Synthetic Esc(1–21) and its diastereomer Esc(1–21)-1c (Table 1) were purchased from Chematek Spa (Milan, Italy). Briefly, each peptide was assembled by step-wise solid-phase synthesis using a standard F-moc strategy and purified via RP-HPLC to a purity of 98%, while the molecular mass was verified by mass spectrometry. Colistin sulphate was purchased from Sigma (St. Louis, MO) and the natural AMP human cathelicidin LL-37 (Table 1) was synthesized by Genscript (Piscataway, NJ). BronchialLife Epithelial Airway Medium was purchased from Lifeline Technology (Frederick, MD). B-ALI Bronchial Air Liquid Interface BulletKit was purchased from Lonza (Walkersville, MD). Eagle’s minimum essential medium (EMEM), was purchased from Sigma (St. Louis, MO), and tryptic soy broth (TSB) and trypitc soy agar (TSA) were purchased from MP Biomedicals (Santa Ana, CA).Table 1Primary structure of the peptides under study.PeptidePrimary structurea Esc(1–21)GIFSKLAGKKIKNLLISGLKG-NH2 Esc(1–21)-1cGIFSKLAGKKIKNLLISGLKG-NH2 LL-37LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES a d-amino acids are in italics and bolded.