Antimicrobial peptides have been isolated and characterized from tissues and organisms representing virtually every kingdom and phylum, ranging from prokaryotes to humans. Yet, recurrent structural and functional themes in mechanisms of action and resistance are observed among peptides of widely diverse source and composition. Biochemical distinctions among the peptides themselves, target versus host cells, and the microenvironments in which these counterparts convene, likely provide for varying degrees of selective toxicity among diverse antimicrobial peptide types. Moreover, many antimicrobial peptides employ sophisticated and dynamic mechanisms of action to effect rapid and potent activities consistent with their likely roles in antimicrobial host defense. In balance, successful microbial pathogens have evolved multifaceted and effective countermeasures to avoid exposure to and subvert mechanisms of antimicrobial peptides. A clearer recognition of these opposing themes will significantly advance our understanding of how antimicrobial peptides function in defense against infection. Furthermore, this understanding may provide new models and strategies for developing novel antimicrobial agents, that may also augment immunity, restore potency or amplify the mechanisms of conventional antibiotics, and minimize antimicrobial resistance mechanisms among pathogens. From these perspectives, the intention of this review is to illustrate the contemporary structural and functional themes among mechanisms of antimicrobial peptide action and resistance.

Mechanisms of Antimicrobial Peptide Action and Resistance

Mechanism of the binding, insertion and destabilization of phospholipid bilayer membranes by alpha-helical antimicrobial and cell non-selective membrane-lytic peptides.

Agents that increase the permeability of the outer membrane.

CONTENTS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 FIVE CHEMICALLY DIFFERENT GROUPS OF PEPTIDE ANTIBIOTICS . . . . . . . . . . .. 63 Linear, Mostly Helical, Peptides Without Cys, With or Without a Hinge . . . . . . . . . . . . . . . 63 Linear Pep tides Without Cys and with a High Proportion of Certain Residues . . . . . . . . . 67 Antibacterial Peptides with One Disulfide Bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Peptides with Two or More S-S Bonds Giving Mainly or Only fJ-Sheet Structures. . . . . . . 69 Antibacterial Peptides Derivedfrom Larger Polypeptides with Other Known Functions . . 72 ANTIBACTERIAL PROTEINS AND ACTIVE FRAGMENTS OF SUCH PROTEINS . . . . 73

Peptide antibiotics and their role in innate immunity.

Water-membrane soluble protein and peptide toxins are used in the defense and offense systems of all organisms, including plants and humans. A major group includes antimicrobial peptides, which serve as a nonspecific defense system that complements the highly specific cell-mediated immune response. The increasing resistance of bacteria to conventional antibiotics stimulated the isolation and characterization of many antimicrobial peptides for potential use as new target antibiotics. The finding of thousands of antimicrobial peptides with variable lengths and sequences, all of which are active at similar concentrations, suggests a general mechanism for killing bacteria rather than a specific mechanism that requires preferred active structures. Such a mechanism is in agreement with the “carpet model” that does not require any specific structure or sequence. It seems that when there is an appropriate balance between hydrophobicity and a net positive charge the peptides are active on bacteria. However, selective activity depends also on other parameters, such as the volume of the molecule, its structure, and its oligomeric state in solution and membranes. Further, although many studies support that bacterial membrane damage is a lethal event for bacteria, other studies point to a multihit mechanism in which the peptide binds to several targets in the cytoplasmic region of the bacteria.

Mode of action of membrane active antimicrobial peptides.

The D enantiomers of three naturally occurring antibiotics--cecropin A, magainin 2 amide, and melittin--were synthesized. In addition, the D enantiomers of two synthetic chimeric cecropin-melittin hybrid peptides were prepared. Each D isomer was shown by circular dichroism to be a mirror image of the corresponding L isomer in several solvent mixtures. In 20% hexafluoro-2-propanol the peptides contained 43-75% alpha-helix. The all-D peptides were resistant to enzymatic degradation. The peptides produced single-channel conductances in planar lipid bilayers, and the D and L enantiomers caused equivalent amounts of electrical conductivity. All of the peptides were potent antibacterial agents against representative Gram-negative and Gram-positive species. The D and L enantiomers of each peptide pair were equally active, within experimental error. Sheep erythrocytes were lysed by both D- and L-melittin but not by either isomer of cecropin A, magainin 2 amide, or the hybrids cecropin A-(1-13)-melittin-(1-13)-NH2 or cecropin A-(1-8)-melittin-(1-18)-NH2. The infectivity of the bloodstream form of the malaria parasite Plasmodium falciparum was also inhibited by the D and L hybrids. It is suggested that the mode of action of these peptides on the membranes of bacteria, erythrocytes, plasmodia, and artificial lipid bilayers may be similar and involves the formation of ion-channel pores spanning the membranes, but without specific interaction with chiral receptors or enzymes.

All-D amino acid-containing channel-forming antibiotic peptides.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2889%2981505-4

Antibacterial and antimalarial properties of peptides that are cecropin-melittin hybrids

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2892%2980377-S

Shortened cecropin A-melittin hybrids. Significant size reduction retains potent antibiotic activity.

Eight new analogs of cecropin A, two new analogs of melittin and 30 hybrid peptides containing sequences from cecropins and melittin have been synthesized. The lengths of the peptides have varied from 37 residues (the length of cecropin A) to 18 residues. The peptides have been assayed for lysis of sheep red blood cells and for antibacterial activity against two Gram negative and three Gram positive bacteria. The best analogs of cecropin A maintained the anti-Escherichia coli activity of the parental peptide, and were not lytic for red blood cells. Melittin and its replacement analogs were all lytic for red blood cells, but an analog with transposed segments was not. Several of the hybrid peptides were found to be both non-hemolytic and highly active against all test bacteria. The data were used to define the structural requirements for antibacterial activity.

Antibacterial peptides designed as analogs or hybrids of cecropins and melittin.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2890%2981239-K

David Wade

Papers

All-D amino acid-containing channel-forming antibiotic peptides.

Antibacterial and antimalarial properties of peptides that are cecropin-melittin hybrids

Shortened cecropin A-melittin hybrids. Significant size reduction retains potent antibiotic activity.

Antibacterial peptides designed as analogs or hybrids of cecropins and melittin.

Hepatotropin mRNA expression in human foetal liver development and in liver regeneration.