Separability of antibacterial and membranolytic activity in the human host defense peptide LL-37

The human CAMP gene product LL-37 is thought to exert direct antimicrobial activity against gram-negative bacteria via membrane disruption. In the course of structure-activity relationship studies of LL-37 aimed at developing peptidomimetic antibiotics, however, we incidentally noted mutations in LL-37 that globally inhibit membrane disruption in both mammalian and gram-negative bacterial cells. Despite their diminished capacity for membranolysis, these variants retained full antibacterial activity against gram-negative bacteria. While testing LL-37 and derivatives thereof against clinical isolates of Pseudomonas aeruginosa from patients with cystic fibrosis, we further noted unusually high rates of elevated minimum inhibitory concentrations for LL-37. Further evaluation of these clinical isolates revealed that they are fully permeabilized by LL-37 without being killed. Thus, we have identified variants of LL-37 that kill gram-negative bacteria without permeabilizing, and gram-negative bacteria that are permeabilized by LL-37 without being killed. This may suggest the existence of one or more mechanisms other than membrane disruption by which LL-37 can kill gram-negative bacteria, which may open up new avenues for antibiotic development based on the naturally evolved, non-membrane targets of human host defense peptides.