Antibiotics Beyond Bactericidal Activity

Summary

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Antimicrobial peptides (AMPs) are essential components of the innate immune system. They act not only by directly killing pathogens but also by modulating the host's immune responses. During infection, endotoxins like Lipopolysaccharide (LPS) released from bacteria can trigger an uncontrolled inflammatory response, leading to sepsis. This thesis focuses on the mechanisms by which natural or synthetic AMPs bind LPS and prevent it from triggering unnecessary immune responses—an essential aspect of their infection-resolving function.

Macrophage models were employed to investigate the ability of AMPs to inhibit cell activation induced by LPS or bacteria. LPS-neutralizing AMPs can play a crucial role in real-time infections by offering a bactericidal activity with minimal or no immune activation, effectively killing bacteria "silently". AMPs were shown not only to bind the toxic LPS but also to influence the intrinsic macrophage functions like phagocytosis. Bacteria can exhibit variable LPS compositions as part of their survival or antimicrobial resistance strategies. It was hypothesized that this structural variability could affect LPS binding and, consequently, the bactericidal activity of AMPs. The results indicated that while LPS binding may influence the anti-inflammatory functions of AMPs, it does not necessarily alter their bacterial killing activity. Synthetic or semi-synthetic peptides inspired by bacterial AMPs are now being developed to address antimicrobial resistance. One such synthetic peptidomimetic compound, derived from polymyxin and murepavadin, was tested for its LPS binding and neutralizing capacity. Notably, this derivative retained its ability to neutralize LPS. Furthermore, a previously unrecognized property of polymyxin was demonstrated: its ability to directly interact with Toll-like receptor 4 (TLR4) and inhibit LPS recognition.

In conclusion, these findings highlight the diverse anti-inflammatory mechanisms of AMPs and underscore their potential as multifunctional compounds for treating Gram-negative bacterial infections, particularly those associated with sepsis.