Phd in CELL AND DEVELOPMENTAL BIOLOGY

Thesis title: Antimicrobial activity of synthetic peptides against Staphylococcus aureus and Pseudomonas aeruginosa

Antibiotic resistance has emerged as a critical concern in a number of fields, including clinical, veterinary, agricultural, environmental, and economic sectors. This is because the discovery of new effective antibiotics is much slower than the rate at which conventional ones lose their effectiveness. In light of this issue, antimicrobial peptides represent a valuable alternative. Given their potential and the challenges they pose, synthetic peptides are of particular interest because they can be chemically modulated to mitigate drawbacks and enhance efficacy. Therefore, the aim of this thesis was to screen eight short facial amphiphilic synthetic peptides, four alone and four identical but conjugated to the bile salt lithocholic acid (LCh), which also displays facial amphiphilic properties. Their potential antimicrobial activity was evaluated against two clinically relevant bacteria: the gram-positive S. aureus and the gram-negative P. aeruginosa. Interestingly, compounds exhibiting antimicrobial activity were identified. The LCh(DV-LK)2 (MIC 7µM; MCB 10µM) and the LCh(DV-LK)4 (MIC 80µM; MBC of 100µM) proved to be the most effective against S. aureus and P. aeruginosa, respectively, and were therefore selected for further investigation into their mechanism of action. The LCh(DV-LK)2 against S. aureus exhibited a non–membrane-disruptive mechanism and no impact on H₂O₂ resistance. It significantly reduced biofilm formation and partially promoted biofilm eradication. The LCh(DV-LK)4 against P. aeruginosa showed both a membrane disruptive mechanism and an effect on the H₂O₂ resistance mechanism. Notably, it was able to affect both biofilm formation and eradication. Regarding their toxicity, neither molecule exhibited toxic effects on S. cerevisiae at antibacterial concentrations, and both were able to extend the lifespan of the pathogen-infected nematode C. elegans. Given the difficulty of tackling nosocomial infections caused by these bacteria, the robustness, consistency and alignment of these results with similar studies reported in the literature is noteworthy. Further investigations are required to assess the potential toxicity and stability of the synthetic peptides analyzed in this study.