Physicochemical Characteristics of Model Membranes Composed of Legionella gormanii Lipids

Abstract

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.

Keywords: Langmuir monolayer technique; Legionella gormanii; phospholipids.

Figure 1

Relative content (%) of fatty acids in the PL classes isolated from the bacteria cultured on the medium without and with choline. a, methyl branch at the anteiso carbon atom; i, methyl branch at the iso carbon atom; n, unbranched acid; c, cyclopropane ring structure; ± standard deviation; * indicates a significant difference in the MW test for choline vs. the lack of choline comparison in each PL. The test was carried out for fatty acids with at least 5% relative content.

Figure 2

Surface pressure-area per molecule ($\mathsf{\pi }-\mathrm{A}$) isotherms registered at 20 °C and 37 °C (a) and determined the compression modulus-surface pressure (${\mathrm{C}}_{\mathrm{S}}^{-1}-\mathsf{\pi }$) dependencies based on them (b) for the monolayers of phospholipids (PL) extracted from L. gormanii bacteria supplemented (PL+choline) or not (PL−choline) with choline.

Figure 3

Surface potential changes $\Delta \mathrm{V}$ vs. the mean molecular area ($\mathrm{A}$) for the PL−choline and PL+choline monolayers at 20 °C and 37 °C.

Figure 4

Surface pressure ($\mathsf{\pi }$), compressibility modulus (${\mathrm{C}}_{\mathrm{S}}^{-1}$) and surface potential changes ($\Delta \mathrm{V}$), vs. the mean molecular area (A) for the obtained monolayers: PL−choline at 20 °C (a), PL−choline at 37 °C (b), PL+choline at 20 °C (c) and PL+choline at 37 °C (d).

Figure 5

BAM images taken for the PL−choline and PL+choline monolayers at 20 °C and 37 °C at given values of surface pressure and mean molecular area.

Figure 6

The effect of molecule shape on packing of the mixed monolayers, composed of phospholipids isolated from L. gormanii bacteria cultured without (a) and with (b) choline.