with metal ions and that the PrAMPs have secondary modes of action affecting the functionality of the bacterial membrane. Combining an ion chelator with PrAMPs may be a novel solution to combat weak antimicrobial activity in ion-rich environments such as host tissues.