Thermodynamic Analysis of Interactions in Langmuir Monolayers Imitating Bacterial Membranes ^†

1. Introduction

Bacteria belonging to the Legionella gormanii species cause respiratory diseases. The key factor in the proper functioning and virulence of these microorganisms is the structure of biological membranes, the main components of which are phospholipids (PL). Their composition in the outer membrane layer of L. gormanii cells can change under various environmental factors, such as the presence of choline in the growth medium. Phospholipid distribution, the quantitative proportions of individual classes and intermolecular interactions define the physicochemical properties of bacterial membranes. The aim of the present research was the thermodynamic analysis of interactions occurring in model L. gormanii membranes with different phospholipid compositions.

2. Methods

Model membranes were created by means of the Langmuir monolayer technique using phospholipids isolated from bacteria grown with (PL + choline) and without (PL − choline) the addition of choline. To characterize the interactions between PL molecules in mixed monolayers, model single-component membranes of specific phospholipids classes’ representatives were analyzed. The dependencies of surface pressure on mean molecular area (π-A isotherms) were obtained. Based on experimental data, the excess area A_exc and excess Gibbs energy of mixing ΔG_exc were determined.

3. Results

The PL − choline membrane, due to its higher content of anionic phospholipids, is characterized by stronger repulsive interactions, while the PL + choline membrane, containing mostly zwitterionic compounds, shows stronger attractive interactions in comparison to single-component monolayers. The increase in repulsive interactions between PL − choline molecules results in greater flexibility of the membrane and limited miscibility of the components. On the contrary, the increase in attractive forces in PL + choline causes the formation of more homogeneous and tightly packed membranes.

4. Conclusions

The determination of interactions occurring in bacterial membranes and their changes induced by external factors can contribute to the development of new methods of treating infections caused by L. gormanii.