Abstract:
Os and Os-C are two novel antimicrobial peptides, derived from a tick defensin, which have been shown
to have a larger range of antimicrobial activity than the parent peptide, OsDef2. The aim of this study
was to determine whether the peptides Os and Os-C are mainly membrane acting, or if these peptides
have possible additional intracellular targets in Escherichia coli and Bacillus subtilis. Transmission electron
microscopy revealed that both peptides adversely affected intracellular structure of both bacteria
causing different degrees of granulation of the intracellular contents. At the minimum bactericidal concentrations,
permeabilization as determined with the SYTOX green assay seemed not to be the principle
mode of killing when compared to melittin. However, fluorescent triple staining indicated that the peptides
caused permeabilization of stationary phase bacteria and TEM indicated membrane effects. Studies
using fluorescently labeled peptides revealed that the membrane penetrating activity of Os and Os-C was
similar to buforin II. Os-C was found to associate with the septa of B. subtilis. Plasmid binding studies
showed that Os and Os-C binds E. coli plasmid DNA at a similar charge ratio as melittin. These studies
suggest membrane activity for Os and Os-C with possible intracellular targets such as DNA. The differences
in permeabilization at lower concentrations and binding to DNA between Os and Os-C, suggest
that the two peptides have dissimilar modes of action.
