Antimicrobial peptides are simultaneously the oldest antibiotics on Earth and the newest frontier in modern drug discovery. They are the molecules that bacteria, fungi, frogs, snails, beetles, sea sponges, and human white blood cells have been using to kill microbes for somewhere between several hundred million and several billion years. They are also the basis of multiple FDA-approved drugs that quietly underpin modern hospital medicine—daptomycin, colistin, polymyxin B, gramicidin, telavancin, oritavancin—and they are at the center of the global push to find new antibiotics that can outpace the rising tide of antimicrobial resistance.
This article walks through what antimicrobial peptides are, how they kill microbes, which ones have made it into clinical use, and why the AMR crisis is forcing the pharmaceutical industry to take a second look at a class of molecules it had largely written off in the antibiotic boom decades.
What Are Antimicrobial Peptides?
Antimicrobial peptides (AMPs) are short, mostly cationic, mostly amphipathic peptides—typically 12 to 50 amino acids long—that disrupt microbial membranes or interfere with intracellular microbial processes. They are produced by essentially every multicellular organism studied and by many single-celled ones, and they are a foundational component of innate immunity. Unlike adaptive immunity (which takes days to weeks to mount a specific response), AMPs are pre-formed or rapidly induced and act within minutes of microbial exposure.
The major mammalian AMP families include:
- Defensins. Small cysteine-rich peptides with three intramolecular disulfide bonds. Humans express α-defensins (HNP1–4 in neutrophils, HD-5 and HD-6 in Paneth cells of the small intestine) and β-defensins (hBD1–4, expressed broadly in epithelial tissues). They are concentrated in the granules of neutrophils and the secretory vesicles of mucosal epithelial cells.
- Cathelicidins. Humans express a single cathelicidin, LL-37, a 37-residue amphipathic helical peptide cleaved from the precursor hCAP18 by neutrophil proteinase 3. LL-37 has direct antimicrobial activity, immunomodulatory effects, and roles in wound healing.
- Histatins. Salivary peptides with antifungal activity, particularly against Candida.
- Hepcidin. A peptide hormone with both antimicrobial activity and a central regulatory role in iron metabolism.
In other organisms, the AMP catalog is even larger. Magainins were isolated from the skin of the African clawed frog Xenopus laevis by Michael Zasloff in 1987, in one of the most influential AMP papers ever published—Zasloff noticed that the frogs healed surgical wounds in non-sterile water without infection and traced the protective activity to peptide secretions in the skin. Cecropins come from the silk moth Hyalophora cecropia, characterized by Hans Boman in the 1980s in foundational work that established insect immunity as a tractable model system for antimicrobial peptide biology. Bacteriocins, defensins, and dozens of other classes appear across the tree of life.
For more on where peptides like these sit in the broader peptide classification, see our overview of natural peptides in the human body.
Mechanism: Why AMPs Kill Microbes (and Mostly Spare Us)
Most AMPs work by selectively disrupting bacterial cell membranes. The selectivity comes from a fundamental difference between bacterial and mammalian membranes:
- Bacterial membranes are rich in negatively charged lipids—phosphatidylglycerol, cardiolipin, lipopolysaccharide on Gram-negatives, lipoteichoic acid on Gram-positives.
- Mammalian outer membrane leaflets are dominated by zwitterionic phospholipids (phosphatidylcholine, sphingomyelin), with cholesterol stabilizing the bilayer.
A cationic amphipathic peptide preferentially binds to the negatively charged bacterial surface, then inserts its hydrophobic face into the lipid bilayer. Once inserted, the peptide can disrupt the membrane through several mechanistic models:
- Barrel-stave pores, where peptides assemble into a transmembrane bundle with hydrophobic outer surfaces and a hydrophilic central pore.
- Toroidal pores, where the membrane lipids tilt and curve around inserted peptides to form a pore lined by both peptide and lipid head groups.
- Carpet model, where peptides accumulate parallel to the membrane surface until they reach a critical density and dissolve the membrane like a detergent.
The result in all cases is loss of membrane integrity, dissipation of the proton motive force, leakage of intracellular contents, and rapid cell death—often within minutes. Some AMPs also have intracellular targets: nucleic acid binding, protein synthesis inhibition, cell wall biosynthesis interference, or interaction with specific enzymes.
The selectivity is not perfect, which is why AMP toxicity to mammalian cells is the single biggest obstacle in developing them as systemic drugs.
Clinical Evidence: AMPs That Made It to Market
Despite the development challenges, several peptide antibiotics have achieved FDA approval and are in active clinical use:
Gramicidin (1939). Isolated from Bacillus brevis by René Dubos, gramicidin was arguably the first commercially produced antibiotic in history—predating penicillin by several years in actual clinical availability. It is too toxic for systemic use but remains in topical antibiotic formulations (Neosporin).
Polymyxin B and Colistin (Polymyxin E). Discovered in the late 1940s, these cyclic lipopeptides target the lipopolysaccharide of Gram-negative bacteria. They were largely abandoned in the 1970s due to nephrotoxicity, then dramatically resurrected in the 2000s as last-line therapy for multidrug-resistant Gram-negative infections (carbapenem-resistant Enterobacterales, Acinetobacter baumannii, Pseudomonas aeruginosa). Colistin in particular is now one of the most important drugs in the AMR-era hospital pharmacy, despite its toxicity.
Daptomycin (Cubicin). FDA-approved in 2003 by Cubist Pharmaceuticals (now Merck), daptomycin is a cyclic lipopeptide derived from Streptomyces roseosporus. It binds bacterial membranes in a calcium-dependent manner, oligomerizes, and depolarizes the membrane, killing Gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci. Daptomycin is approved for complicated skin and skin structure infections, S. aureus bacteremia, and right-sided endocarditis. It is the most commercially successful modern peptide antibiotic and a textbook example of how natural-product peptide drug discovery still works in 2026.
Telavancin, Dalbavancin, and Oritavancin. These are semi-synthetic lipoglycopeptides—engineered derivatives of vancomycin with lipid tails that anchor them to the bacterial membrane and dramatically extend their half-lives. Oritavancin and dalbavancin can be given as single intravenous doses for skin infections, an enormous practical advantage in outpatient antibiotic therapy.
Bacitracin. A topical peptide antibiotic from Bacillus subtilis, used in ointments for minor skin infections.
Vancomycin. Technically a glycopeptide rather than a pure AMP, but mechanistically related and historically the first-line drug for serious Gram-positive infections.
In the development pipeline, omiganan (a cathelicidin-derived topical peptide from MX Pharmaceuticals) has been studied for catheter-related infections and rosacea, murepavadin targets Pseudomonas outer membrane proteins, and brilacidin is a small-molecule defensin mimetic in clinical trials. Several startups (including teams at Genome Protein Logic, deepCDR Bio, and academic labs using AlphaFold-driven peptide design) are now using machine learning to generate novel AMP candidates at scale.
Approved Uses and AMR Context
The peptide antibiotics already on the market are concentrated in two clinical niches:
- Last-line Gram-negative therapy. Colistin and polymyxin B are reserved for multidrug-resistant Gram-negative infections, especially in ICU patients, where no other agent retains activity.
- Resistant Gram-positive infections. Daptomycin, vancomycin, and the lipoglycopeptides are workhorses against MRSA, vancomycin-resistant enterococci, and other resistant Gram-positives.
The broader strategic role of AMPs in the AMR crisis is increasingly recognized. Resistance to existing peptide antibiotics is rising (mcr-1, the plasmid-encoded colistin resistance gene, was identified in China in 2015 and has since spread globally), but the natural AMP repertoire is so vast and structurally diverse that it remains one of the most promising sources of novel scaffolds for the next generation of antibacterials. Several governments and the BARDA antimicrobial program are actively funding peptide antibiotic development.
Safety and Side Effects
AMP toxicity is the field's persistent obstacle. Membrane-disrupting mechanisms that work on bacteria can also damage mammalian cells, and the most common adverse-effect profile across the class includes:
- Nephrotoxicity (especially colistin, polymyxin B, and to a lesser extent daptomycin).
- Neurotoxicity (paresthesias, neuromuscular blockade, with the polymyxins).
- Myopathy and rhabdomyolysis (daptomycin, particularly with statin co-administration).
- Eosinophilic pneumonia (rare but recognized with daptomycin).
- Infusion-related reactions (vancomycin red-man syndrome and similar phenomena).
Topical and locally delivered AMPs avoid most systemic toxicity, which is why so many AMP development programs target topical and ophthalmic applications first.
Connection to Gene Editing and Modern Peptide Therapy
AMPs are deeply entangled with the gene-editing era in three ways. First, CRISPR screens are now being used to map bacterial resistance pathways for peptide antibiotics—identifying the genetic determinants that allow bacteria to evade colistin, daptomycin, or experimental AMPs. Those screens are accelerating mechanism-of-resistance work that used to take years.
Second, synthetic biology and AI-driven peptide design are reshaping AMP discovery. Machine-learning models trained on the existing AMP literature can now propose novel sequences with predicted activity, and labs at MIT, Penn, and elsewhere have demonstrated AI-designed AMPs with potency competitive against natural templates. Several of those programs use the same generative protein modeling toolkit (AlphaFold, RFDiffusion, ESMFold) that is reshaping biologic drug design more broadly.
Third, AMP delivery is increasingly converging with the same nanoparticle and conjugation strategies used for gene-editing payloads. See our peptide delivery challenges and nanoparticles article for the broader context.
FAQ
What is the most successful FDA-approved antimicrobial peptide?
Daptomycin (Cubicin), approved in 2003 for complicated skin infections, S. aureus bacteremia, and right-sided endocarditis. It is the most commercially significant modern peptide antibiotic.
What is the difference between defensins and cathelicidins?
Defensins are cysteine-rich peptides with three disulfide bonds, expressed in neutrophils and epithelial cells. Cathelicidins (LL-37 in humans) are amphipathic helical peptides cleaved from a precursor protein, with both direct antimicrobial and immunomodulatory functions.
Why don't antimicrobial peptides damage human cells more often?
Selectivity comes from the difference in membrane composition. Bacterial membranes have negatively charged phospholipids on their outer leaflet, attracting cationic AMPs. Mammalian outer membranes are dominated by neutral phospholipids and stabilized by cholesterol, which makes them less vulnerable.
Can bacteria develop resistance to antimicrobial peptides?
Yes. The plasmid-encoded mcr-1 gene confers colistin resistance and has spread globally since its 2015 discovery. Some bacteria modify their lipopolysaccharide to reduce negative charge, others express AMP-degrading proteases, and some pump AMPs out via efflux systems.
Why is the AMR crisis reviving interest in peptide antibiotics?
Because the natural AMP repertoire is enormous, structurally diverse, and largely unexplored, and because some AMP mechanisms (membrane disruption) are difficult for bacteria to evolve around without compromising fitness. AI-driven design is also lowering the cost of finding novel scaffolds.
Is colistin still used today?
Yes. Despite its nephrotoxicity, colistin is one of the most clinically important last-resort antibiotics for multidrug-resistant Gram-negative infections in 2026.
