Antimicrobial Peptides: Mechanisms of Action and Resistance

Abstract

More than 40 antimicrobial peptides and proteins (AMPs) are expressed in the oral cavity. These AMPs have been organized into 6 functional groups, 1 of which, cationic AMPs, has received extensive attention in recent years for their promise as potential antibiotics. The goal of this review is to describe recent advances in our understanding of the diverse mechanisms of action of cationic AMPs and the bacterial resistance against these peptides. The recently developed peptide GL13K is used as an example to illustrate many of the discussed concepts. Cationic AMPs typically exhibit an amphipathic conformation, which allows increased interaction with negatively charged bacterial membranes. Peptides undergo changes in conformation and aggregation state in the presence of membranes; conversely, lipid conformation and packing can adapt to the presence of peptides. As a consequence, a single peptide can act through several mechanisms depending on the peptide's structure, the peptide:lipid ratio, and the properties of the lipid membrane. Accumulating evidence shows that in addition to acting at the cell membrane, AMPs may act on the cell wall, inhibit protein folding or enzyme activity, or act intracellularly. Therefore, once a peptide has reached the cell wall, cell membrane, or its internal target, the difference in mechanism of action on gram-negative and gram-positive bacteria may be less pronounced than formerly assumed. While AMPs should not cause widespread resistance due to their preferential attack on the cell membrane, in cases where specific protein targets are involved, the possibility exists for genetic mutations and bacterial resistance. Indeed, the potential clinical use of AMPs has raised the concern that resistance to therapeutic AMPs could be associated with resistance to endogenous host-defense peptides. Current evidence suggests that this is a rare event that can be overcome by subtle structural modifications of an AMP.

Keywords: antibacterial agents; antibiotic resistance bacterial; cell membrane; cell wall; gram negative bacteria; gram positive bacteria.

Figure 1.

Amphipathic 3-dimensional structures of cationic antimicrobial peptides: (A) cathelicidin LL-37 (residues 2 to 30; pdb 2K6O), (B) magainin 2 (2MAG), (C) lactoferrin (residues 1 to 11; 1XV4), (D) indolicidin (1G89), (E) human defensin 5 (2LXZ), (F) helical model of GL13K (residues 5 to 13). The positively (blue) and negatively (red) charged residues are highlighted. The structures, except that of defensin 5, were obtained in the presence of detergent micelles by solution nuclear magnetic resonance spectroscopy. All low-energy conformations of the PDB files are included in the image to obtain a better view of the full conformational space. Notably, the peptides exhibit different degrees of hydrophobicity, hydrophobic moment, and amphipathicity and consequently differ in their membrane interactions. Space-filling models of cationic amphipathic peptides were created with the Cn3D software (Wang et al. 2000).

Figure 2.

Models illustrating the membrane interactions of antimicrobial peptides. The peptide (orange and yellow) is illustrated as a stack of arrows (beta-sheet aggregate) (A), a random coil string (B), or a cylinder representing a helical structure (yellow, side view; orange, end-on view; B, C). Whereas at low peptide density, the soft membranes adjust to maintain the membrane integrity (B), at higher local peptide concentrations, the peptide-imposed curvature strain on the lipid bilayer causes transient openings (C). Multiple equilibria govern the membrane-association processes, including peptide in aqueous solution (random coil; B) ⇌ amphipathic monomers at the membrane surface ⇌ peptide-lipid supramolecular assemblies causing membrane openings. Additionally, depending on the peptide, beta-sheet membrane oligomers (A) or aggregated peptide structures form in solution. Panel B represents the adaptation of soft membranes to external stimuli (SMART model: Soft Membranes Adapt and Respond, also Transiently, in the presence of antimicrobial peptides), the alignment along the surface being a preliminary state to the carpet model, where a high peptide density causes membrane lysis. At intermediate peptide concentrations, transient openings form that have a toroidal shape made of lipids and peptide. To add another layer of complexity, cationic amphipathic AMPs have recently been shown to arrange in mesophase structures along the surface of charged lipid bilayers (Aisenbrey and Bechinger 2014).

Figure 3.

Bacterial resistance components against antimicrobial peptides as discussed in the text: secreted bacterial proteases (e.g., gingipains), lipopolysaccharides in the outer membrane of gram-negative bacteria (Rhee 2014), wall teichoic acid and lipoteichoic acid in the cell wall of gram-positive bacteria (Carvalho et al. 2014), D-alanine modification of teichoic and lipoteichoic acids, multidrug efflux pumps (Alvarez-Ortega et al. 2013), and extracellular biofilm matrix (Verma-Gaur et al. 2015). Symbols are from the references listed and used under Creative Commons licenses (https://creativecommons.org/licenses/); they have been modified for size and cropped to fit the figure.