Human skin makes its own built-in antibiotic

Researchers at the University of California, San Diego studied a peptide called cathelicidin, present in human skin as LL-37 and in mouse skin as a related form called CRAMP, to see whether it actually protected against infection or was just a bystander.

In the peer-reviewed study, the team bred mice lacking the CRAMP gene and exposed both normal and CRAMP-deficient mice to Group A Streptococcus, the bacterium behind flesh-eating skin infections. Mice without the peptide developed far larger, more severe skin lesions than normal mice, showing the native peptide was actively holding the infection back.

The finding established cathelicidins as a genuine first line of defense built into skin itself, not simply a leftover of immune cells working elsewhere. It helped launch a search for synthetic versions of LL-37 or related peptides as topical treatments for resistant skin infections, a search still active more than two decades later.

Skin already carries its own built-in antibiotic.