Action Mechanism Of Antimicrobial Peptides Research Articles

Interactions of two enantiomers of a designer antimicrobial peptide with structural components of the bacterial cell envelope.

Antimicrobial peptides (AMPs) have great potential in treating multi-drug resistant bacterial infections. The antimicrobial activity of d-enantiomers is significantly higher than l-enantiomers and sometimes selectively enhanced against Gram-positive bacteria. Unlike phospholipids in the bacterial plasma membrane, the role of other bacterial cell envelop components is often overlooked in the mode of action of AMPs. In this work, we explored the structural interactions between the main different structural components in Gram-negative/Gram-positive bacteria and the two enantiomers of a designer AMP, GL13K. We observed that both l-GL13K and d-GL13K formed self-assembled amyloid-like nanofibrils when the peptides interacted with lipopolysaccharide and lipoteichoic acid, components of the outer membrane of Gram-negative bacteria and cell wall of Gram-positive bacteria, respectively. Another cell wall component, peptidoglycan, showed strong interactions exclusively with d-GL13K and formed distinct laminar structures. This specific interaction between peptidoglycans and d-GL13K might contribute to the enhanced activity of d-GL13K against Gram-positive bacteria as they have a much thicker peptidoglycan layer than Gram-negative bacteria. A better understanding of the specific role of bacterial cell envelop components in the AMPs mechanism of action can guide the design of more effective Gram-selective AMPs.

An In-Cell Solid-State NMR Portrayal of the Action Mechanism of Antimicrobial Peptides with Intact Bacteria

Antimicrobial peptides (AMPs) are promising candidates to act against drug-resistant bacteria since they can disrupt the bacterial lipid barrier, leading to cell death. 2H solid-state NMR is a valuable tool to study at a molecular level the action of AMPs on the lipid organization and dynamics within the membrane bilayer. While these studies are usually performed with model phospholipid membranes, the complexity of the bacterial cell wall prompts the development of in-cell NMR techniques to take into account interactions of all constituents. In particular, Gram(−) bacteria are protected by lipopolysaccharides in their outer membrane while a thick peptidoglycan covers the lipid bilayer of Gram(+) bacteria. We have thus developed novel 2H solid-state NMR tools to study in vivo the bactericidal action mechanism of aurein 1.2 and caerin 1.1, AMPs excreted by frog skin. Using deuterated palmitic acid, we have established a protocol to 2H-label phospholipids in Gram(+) Bacillus subtilis, and optimized the deuteration protocol of Gram(−) Escherichia coli. Via a combination of static and magic-angle spinning (MAS) experiments, our results support the interaction of the AMPs with membrane lipids leading to decrease in lipid chain order. This effect is, however, observed at higher peptide concentration with B. subtilis and most likely attributable to an interaction with negatively-charged components in its cell walls, such as teichoic acids. Our study shows that the nature of the cell wall plays a role upon the actual concentration of AMPs on the bacterial membrane and highlights the importance of studying membrane interactions with intact bacteria.

Focal Targeting of the Bacterial Envelope by Antimicrobial Peptides.

Antimicrobial peptides (AMPs) are utilized by both eukaryotic and prokaryotic organisms. AMPs such as the human beta defensins, human neutrophil peptides, human cathelicidin, and many bacterial bacteriocins are cationic and capable of binding to anionic regions of the bacterial surface. Cationic AMPs (CAMPs) target anionic lipids [e.g., phosphatidylglycerol (PG) and cardiolipins (CL)] in the cell membrane and anionic components [e.g., lipopolysaccharide (LPS) and lipoteichoic acid (LTA)] of the cell envelope. Bacteria have evolved mechanisms to modify these same targets in order to resist CAMP killing, e.g., lysinylation of PG to yield cationic lysyl-PG and alanylation of LTA. Since CAMPs offer a promising therapeutic alternative to conventional antibiotics, which are becoming less effective due to rapidly emerging antibiotic resistance, there is a strong need to improve our understanding about the AMP mechanism of action. Recent literature suggests that AMPs often interact with the bacterial cell envelope at discrete foci. Here we review recent AMP literature, with an emphasis on focal interactions with bacteria, including (1) CAMP disruption mechanisms, (2) delocalization of membrane proteins and lipids by CAMPs, and (3) CAMP sensing systems and resistance mechanisms. We conclude with new approaches for studying the bacterial membrane, e.g., lipidomics, high resolution imaging, and non-detergent-based membrane domain extraction.