Abstract
Our long-term goal is to identify new antimicrobial peptides that might be effective against pneumonic Francisella infection in mice. Previously, our group searched the peptidome of the American alligator for novel cationic antimicrobial peptides and identified a naturally-occurring C-terminal fragment of apolipoprotein C-1, which we called Apo6. This peptide was found to have antibacterial activity against the ESKAPE pathogens, including those exhibiting multi-drug resistance. In this work, we tested Apo6 and synthetic derivatives for antibacterial activity against Francisella tularensis including the virulent strain F. tularensis SchuS4. Francisella is inherently highly resistant to the cyclic peptide polymyxin antibiotics and beta-lactam antibiotics. We found that our synthetic peptide derivatives (called GATR peptides), designed with increased hydrophobicity and charge, had generally stronger in vitro antimicrobial activity against Francisella than the parent peptide Apo6. The GATR peptides had a greater effect on the bacterial membrane than the Apo6 peptide and were able to bind Francisella LPS, suggesting their mechanism of action against Francisella. Cytotoxicity experiments showed low cytotoxicity for most of the GATR peptides, and whole organism toxicity studies in the waxworm allowed us to down-select to two our lead peptides, GATR-3 and GATR-6. These peptides were tested in a murine pulmonary tularemia model. We found that the GATR-3 peptide rescued 50-60% of mice from lethal tularemia infection when administered systemically through the intraperitoneal route. This peptide is a candidate for further pre-clinical studies for a potential peptide-based approach to tularemia.
