The human antimicrobial peptide LL-37 killed Staphylococcus aureus at nanomolar concentrations — far lower than conventional antibiotics — and was uniquely effective against bacteria hiding inside host cells, which standard drugs struggle to reach.
Effective at nanomolar vs. millimolar for antibioticsLL-37 killed extracellular S. aureus at concentrations roughly 1,000 to 1,000,000 times lower than the conventional antibiotics doxycycline and cefazolin
What the researchers found
LL-37 was effective at killing extracellular S. aureus at nanomolar concentrations, while lactoferricin B required micromolar concentrations and the conventional antibiotics doxycycline and cefazolin required millimolar concentrations — a potency difference of roughly three orders of magnitude.
Critically, LL-37 was superior to conventional antibiotics at eliminating intracellular S. aureus within osteoblasts. Kinetic studies showed LL-37 acted rapidly against both extra- and intracellular bacteria. An unexpected finding was that LL-37 killed a clinical S. aureus strain more effectively than a standard laboratory (ATCC) strain, suggesting strong real-world applicability.
Why it matters
Intracellular bacterial persistence is a major driver of chronic bone infections (osteomyelitis), prosthetic joint infections, and recurrent Staph infections. Conventional antibiotics frequently fail because they cannot efficiently penetrate host cell membranes to reach hiding bacteria. LL-37's ability to kill both extracellular and intracellular S. aureus at very low concentrations makes it a promising candidate for addressing antibiotic-resistant and recurrent infections — one of the most urgent challenges in modern medicine.
How the study worked
The researchers used bacterial killing assays to measure LL-37's extracellular efficacy compared to lactoferricin B, doxycycline, and cefazolin against both an ATCC reference strain and a clinical isolate of S. aureus. An osteoblast infection model was established to test intracellular killing — osteoblasts were infected with S. aureus, then treated with LL-37 or antibiotics, and surviving intracellular bacteria were counted. Kinetic studies tracked the speed of bacterial elimination over time.
What this study cannot tell us
This is an in vitro study using cell-based models, and results may not directly translate to in vivo efficacy where factors like peptide stability, serum binding, tissue penetration, and host toxicity come into play. LL-37 can be cytotoxic at higher concentrations, and the therapeutic window between antimicrobial efficacy and host cell damage was not thoroughly characterized. The study tested only S. aureus; efficacy against other intracellular pathogens is unknown from this work. The osteoblast model, while relevant to bone infections, represents only one of many tissues where intracellular infection occurs.
How to read the evidence
This is a preclinical in vitro study using cell-based infection models. The results are compelling and well-controlled but have not been validated in animal models or human trials, so translation to clinical use remains uncertain.
When this study was published
Published in 2013, this study is over a decade old. LL-37 research has continued to advance, with ongoing efforts to develop stable, deliverable formulations for clinical use, though no LL-37-based drug has yet reached the market.
The bigger picture
With antibiotic resistance recognized by the WHO as one of the top global health threats, antimicrobial peptides represent a promising alternative or complement to conventional antibiotics. LL-37, the only human cathelicidin, has been extensively studied for its broad-spectrum antimicrobial activity. This study's demonstration that it can eliminate intracellular bacteria — a sanctuary that shields pathogens from most drugs — adds a particularly valuable capability to its profile. The challenge remains translating LL-37's in vitro potency into clinically viable treatments, addressing issues of stability, delivery, and potential toxicity at therapeutic doses.
Questions still open
- Can LL-37's intracellular killing ability be maintained in vivo without unacceptable toxicity to host tissues?
- Why is LL-37 more effective against the clinical Staph strain than the laboratory strain, and what does this imply about resistance mechanisms?
- Could LL-37 be combined with conventional antibiotics to create synergistic regimens that clear both extracellular and intracellular reservoirs?
Common questions
What is LL-37 and where does it come from?
Why are intracellular bacteria such a problem for treating infections?
Read the original research
Cationic antimicrobial peptide LL-37 is effective against both extra- and intracellular Staphylococcus aureus.
Antimicrobial agents and chemotherapy, 57(3), 1283-90
Citation
Noore, Jabeen; Noore, Adly; Li, Bingyun. (2013). Cationic antimicrobial peptide LL-37 is effective against both extra- and intracellular Staphylococcus aureus.. Antimicrobial agents and chemotherapy, 57(3), 1283-90. https://doi.org/10.1128/AAC.01650-12