In vitro effect of the acidic pH on the susceptibility of the antimicrobial peptide Esc(1-21) against respiratory pathogens in cystic fibrosis lung environment

In healthy individuals, to maintain lung function and sterility, a thin sheet of fluid that covers the airway epithelium (airways surface liquid, ASL) is present. ASL contains several innate defenses, including a complex mixture of antimicrobials that kill bacteria, and contributes to the mucociliary clearance system that carries pathogens out of the lungs. Recently, in vivo studies on airway epithelia have shown that defective Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), as found in cystic fibrosis (CF) patients, reduces ASL pH, compromising its antimicrobial activity. This causes a pathologic process characterized by chronic infection, inflammation, mucus obstruction, and bronchiectasis [1]. Antimicrobial peptides (AMPs) are components of the innate immunity in all life forms and represent promising molecules for the development of new antibacterial drugs. Recently, a frog skin derived AMP, i.e. Esc(1-21), has been characterized for its bactericidal activity against the planktonic and biofilm forms of the Gram-negative bacterium Pseudomonas aeruginosa, which is responsible for more than 80% of drug-resistant infections in CF patients [2]. We carried out studies to evaluate the in vitro antimicrobial efficacy of Esc(1-21), compared to a conventional antibiotic, i.e. tobramycin, when used in CF-like acidic lung environment against the planktonic and biofilm forms of a panel of multidrug resistant Gram-negative clinical isolates. In parallel, the effect of Esc(1-21) on the viability of eukaryotic cells, under acidic conditions, was also investigated. Our results have shown that Esc(1-21) is able to preserve its antimicrobial activity when used in acidic pH conditions. In addition, its ability to kill bacterial biofilm is maintained at low pH and appears to be even more effective than when tested under non-CF conditions. On the contrary, tobramycin showed a significant weaker activity when tested in acidic environment at a wide range of concentrations against most of the selected strains. Finally, Esc(1-21) did not markedly reduce the percentage of viable eukaryotic cells in acidic conditions. Altogether, these results highlight Esc(1-21) as an attractive candidate for the production of new antimicrobial compounds against Gram-negative respiratory infections, especially in CF patients. [1] Pezzulo AA et al. (2012) Nature 487, 109 [2] Luca V et al. (2013) Cell Mol Life Sci 70, 27732 This research was supported by EU funding within the NextGeneration EU-MUR PNRR Extended Partnership initiative on Emerging Infectious Diseases (Project no. PE00000007, INF-ACT) to M.L.M and Fondazione Italiana per la Ricerca Cistica (Project FFC#4/2022) Delegazione FFC Ricerca di Roma e della Franciacorta e Val Camonica