Thesis presented October 13, 2023

Abstract:
The emergence of antibiotic-resistant bacterial strains is considered one of the major health threats of our modern era. Aiming to find alternatives to the commercialized antibiotic molecules, different therapeutic possibilities are being studied such as the antimicrobial peptides (AMPs). One type of AMPs is the Ribosomally synthesized and Post-translationally modified Peptides (RiPPs). In our project, we precisely focused on thioether bond-containing RiPPs. The work tackled two types of peptides produced by a dominant member of the human gut microbiota Ruminococcus gnavus E1: RumCs sactipeptides and RgSCIFF ranthipeptide. The five RumCs isoforms (RumC1-5) were heterologously produced and their unique compact bihairpin structure was characterized. A potent antibacterial activity was observed for all RumCs against different Gram–positive and negative bacteria, except in the case of RumC2. The peptides resulted in no cellular toxicity when tested with different eukaryotic cell models. In addition, this work proved that the active RumCs peptides possess a similar mode of action that is not pore forming. They are exclusively active against dividing cells, and they inhibit the ATP synthesis. Further work is in progress to decipher the mode of action and to identify the cellular target. On the other hand, this Ph.D. project dealt also with the biochemical characterization of RgSCIFF ranthipeptide and its corresponding radical-SAM modifying enzyme. Interestingly, the peptide presents an unusual structure with characteristic thioether bonds of both ranthipeptides and sactipeptides, thus being the first example of a hybrid peptide.

Keywords:
Antibiotic resistance, Antimicrobial peptides, RiPPs, Sactipeptides, Ranthipeptides, Radical-SAM Enzymes, Ruminococcus gnavus