Antibacterial action of structurally diverse cationic peptides on gram-positive bacteria

Antimicrobial cationic peptides are ubiquitous in nature and are thought to be a component of the first line of defense against infectious agents. It is widely believed that the killing mechanism of these peptides on bacteria involves an interaction with the cytoplasmic membrane. Cationic peptides f rom different structural classes were used in experiments with Staphylococc us aureus and other medically important gram-positive bacteria to gain insi ght into the mechanism of action. The membrane potential-sensitive fluoroph ore dipropylthiacarbo-cyanine was used to assess the interactions of select ed antimicrobial peptides with the cytoplasmic membrane of S. aureus. Study of the kinetics of killing and membrane depolarization showed that, at ear ly time points, membrane depolarization was incomplete, even when 90% or mo re of the bacteria had been killed, CP26, a 26-amino-acid alpha-helical pep tide with a high MIC against S, aureus, still had the ability to permeabili ze the membrane. Cytoplasmic-membrane permeabilization was a nidespread abi lity and an action that may be necessary for reaching an intracellular targ et but in itself did not appear to be the killing mechanism. Transmission e lectron microscopy of S. aureus and Staphylococcus epidermidis treated with CP29 (a 26-amino-acid alpha-helical peptide), CP11CN (a 13-amino-acid, pro line- and tryptophan-rich peptide), and Bac2A-NH2 (a linearized version of the 12-amino acid loop peptide bactenecin) showed variability in effects on bacterial structure. Mesosome-like structures were seen to develop in S. a ureus, whereas cell wall effects and mesosomes were seen with S, epidermidi s. Nuclear condensation and abherrent septation were occasionally seen in S . epidermidis. Our experiments indicated that these peptides vary in their mechanisms of action and that the mechanism of action likely does not solel y involve cytoplasmic-membrane permeabilization.