This educational resource provides a comprehensive introduction to antimicrobial peptides, the small host defence molecules that play a central role in innate immunity. It covers their structural features, major classes, mechanisms of membrane disruption and intracellular actions, immunomodulatory…
Antimicrobial peptides AMPs are a large and diverse family of molecules that serve as a first line of defense in the innate immune system. They are sometimes called host defence peptides because their functions extend beyond direct killing of microbes to include modulation of immune responses. The innate immune system is the branch of immunity that is present from birth, acting quickly whether an infection has been encountered before or not. AMPs are found in essentially all classes of life, from bacteria and fungi to plants, invertebrates, and vertebrates, including humans.
The activity profile of AMPs is broad. They have demonstrated effectiveness against Gram-negative bacteria, which have an outer membrane, and Gram-positive bacteria, which lack that outer membrane. In addition, they show activity against enveloped viruses, fungi, and, in some experimental contexts, cancerous cells. This broad range has made AMPs a subject of intense interest for drug discovery, particularly at a time when conventional antibiotics are losing effectiveness due to resistance.
Most AMPs are small peptides, typically 12 to 50 amino acids in length. Human defensins, for example, fit within this range. To carry out their function, they usually contain two or more positively charged residues. These positive charges are supplied by the amino acids arginine, lysine, or histidine. The positive charge gives the peptide an affinity for negatively charged microbial membranes.
In addition to the positively charged residues, AMPs contain a substantial amount of hydrophobic amino acids, often more than half of the peptide. A hydrophobic component means the peptide has regions that avoid water and can insert into lipid membranes. This combination of positive charge and hydrophobic character is what allows AMPs to interact with and disrupt bacterial cell membranes while leaving many host cells relatively untouched.
AMPs adopt a limited number of structural themes. The first is the alpha-helical structure, where the peptide forms a helical shape. The second involves beta-stranded structures that are stabilized by disulfide bonds, covalent links between sulfur atoms in cysteine residues. The third theme is the beta-hairpin or loop structure. The fourth is an extended or flexible structure that does not fold into a defined pattern. The structure influences how the peptide interacts with membranes, but many AMPs are flexible and can adopt different conformations depending on their environment.
Researchers classify AMPs into several broad families based on amino acid composition and structure. These categories help scientists predict how a new peptide might behave.
Anionic peptides carry a net negative charge. Examples of this group include maximin H5, which comes from frog skin, and dermcidin, which is produced in human sweat glands. Their negative charge seems contradictory given the general rule that AMPs are cationic. Both positively and negatively charged peptides contribute to host defence.
Linear cationic alpha-helical peptides are one of the most studied groups. This class includes cecropins, first isolated from insects, melittin from bee venom, magainin from frog skin, and LL-37, which is the human cathelicidin. These peptides are unstructured in solution but fold into an alpha-helix when they contact a membrane.
Cationic peptides that are enriched for specific amino acids form another group. Some are rich in proline, glycine, or other residues. The enrichment may influence the mechanism, with some peptides targeting intracellular molecules rather than lysing membranes.
The final major class is composed of disulfide bond-forming peptides, in which cysteine residues form stabilizing bonds. Well known members include defensins, which are widely expressed in humans and other animals, protegrins from pigs, and tachyplesins from horseshoe crabs. The disulfide bonds hold these peptides in a rigid structure that is relatively stable.
The most studied mechanism by which AMPs kill bacteria is disruption of the cell membrane. Four models have been proposed to describe how peptides assemble and damage the lipid bilayer. These models are not mutually exclusive, and a single peptide may act through more than one, depending on its concentration, the lipid composition of the target membrane, and other conditions.
In the barrel-stave model, peptide molecules insert into the membrane and align like the staves of a barrel to form a transmembrane channel. This channel creates a tube through which ions and other small molecules can pass, ultimately causing the cell to lose essential ions and molecules and die.
In the carpet model, peptides accumulate on the surface of the membrane in a dense layer, like a carpet, and cover a large area. When the peptide concentration becomes high enough, the membrane is permeabilized and breaks apart into micelles. This is a detergent-like action that destroys the barrier function.
In the toroidal model, peptide insertion causes the lipid membrane to bend inward, forming a pore that is lined by both peptide and lipid headgroups. The structure is called toroidal because of its doughnut-like shape when viewed in cross-section. These pores allow leakage of cellular contents.
A more recent version of this is the disordered toroidal-pore model. In this model, the toroidal pore is not a uniform, well-defined structure. Instead, peptides and lipids rearrange in a dynamic, disordered manner, forming transient openings. The disordered nature of these pores makes them harder to study and may explain variability in experimental observations.
Membrane disruption is not the only way AMPs can act. A growing body of evidence indicates that some AMPs can pass through the membrane and interact with intracellular targets. Once inside the cell, they may interfere with DNA and protein synthesis, either by binding directly to nucleic acids or by blocking the protein production machinery.
AMPs can also interfere with protein folding, preventing proteins from reaching their proper three-dimensional shape, which is essential for their function. Additionally, some peptides inhibit cell wall synthesis, disrupting construction of the bacterial cell wall. Because a functional cell wall is critical for the stability of many bacteria, blocking its synthesis can be lethal.
The ability to act on multiple targets is significant from a therapeutic perspective. It may make it harder for bacteria to develop resistance, because a single mutation is unlikely to protect against a peptide that attacks the membrane and several intracellular pathways simultaneously.
The term host defence peptides reflects the fact that AMPs do more than directly kill microorganisms. They can alter host gene expression, meaning they can change which genes are switched on or off in cells of the immune system. This can shift the overall immune response in ways that help clear an infection.
AMPs can also act as chemokines, which are signaling proteins that guide immune cells to the site of an infection or injury. By attracting neutrophils, monocytes, and other cells to the right place, AMPs help coordinate a rapid response. Some AMPs promote wound healing, either by directly encouraging skin cells to migrate and divide or by stimulating the release of growth factors.
Dendritic cells are antigen-presenting cells that bridge the innate and adaptive immune systems. AMPs can modulate the responses of dendritic cells, influencing whether they promote inflammation or tolerance. This means that AMPs can shape the direction of the whole immune response, which has implications for both infection and autoimmune disease.
A key feature of AMPs is that they tend to target bacterial cells over the cells of the host. This selectivity is largely due to differences in membrane composition. The membranes of bacteria are rich in acidic phospholipids, negatively charged molecules that attract the positively charged AMPs. In contrast, the surfaces of mammalian cells are composed mostly of zwitterionic phospholipids, molecules that carry both positive and negative charges but have a net neutral charge.
The difference in charge creates an electrostatic preference. AMPs bind more strongly to negatively charged bacterial membranes, and this selective affinity reduces their toxicity to host cells, although it does not eliminate it. Some AMPs are highly selective, while others need to be modified in order to reduce damage to mammalian cells.
Several AMPs have reached clinical use as conventional drugs. As of January 2018, the clinically used AMPs included bacitracin, daptomycin, enfuvirtide, teicoplanin, and vancomycin. Bacitracin is used topically to treat skin infections and is a common ingredient in over-the-counter antibiotic ointments. Daptomycin is an intravenous lipopeptide antibiotic used for serious infections caused by Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus. Enfuvirtide is a peptide drug used in the treatment of HIV infection, functioning as a fusion inhibitor that prevents the virus from entering host cells. Teicoplanin is a glycopeptide antibiotic used against resistant Gram-positive infections. Vancomycin is a well known glycopeptide antibiotic used when other antibiotics have failed.
Nisin is an antimicrobial peptide that stands out for its regulatory status. It is the only FDA approved antimicrobial peptide and is widely used as a food preservative. Nisin is produced by bacteria and is added to cheese, dairy products, and canned goods to prevent the growth of Gram-positive bacteria. Its approval for food use, rather than as a drug, reflects its long history of safe use and its effectiveness in that setting.
It is important to note that many other AMPs are in preclinical development or clinical trials for various applications, but they have not necessarily received regulatory approval. The use of AMPs beyond approved indications is a research area, not an established clinical practice.
Research has revealed that some AMPs have effects that go beyond infectious diseases. Certain cecropins have been reported to possess anticancer properties. These observations are mainly from laboratory studies, where the peptides have been shown to kill cancer cells or inhibit their growth. The mechanisms are not fully understood, but the positive charge of AMPs may allow them to bind to cancer cell membranes, which also carry a more negative surface charge than normal cells.
The fruit fly defensin has been shown to prevent tumor growth in experimental models. Defensins from insects and mammals have a wide range of activities beyond direct killing, including the ability to influence cell division and survival.
AMPs also demonstrate antiviral effects, particularly against enveloped viruses. Enveloped viruses have a lipid membrane derived from the host cell, and AMPs can disrupt this membrane. Finally, there are roles for AMPs in neurology. Some peptides are expressed in the nervous system and can modulate neural cell activity, though understanding of these roles is still developing. These observations are largely at the research stage, and clinical applications in oncology or neurology would require substantial further study.
Bacteria are not passive targets. They have evolved a number of strategies to reduce the effectiveness of AMPs. One common strategy is alteration of surface charge. If a bacterium reduces the number of negatively charged molecules on its surface, the positively charged AMPs bind less well. This is an example of a common theme in resistance: change the target so the attacker cannot recognize it.
Some bacteria produce a capsule, a polysaccharide layer covering the cell surface. This capsule can interfere with the ability of AMPs to reach the membrane. Others modify the fluidity of their membrane, making it more or less flexible in ways that reduce the ability of peptides to insert.
Once an AMP is bound, bacteria can internalize and degrade the peptide using enzymes. Efflux pumps can also be deployed to actively transport the peptide out of the cell before it can cause damage. In addition, many bacteria produce proteases, enzymes that break down proteins and peptides, which can cleave AMPs into inactive fragments. Finally, bacteria can release outer membrane vesicles that act as decoys. These small membrane blebs bind the AMPs and absorb them, reducing the amount that reaches the bacterial surface.
The existence of these resistance mechanisms is important for drug development. A peptide candidate that works only on the membrane can be defeated by surface charge changes, while one with multiple modes of action may be harder for bacteria to resist.
Because AMPs are so diverse and numerous, researchers rely on curated databases to organize information. Prominent databases include the Antimicrobial Peptide Database APD , ADAM, CAMP, DBAASP, and DRAMP. These databases collect information on peptide sequences, structures, and activities. They help researchers search for peptides with particular properties and avoid re-discovering known compounds.
In addition to databases, there are prediction tools that use computational methods to identify potential antimicrobial peptides. PeptideRanker is one such tool. Given a peptide sequence, it scores the likelihood that the peptide has antimicrobial activity. These tools are useful for triaging the huge number of possible peptides that could be examined experimentally. They are not perfect, and predictions must be confirmed in the laboratory.
Many AMPs are natural molecules that have been studied for decades, but few have reached regulatory approval as drugs. The difference between topical use, food use, and systemic drug use is significant. A peptide that is safe on the skin may not be safe when injected. Clinical trial evidence is required to establish safety and efficacy for each new route of administration and disease indication.
Regulatory agencies such as the FDA in the United States and the European Medicines Agency in Europe review peptide drugs using the same standards as other medicines. The approval of nisin as a food preservative is not the same as approval for therapeutic use. Any claims that AMPs can treat cancer, viral infections, or neurological diseases should be regarded as experimental unless supported by properly conducted clinical trials and regulatory approvals.
This educational resource is for general informational purposes only. It is not medical advice, and it should not be used to make decisions about treatment. Many of the antimicrobial, anticancer, and immune-related applications described in this article are under investigation and have not been approved by regulatory authorities for clinical use. Consult a qualified healthcare professional for advice about any specific medical condition.