Legionella gormanii is a fastidious, Gram-negative bacterium known to be the etiological agent of atypical community-acquired pneumonia. The human cathelicidin LL-37 exhibits a dose-dependent bactericidal effect on L. gormanii. The LL-37 peptide at the concentration of 10 µM causes the bacteria to become viable but not cultured. The antibacterial activity of the peptide is attributed to its effective binding to the bacterial membrane, as demonstrated by the fluorescence lifetime imaging microscopy. In this study, to mimic the L. gormanii membranes and their response to the antimicrobial peptide, Langmuir monolayers were used with the addition of the LL-37 peptide to the subphase of the Langmuir trough to represent the extracellular fluid. The properties of the model membranes (Langmuir monolayers) formed by phospholipids (PL) isolated from the L. gormanii bacteria cultured on the non-supplemented (PL−choline) and choline-supplemented (PL+choline) medium were determined, along with the effect of the LL-37 peptide on the intermolecular interactions, packing, and ordering under the monolayer compression. Penetration tests at the constant surface pressure were carried out to investigate the mechanism of the LL-37 peptide action on the model membranes. The peptide binds to the anionic bacterial membranes preferentially, due to its positive charge. Upon binding, the LL-37 peptide can penetrate into the hydrophobic tails of phospholipids, destabilizing membrane integrity. The above process can entail membrane disruption and ultimately cell death. The ability to evoke such a great membrane destabilization is dependent on the share of electrostatic, hydrogen bonding and Lifshitz–van der Waals LL-37−PL interactions. Thus, the LL-37 peptide action depends on the changes in the lipid membrane composition caused by the utilization of exogenous choline by the L. gormanii.

Legionella gormanii is one of the species belonging to the genus Legionella, which causes atypical community-acquired pneumonia. The most important virulence factors that enable the bacteria to colonize the host organism are associated with the cell surface. Lipids building the cell envelope are crucial not only for the membrane integrity of L. gormanii but also by virtue of being a dynamic site of interactions between the pathogen and the metabolites supplied by its host. The utilization of exogenous choline by the Legionella species results in changes in the lipids’ composition, which influences the physicochemical properties of the cell surface. The aim of this study was to characterize the interfacial properties of the phospholipids extracted from L. gormanii cultured with (PL+choline) and without exogenous choline (PL−choline). The Langmuir monolayer technique coupled with the surface potential (SPOT) sensor and the Brewster angle microscope (BAM) made it possible to prepare the lipid monomolecular films (model membranes) and study their properties at the liquid/air interface at 20 °C and 37 °C. The results indicate the effect of the choline addition to the bacterial medium on the properties of the L. gormanii phospholipid membranes. The differences were revealed in the organization of monolayers, their molecular packing and ordering, degree of condensation and changes in the components’ miscibility. These findings are the basis for further research on the mechanisms of adaptation of this pathogen, which by changing the native composition and properties of lipids, bypasses the action of antimicrobial compounds and avoids the host immune attack.

Legionella dumoffii is an intracellular pathogen of freshwater protozoans capable of infecting and multiplying in mammalian cells, causing a severe respiratory disease called Legionnaires’ disease. The pathomechanism of infection development is very complex and depends on many factors, including the structure and properties of macromolecules that build the components of the L. dumoffii cell envelope. Phospholipids (PLs) forming biological membranes have a significant impact on the integrity of the membrane as well as on the interactions with the host cells. L. dumoffii changes its lipid profile under the influence of external factors, which allows it to adapt to the living environment. One of the factors altering the PL composition is the presence of exogenous choline. The aim of this study was to determine the physicochemical properties of the model bacterial membranes adsorbed at the air–liquid interface (Langmuir monolayers). They were composed of phospholipids isolated from L. dumoffii cultured with (PL+choline) and without (PL−choline) choline. Moreover, the effect of the human cathelicidin (LL-37 peptide) added to the subphase on these monolayers was analyzed in terms of phospholipid–peptide interactions. The results indicated that the monolayers of PL+choline were slightly more condensed than PL−choline. In the presence of LL-37, the elasticity of both monolayers increased; thus, their molecular packing and ordering decreased. The disturbing effect was related to the peptide’s antibacterial activity.