Abstract
Monolayers composed of bacterial phospholipids were used as model membranes to study interactions of the naturally occurring phenolic compounds 2,5-dihydroxybenzaldehyde and 2-hydroxy-5-methoxybenzaldehyde, and the plant essential oil compounds carvacrol, cinnamaldehyde, and geraniol, previously found to be active against both Gram-positive and Gram-negative pathogenic microorganisms. The lipid monolayers consist of 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (DPPE), 1,2-dihexa- decanoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG), and 1,1',2,2'-tetratetradecanoyl cardiolipin (cardiolipin). Surface pressure–area (π-A) and surface potential–area (Δψ-A) isotherms were measured to monitor changes in the thermodynamic and physical properties of the lipid monolayers. Results of the study indicated that the five compounds modified the three lipid monolayer structures by integrating into the monolayer, forming aggregates of antimicrobial –lipid complexes, reducing the packing effectiveness of the lipids, increasing the membrane fluidity, and altering the total dipole moment in the monolayer membrane model. The interactions of the five antimicrobial compounds with bacterial phospholipids depended on both the structure of the antimicrobials and the composition of the monolayers. The observed experimental results provide insight into the mechanism of the molecular interactions between naturally-occurring antimicrobial compounds and phospholipids of the bacterial cell membrane that govern activities.
Highlights
Natural compounds of plant origin have been known for their effective antimicrobial activity since ancient time [1] and are still widely used for treatment and prevention of infections by millions of people in many parts of the world
Published studies focus on investigating the cellular responses of bacteria that are subjected to non-lethal treatments with potential antimicrobial agents, such as changes in membrane integrity indicated by the leakage of intracellular constituents into the extracellular environment, changes in intracellular pH, and changes in the ability to perform ATP synthesis [2,3,4]
We evaluated the interaction between the naturally occurring compounds and bacterial membrane lipids on model monolayers composed of bacterial phospholipids, DPPE, DPPG, and cardiolipin, using surface pressure and surface potential isotherms
Summary
Natural compounds of plant origin have been known for their effective antimicrobial activity since ancient time [1] and are still widely used for treatment and prevention of infections by millions of people in many parts of the world. Published studies focus on investigating the cellular responses of bacteria that are subjected to non-lethal treatments with potential antimicrobial agents, such as changes in membrane integrity indicated by the leakage of intracellular constituents into the extracellular environment, changes in intracellular pH, and changes in the ability to perform ATP synthesis [2,3,4] These experiments, involve the use of complex living organisms and because the antimicrobials may induce multiple effects on bacterial cells, the exact molecular mechanisms are not readily defined [9]. We evaluated the interaction between the naturally occurring compounds and bacterial membrane lipids on model monolayers composed of bacterial phospholipids, DPPE (zwitterionic lipid), DPPG, and cardiolipin (both anionic lipids), using surface pressure and surface potential isotherms.
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