Antimicrobial peptides (AMPs) have been identified as important therapeutic agents that can be developed as new multifunctional antibiotic compounds, which may address antibiotic resistance. AMPs have a wide range of bioactivities, including antimicrobial, antioxidant, anti-inflammatory and anticancer properties. Os and Os-C (a derivative of Os, lacking cysteine residues) are two synthetic AMPs derived from the tick defensin OsDef2 which have been shown to have antibacterial, antioxidant and anti-inflammatory activity. Differences in bacterial killing times between these peptides indicate differences in the modes of bacterial killing.
For the further development of Os and Os-C for therapeutic application, the aim of this study was to establish the mode of bacterial killing, to determine if these peptides are cytotoxic to human erythrocytes and leukocytes. Lastly, to determine if these peptides have additional beneficial cellular effects such as antioxidant activity.
Ultrastructural analysis with electron microscopy techniques revealed that both peptides adversely affected the membranes and intracellular structures of both Gram-negative Escherichia coli and Gram-positive Bacillus subtilis bacteria. Effects included membrane ruffling, cytoplasmic retraction, intracellular granulation and the formation of dense fibres. At the minimum bactericidal concentrations (MBCs) of 0.77 μM for Os and 1.74 μM for Os-C membrane permeabilisation measured with the SYTOX green assay was found not to be the principle mode of action. In stationary phase bacteria, fluorescent triple staining showed that both peptides caused permeabilisation. Studies using fluorescently labelled peptides revealed that the membrane penetrating activities of Os and Os-C were similar to buforin II, a cell-penetrating peptide. Os was able to enter stationary phase E. coli and B. subtilis while Os-C was unable to enter E. coli cells and accumulated on B. subtilis septa. Using plasmid binding and fluorescence displacement assays both peptides could bind DNA, while a dosage effect was only observed for Os.
Evaluation of cytotoxicity revealed that Os and Os-C caused no erythrocyte haemolysis or changes to erythrocyte morphology. Only the highest concentration of Os (100 μM), which is 130 fold greater than the MBC for E. coli and B. subtilis, caused cellular damage to peripheral mononuclear (MN) and polymorphonuclear (PMN) cells. In contrast, Os-C caused leukocyte activation identified by associated morphological features and reactive oxygen species (ROS) formation.
Chemical and erythrocyte antioxidant assays indicated that both Os and Os-C had antioxidant activity. Both peptides provided extracellular protection of erythrocytes against 2,2'-azobis(2-amidinopropane) dihydrochloride induced oxidative damage. In MN and PMN cells Os showed low levels of antioxidant activity while Os-C had minimal activity.
In conclusion, both peptides showed a dual mechanism of bacterial killing, targeting both the membrane and intracellular elements. Os had a predominant membrane effect while Os-C targeted the septa of B. subtilis and had a higher affinity for DNA. Cytotoxicity in erythrocytes and leukocytes was minimal. In addition, Os exhibited antioxidant properties while Os-C caused leukocyte activation. Both peptides have been identified as promising therapeutic agents although activity in plasma and the effect on coagulation must still be determined.