Gram-negative outer and inner membrane models: insertion of cyclic cationic lipopeptides

Area of Science:

• Microbiology
• Biochemistry
• Drug Discovery

Background:

• Polymyxin B (PxB) is a critical antibiotic for treating Gram-negative bacterial infections.
• Understanding PxB's mechanism of action is crucial for developing new antibiotics with improved therapeutic indexes.
• PxB interacts with both the outer membrane (OM) and inner membrane (IM) of bacteria.

Purpose of the Study:

• To elucidate the key structural features of Polymyxin B essential for its antibacterial activity.
• To investigate the role of specific domains, such as the hydrophobic region and positive charges, in PxB's interaction with bacterial membranes.
• To understand how PxB induces membrane disruption leading to bacterial cell death.

Main Methods:

• Mimicking bacterial outer and inner membranes using lipopolysaccharide (LPS) and anionic lipid (POPG) monolayers.
• Utilizing penetration assays, pressure/area curve analysis, and Brewster angle microscopy to study peptide-membrane interactions.
• Assessing peptide-induced membrane-membrane lipid exchange using phospholipid unilamellar vesicles.

Main Results:

• Synthetic analogues confirmed the importance of the hydrophobic domain for LPS binding and the role of positive charges in membrane insertion.
• The N-terminal acyl chain is crucial for high-affinity LPS interaction, while its absence prevents inner membrane insertion.
• Cationic amphipathicity was identified as the primary determinant of PxB's antimicrobial activity, driving membrane disruption and lipid exchange.

Conclusions:

• Specific structural elements of Polymyxin B dictate its interaction with bacterial membranes.
• Cationic amphipathicity is the key driver of PxB's membrane-disrupting mechanism of action.
• These findings provide a basis for designing novel PxB-based antibiotics with enhanced efficacy.