Human Antimicrobial Peptide LL-37 Kills Staph Bacteria Both Outside and Inside Cells — Outperforming Conventional Antibiotics

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.

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.

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.

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.