The human LL-37 peptide exerts antimicrobial activity against Legionella micdadei interacting with membrane phospholipids.

Legionella micdadei is responsible for community- or nosocomial-acquired pneumonia as well as the influenza-like illness Pontiac fever. The aim of this study was to investigate the ability of L. micdadei to utilize extracellular choline for phosphatidylcholine (PC) synthesis and its consequences for the phospholipid composition of its membrane system and the interaction with the human LL-37 peptide. Comparative analysis of the PC content using isotopic labeling revealed that in presence of exogenous choline 98% of the total PC was synthesized via the Pcs pathway while the remaining 2% were generated via the PE-methylation (PmtA) pathway. PC species were to a greater extent defined by the Pcs pathway in the outer membrane than in the inner membrane. While no major changes in the bacterial lipid content were observed using 31P NMR, indication for utilization of longer acyl chains and slight increase of PG in response to choline addition was observed by a top-down lipidomics screen. The LL-37 peptide inhibited L. micdadei growth in a dose-dependent manner. Bacteria cultured with exogenous choline were more sensitive to the LL-37 peptide when compared to the standard culture condition. Our biophysical investigations show that the peptide perturbs bacterial-derived phospholipid monolayers and this interaction is dependent on the molar portion of PC. This interaction is responsible for the observed changes in the anti-L. micdadei activity of the LL-37 peptide.