How LL-37 Disrupts Bacterial Membranes and Biofilms
LL-37 and Cathelicidins
0.5 μg/ml
The concentration at which LL-37 blocks Pseudomonas aeruginosa biofilm formation, 128-fold below its minimum inhibitory concentration.
Overhage et al., Infection and Immunity, 2008
Overhage et al., Infection and Immunity, 2008
View as imageYour body manufactures its own antibiotic. LL-37, the only cathelicidin produced by human cells, kills bacteria by punching holes in their membranes and dismantles the protective biofilm colonies that make infections so difficult to treat. A 2025 review cataloged LL-37's activity against more than 38 bacterial species, 16 fungi, and 16 viruses.[1] What makes this peptide unusual is its dual capability: it destroys individual bacteria through direct membrane disruption, and it prevents bacterial communities from forming at concentrations 128-fold lower than what is needed to kill. For a broader look at how vitamin D regulates LL-37 production, see our pillar article on the sunshine-antibiotic connection.
Key Takeaways
- LL-37 inhibits P. aeruginosa biofilm formation at 0.5 μg/ml, 128-fold below its 64 μg/ml minimum inhibitory concentration (Overhage et al., 2008)
- The peptide switches between pore formation and nanofibre assembly depending on the lipid composition of the target membrane (Shahmiri et al., 2016)
- LL-37 forms tetrameric channels with a strongly charged core when crystallized with membrane mimics, explaining its membrane-perforating activity (Sancho-Vaello et al., 2020)
- LL-37 kills both extracellular and intracellular S. aureus, with 90% reduction of intracellular bacteria at 25 μg/ml within 1 hour (Noore et al., 2013)
- Biofilm disruption works through at least five separate mechanisms including reduced attachment, quorum sensing interference, and biofilm matrix degradation (Memariani and Memariani, 2023)
What LL-37 Is and Why It Matters
LL-37 is a 37-amino-acid peptide cleaved from the precursor protein hCAP-18 (human cationic antimicrobial protein 18). It belongs to the cathelicidin family of host defense peptides, and it is the only cathelicidin humans produce.[2] Neutrophils, macrophages, and epithelial cells throughout the skin, lungs, and gut all release LL-37 in response to infection. For a deeper look at what makes this peptide unique among species, see how cathelicidins compare across the animal kingdom.
The peptide carries a net positive charge of +6 at physiological pH. This charge is central to its selectivity: bacterial membranes are rich in negatively charged phospholipids (phosphatidylglycerol, cardiolipin), while human cell membranes are dominated by neutral zwitterionic phospholipids (phosphatidylcholine, sphingomyelin).[3] The electrostatic mismatch means LL-37 accumulates on bacterial surfaces while largely ignoring host cells. Cholesterol in mammalian membranes further reduces LL-37's membrane-disrupting ability, adding another layer of selectivity.
In solution, LL-37 is unstructured. Upon contact with a lipid membrane, it folds into an amphipathic alpha-helix, with hydrophobic residues on one face and cationic residues on the other.[3] This structural transition is the first step in both its membrane-killing and antibiofilm activities.
How LL-37 Destroys Bacterial Membranes
Electrostatic Binding and Initial Contact
The killing sequence begins with electrostatic attraction. LL-37's cationic residues are drawn to the anionic headgroups of bacterial phospholipids. Pastuszak et al. (2023) demonstrated this using Langmuir monolayer models of Legionella gormanii membranes: the peptide bound preferentially to anionic lipid mixtures and barely perturbed neutral phosphatidylcholine or phosphatidylethanolamine monolayers.[4] At 10 micromolar concentration, LL-37 rendered L. gormanii viable but non-culturable, a state indicating severe membrane compromise without immediate lysis.[4] This dose-dependent effect suggests LL-37 progressively compromises membrane integrity across a range of concentrations rather than triggering instant cell death at a single threshold.
Bucki et al. (2010) described an additional function during this initial binding phase: LL-37 neutralizes lipopolysaccharide (LPS) from gram-negative bacteria and lipoteichoic acid from gram-positive species.[2] This is relevant because it means LL-37 does not just kill bacteria; it also neutralizes the toxic cell wall fragments released during bacterial lysis, preventing the septic inflammatory cascade that can follow antibiotic treatment.
The Carpet Model: Surface Dissolution
Henzler Wildman et al. (2003) used solid-state NMR spectroscopy with site-specifically labeled LL-37 in oriented lipid bilayers and made three key findings about how the peptide kills.[3]
First, LL-37 lies parallel to the membrane surface. Nitrogen-15 chemical shift and dipolar-shift spectroscopy showed the amphipathic helix oriented flat against the bilayer rather than spanning it vertically. This orientation persisted across anionic and zwitterionic bilayers and at varying peptide concentrations and temperatures.
Second, phosphorus-31 NMR showed the peptide induced positive curvature strain in the lipid bilayer, increasing the lamellar-to-inverted-hexagonal phase transition temperature in both model systems and E. coli lipids.
Third, no small, rapidly tumbling membrane fragments formed, ruling out a simple detergent-like dissolution. Instead, the evidence pointed to a toroidal pore mechanism at the concentrations tested, where accumulated peptides and lipid headgroups curve inward together to create pores that leak the cell's contents, collapse the transmembrane potential, and trigger death.
Pore vs. Nanofibre: The Membrane Decides
Shahmiri et al. (2016) revealed that LL-37's killing mechanism is not fixed. It switches based on the structure of lipids in the membrane core, not just the headgroup charge.[5] In bilayers with unsaturated phospholipids (kinked acyl chains that create a more fluid membrane), LL-37 forms discrete pores. In bilayers with saturated phospholipids (straight chains, tighter packing), the peptide assembles into nanofibres on the membrane surface instead of inserting as pores.
This finding was significant because it showed the target membrane itself determines how LL-37 kills. Different bacterial species have different lipid compositions, which may explain why LL-37's potency varies across pathogens. A bacterium with predominantly unsaturated membrane lipids would be attacked through pore formation, while one with more saturated lipids would face nanofibre-mediated surface disruption. For a broader explanation of how antimicrobial peptides form pores, see our dedicated article on the topic.
Channel Formation at the Atomic Level
Sancho-Vaello et al. (2020) crystallized LL-37 in the presence of membrane-mimicking detergents and resolved a narrow tetrameric channel structure.[6] Four LL-37 molecules assemble into a functional unit with a strongly charged interior that would allow ions and small molecules to flow uncontrollably across the membrane, collapsing the transmembrane potential. This structural work provides a molecular explanation for why LL-37 can depolarize bacterial membranes at concentrations where it does not fully lyse cells: even a few open channels per cell are enough to disrupt ion homeostasis.
These mechanisms are not mutually exclusive. A bacterium exposed to rising LL-37 concentrations likely experiences surface carpet effects first, followed by pore insertion or nanofibre assembly as local peptide density increases. The diversity of cathelicidins across species shows that evolution has tuned this balance differently across organisms. LL-37's ability to switch modes based on membrane composition gives it versatility that synthetic antibiotics, which typically have a single molecular target, lack.
How LL-37 Prevents Biofilm Formation
Killing individual bacteria is one thing. Preventing them from organizing into biofilms, the structured communities encased in a self-produced matrix that resist antibiotic treatment, is another. LL-37 does both, and at very different concentrations.
Sub-MIC Antibiofilm Activity
The landmark study by Overhage et al. (2008) demonstrated that LL-37 inhibits Pseudomonas aeruginosa biofilm formation at 0.5 μg/ml, a concentration 128-fold below its minimum inhibitory concentration (MIC) of 64 μg/ml.[7] At this sub-lethal concentration, LL-37 is not killing bacteria. It is changing their behavior. LL-37 also affected pre-existing, pre-grown P. aeruginosa biofilms, demonstrating activity against both forming and established communities.
Using microarray analysis, Overhage and colleagues identified three distinct anti-biofilm mechanisms: LL-37 decreased the attachment of bacterial cells to surfaces (the essential first step in biofilm formation), stimulated type IV pilus-dependent twitching motility (promoting migration rather than settlement), and downregulated both the Las and Rhl quorum sensing systems, preventing bacteria from collectively switching to the biofilm phenotype.[7]
Hell et al. (2010) extended this observation to Staphylococcus epidermidis, showing that just 1 mg/L of LL-37 significantly decreased both the attachment capability and the biofilm mass of this gram-positive organism.[8] Jacobsen and Jenssen (2012) confirmed the pattern across additional species and noted that LL-37's antibiofilm activity bridges innate and adaptive immune responses, making it fundamentally different from conventional antibiotics that only target metabolically active bacteria.[9]
Five Mechanisms of Biofilm Disruption
Memariani and Memariani (2023) published the most comprehensive review of LL-37's antibiofilm effects, identifying five distinct mechanisms active across at least 12 bacterial species.[10]
Inhibition of bacterial adhesion. LL-37 interferes with the initial attachment of bacteria to surfaces, the critical first step in biofilm formation. Without attachment, the biofilm never gets started.
Downregulation of biofilm-associated genes. In Pseudomonas aeruginosa, LL-37 alters the expression of genes encoding Type IV pili (required for twitching motility and microcolony formation) and rhamnolipid biosynthesis (involved in biofilm architecture).
Suppression of quorum-sensing pathways. Quorum sensing is the bacterial communication system that coordinates biofilm development. LL-37 disrupts the Las quorum-sensing system in P. aeruginosa, interfering with the molecular signals bacteria use to know when they have reached the population density needed to form a biofilm.[10] Khan et al. (2026) placed LL-37's quorum sensing disruption in the broader context of peptide-based inhibitors and noted that peptides can simultaneously disrupt membrane integrity and signaling pathways, providing a multi-hit attack harder for bacteria to resist through single mutations.[11]
Degradation of biofilm matrix. Even after a biofilm has formed, LL-37 can penetrate and degrade the extracellular polymeric substance (EPS) matrix that protects biofilm-resident bacteria.
Eradication of biofilm-residing cells. At higher concentrations, LL-37 kills bacteria embedded within established biofilms, a task that most antibiotics cannot accomplish because the biofilm matrix blocks drug penetration.
The bacteria vulnerable to these effects span both gram-positive and gram-negative organisms: Acinetobacter baumannii, Burkholderia thailandensis, Cutibacterium acnes, Escherichia coli, Francisella tularensis, Helicobacter pylori, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus pyogenes, among others.[10] This broad activity connects to the wider field of antimicrobial peptides and microbiome balance.
Fighting Drug-Resistant Bacteria: LL-37 Against S. aureus and MRSA
LL-37's membrane-disruption mechanism gives it activity against bacteria that have evolved resistance to conventional antibiotics, because membrane composition is harder to modify than the specific protein targets that most antibiotics attack.
Noore et al. (2013) tested LL-37 against both extracellular and intracellular Staphylococcus aureus.[12] The intracellular component matters: S. aureus can hide inside host cells like macrophages, evading conventional antibiotics that cannot cross mammalian cell membranes effectively. LL-37 at 25 μg/ml reduced intracellular S. aureus by 90% within 1 hour. The researchers attributed this to LL-37's ability to enter mammalian cells (facilitated by its cationic, amphipathic structure) and kill bacteria within the intracellular compartment, a capacity most conventional antibiotics lack.
Nibbering et al. (2019) extended this to clinical application by testing P10, an LL-37-derived peptide, against methicillin-resistant S. aureus (MRSA) on human skin explants.[13] P10 eradicated MRSA from human skin when formulated in pharmaceutical ointments, demonstrating that LL-37-based compounds can overcome the formulation challenges that have limited clinical translation. This line of research connects to the broader work on antimicrobial peptides in wound care.
Why Bacteria Struggle to Resist Membrane-Targeting Peptides
Bacteria can partially reduce susceptibility to LL-37 by modifying their membrane lipid composition, adding positively charged groups to LPS, or producing proteases that degrade the peptide.[2] Some pathogens, including Group A Streptococcus, produce surface proteins that sequester cathelicidins before they reach the membrane. These partial resistance mechanisms are documented, and our article on whether bacteria can become resistant to antimicrobial peptides covers them in detail.
Full resistance is difficult to evolve because LL-37 attacks a fundamental structural feature (the negatively charged lipid bilayer) rather than a specific protein target. Changing membrane composition enough to fully resist LL-37 would compromise the membrane's basic functions: permeability barrier, protein anchoring, and energy transduction. This structural constraint is why antimicrobial peptides have remained effective over hundreds of millions of years of host-pathogen coevolution, while resistance to conventional antibiotics can emerge within years of clinical use.[11]
The multi-target nature of LL-37's action, combining membrane disruption with quorum sensing interference and intracellular killing, creates additional barriers. A bacterium would need to simultaneously modify its membrane, its quorum sensing receptors, and its intracellular environment, a combination of mutations far less probable than the single-step mutations that commonly confer resistance to conventional antibiotics.
Current Limitations and the Search for Better Derivatives
The native LL-37 molecule has practical limitations for therapeutic use. Yuan et al. (2025) systematically reviewed these challenges: high production costs (37 amino acids is long for a synthetic peptide), reduced efficacy under physiological salt concentrations that screen out essential electrostatic interactions, susceptibility to proteolytic degradation by bacterial and host enzymes, and toxicity to mammalian cells at concentrations needed for direct bactericidal activity.[14]
The anti-biofilm activity at 0.5 μg/ml is well below the toxic range, making biofilm prevention a more feasible clinical application than direct bactericidal therapy. Multiple research groups are developing shortened LL-37 fragments and modified derivatives that retain the membrane-disrupting and anti-biofilm properties while reducing length, improving salt stability, and lowering host cell toxicity.[14]
Wuersching et al. (2021) demonstrated another approach: combining LL-37 with conventional antibiotics. LL-37 and human lactoferricin improved the efficacy of amoxicillin, clindamycin, and metronidazole against anaerobic biofilms associated with oral diseases.[15] The peptide degrades the biofilm matrix and permeabilizes bacteria within, making them vulnerable to antibiotics that would otherwise fail. The different attack mechanisms reduce the probability of bacteria developing resistance to both simultaneously.
Most studies of LL-37's membrane disruption have used model lipid bilayers, bacterial monocultures, or simplified in vitro biofilm systems. In the body, serum proteins bind LL-37 and reduce its activity, physiological salt concentrations diminish its antimicrobial potency, and the extracellular matrix of mature in vivo biofilms is thicker and more heterogeneous than laboratory biofilms. Clinical translation has been slow. A randomized trial demonstrated that topical LL-37 healed chronic venous leg ulcers (Gronberg et al., 2014), but systemic applications face stability, dosing, and potential inflammatory side effect challenges. The unique versatility of LL-37 as the only human cathelicidin is both its greatest asset and its greatest complication for drug development.
The Bottom Line
LL-37 kills bacteria through multiple membrane disruption mechanisms (carpet-like dissolution, pore formation, and nanofibre assembly) that switch based on the target membrane's lipid composition. Its anti-biofilm activity operates separately at concentrations 128-fold below its bactericidal MIC, working through reduced attachment, stimulated motility, quorum sensing disruption, matrix degradation, and embedded cell killing. The native peptide has practical limitations for clinical use, but LL-37-derived compounds and combination therapies with conventional antibiotics are advancing toward applications in wound care and drug-resistant infections.