Investigating the antibacterial activity and mode of action of antimicrobial peptides Os and Os-C against Staphylococcus epidermidis

Abstract

Staphylococcus epidermidis (S. epidermidis) is a resident member of the skin microbiota, however, this organism acts opportunistically by causing moderate to severe infection further complicated by biofilm development. With the development of antimicrobial resistance, the number of antibiotics available for treatment are limited and increasingly, there is a need for new drugs with novel modes of action. Antimicrobial peptides (AMPs) have emerged as promising candidates to conventional antibiotics due to their broad spectrum of activity and varied mechanisms of killing. In this study the antibacterial activity and mode of action of two synthetic AMPs, Os and Os-C, derived from OsDef2 defensin, was investigated. The antibacterial activities of the synthetic peptides were evaluated using planktonic S. epidermidis because of the high propensity of this microorganism to form biofilms. The inhibitory activity of Os and Os-C was determined in 1% TSB for 6 hours and had a minimum inhibitory concentration (MIC) of 10 and 3.34 µM respectively. Both peptides had a minimum bactericidal concentration (MBC) of 10 µM with 99.5% killing after 3 hours. The mode of action of Os and Os-C was further investigated using the Live/Dead permeability and the membrane depolarisation assays. At concentrations below the MIC of Os (7 µM), Os-C (20 µM), and the control melittin (0.4 µM), Os and melittin to the same extent permeabilised the cell membrane of S. epidermidis. In contrast, Os-C had no effect. Os caused some membrane depolarisation while membrane depolarisation was identified as a key event in the mode of action of Os-C. Subsequent morphological analysis of S. epidermidis cells using scanning electron microscopy revealed that melittin, Os, and Os-C induced the formation of membrane blebs and indents. Deflated cells, ghost cells, as well as cellular debris were also present. In addition, Os-C induced the formation of cellular extensions/protrusions. Using transmission electron microscopy, it was demonstrated that melittin caused membrane indentation, cytoplasmic retraction, cell wall fragmentation, as well as the formation of irregular septa and intracellular lamellae. Os caused intracellular coagulation, lysis with extruded intracellular material, cell wall fragmentation, as well as irregular septa formation. In contrast, Os-C caused membrane invagination and the formation of irregular septa in addition to mesosome sacs and tubules. These morphological features identify the membrane lytic effects of Os whereas Os-C stimulated mesosome formation. Both Os and Os-C have MIC and MBC values within the micromolar range with a killing time of 3 hours. Although both peptides interact with the S. epidermidis membrane, the mode of action is different with Os causing direct membrane disruption while Os-C induced membrane depolarisation and mesosome formation usually associated with the formation of reactive oxygen species. It was thus concluded that these two peptides could be developed for topical use given the nature of S. epidermidis infections and the sensitivity of these peptides to high salt environments.