Interactions of Bacterial Quorum Sensing Signals with Model Lipid Membranes: Influence of Acyl Tail Structure on Multiscale Response

Abstract

Many common bacteria use amphiphilic N-acyl-Lhomoserine lactones (AHLs) as signaling molecules to coordinate group behaviors at high cell densities. Past studies demonstrate that AHLs can adsorb to and promote the remodeling of lipid membranes in ways that could underpin cell−cell or host−cell interactions. Here, we report that changes in AHL acyl tail group length and oxidation state (e.g., the presence or absence of a 3-oxo group) can lead to differences in the interactions of eight naturally occurring AHLs in solution and in contact with model lipid membranes. Our results reveal that the presence of a 3-oxo group impacts remodeling when AHLs are placed in contact with supported lipid bilayers (SLBs) of the phospholipid 1,2dioleoyl-sn-glycero-3-phosphocholine (DOPC). Whereas AHLs that have 3-oxo groups generally promote the formation of microtubules, AHLs that lack 3-oxo groups generally form hemispherical caps on the surfaces of SLBs. These results are interpreted in terms of the time scales on which AHLs translocate across bilayers to relieve asymmetrical bilayer stress. Quartz crystal microbalance with dissipation measurements also reveal that 3-oxo AHLs associate with DOPC bilayers to a greater extent than their non-3-oxo analogues. In contrast, we observed no monotonic relationship between AHL tail length and bilayer reformation. Finally, we observed that 3-oxo AHLs facilitate greater transport or leakage of molecular cargo across the membranes of DOPC vesicles relative to AHLs without 3-oxo groups, also suggesting increased bilayer disruption and destabilization. These fundamental studies hint at interactions and associated multiscale phenomena that may inform current interpretations of the behaviors of AHLs in biological contexts. These results could also provide guidance useful for the design of new classes of synthetic materials (e.g., sensor elements or drug delivery vehicles) that interact with or respond selectively to communities of bacteria that use 3-oxo AHLs for cell−cell communication.

Interactions of Bacterial Quorum Sensing Signals with Model Lipid Membranes: Influence of Membrane Composition on Membrane Remodeling

“…Analogous fluorescence microscopy experiments to characterize the interactions of 3-oxo-C12-AHL (Figure B and Figure S6C) with these DOPE-containing bilayers revealed the immediate and apparently direct formation of hemispherical caps on the surfaces of the bilayers; that is, we did not observe these caps to form by the initial formation and subsequent retraction of extended tubule structures. The introduction of 3-oxo-C12-HS at a concentration of 50 μM (Figure F and Figure S6C) appeared to lead to the formation of hemispherical caps that appeared to protrude more from the surface of the bilayer than those reported in our previous studies. ,, Additional studies will be required to investigate possible differences in these structures more completely.…”

“…We recently reported on the influence of AHL head- and tailgroup structure on membrane remodeling in single-component SLBs of DOPC formed using either a vesicle fusion or bicelle formation technique. Other groups have reported that it can be difficult to form some compositionally complex, mixed-lipid SLBs using the vesicle fusion technique , and that even single-component bilayers are often nonhomogeneous and can contain unruptured vesicles that appear as optical defects .…”

“…The work reported here was motivated broadly by recent studies by our group demonstrating that N -acyl l -homoserine lactones (AHLs), a class of nonionic, amphiphilic small-molecule signals used by common Gram-negative bacteria to coordinate collective behavior with cell density (i.e., quorum sensing), − can interact with and promote large-scale reformations in model single-component lipid membranes composed of the phospholipid 1,2-dioleoyl- sn -glycero-3-phospho­choline (DOPC). , In this study, we sought to characterize interactions of AHLs with more complex, multicomponent lipid membranes as a first step toward understanding how differences in membrane composition and properties (e.g., fluidity, etc.) can impact subsequent multiscale remodeling in response to these bacterial signals.…”

“…AHLs possess a relatively polar homoserine lactone headgroup and an aliphatic tail of varying length (e.g., 3-oxo-C12-AHL, Figure ) , and have previously been demonstrated to undergo molecular self-assembly in solution , and interact with other self-assembled structures, including lipid bilayers, ,,− in ways that are similar to those of other classes of nonionic surfactants. − , The homoserine lactone headgroup of AHLs is hydrolyzed in aqueous media (over hours to days, depending on the tail structure), leading to anionic, ring-opened homoserine (HS) headgroup products , (e.g., 3-oxo-C12-HS, Figure ) that have also been demonstrated to interact with lipid membranes . Our group recently reported that 3-oxo-C12-AHL, 3-oxo-C12-HS, and other structurally related long-chained AHLs can interact with supported lipid bilayers (SLBs) composed of DOPC and promote the formation of extended micrometer-scale tubules and hemispherical “caps” on the surfaces of the bilayers. , That past work also demonstrated that changes in the head- and tailgroup structures of AHLs can impact the nature of these remodeling events substantially (e.g., leading to changes in the dynamics or relative proportions of these structures). , The results of those studies provide useful insight into fundamental factors, such as the time scales on which amphiphilic molecules translocate across lipid bilayers, that influence multiscale remodeling and provide guidance for the design of synthetic materials that can respond to chemical products produced by bacterial communities. The single-component membrane models used in those studies do not, however, reflect the compositional complexities that are typical of many biological membranes (or many synthetic soft materials systems) − and that likely underlie many unexplored or currently unappreciated roles that AHLs (and their byproducts) could play in mediating the interactions of bacteria with those systems.…”

Rigid Polymer Network-Based Autonomous Photoswitches Working in the Solid State Encoded by Room-Temperature Phosphorescence

“…Organic functional luminescent materials have received much attention in recent years since they have exhibited a great research value in the fields of bioimaging, sensors, display, and information technology. − In particular, it is worth noting that the emissive intensity or wavelength of the materials can be typically regulated through external stimulation such as pH, temperature, electricity, light, mechanical force, solvent polarity, and so forth, reflecting the switching behaviors of the functional molecules. − Among these cases, the light-controlled molecular switch continues to be one of the most significant research motives because light does not easily produce additional byproducts in the system and can also be operated rapidly, accurately, and remotely. − Generally, the principle of a light-controlled molecular switch is based on photochemical reactions, such as photocyclization and photoisomerization, and there are still few designs for molecular switches based on the conformational conversion. Thus, the reversibility of these switch systems needs an additional chemical or energy imposition. − In contrast, several nonequilibrium systems based on the photoexcitation principle have been developed as autonomous photoswitches (resettable without a secondary stimuli), which can achieve self-recovery through relaxation when the light stimulation is cut off. − However, the autonomous photoswitch can easily work in solution or in some well-defined crystalline systems. − To create an autonomously photoswitchable material that can work in the solid state, even in the amorphous form, remains a challenge.…”