Antimicrobial activity and salt sensitivity of recombinant HBD-1 and HBD-2 peptides. (A) E. coli luminescence assay. HBD-1 and HBD-2 both kill E. coli in a dose-dependent fashion. The activity of HBD-2 was ≈10-fold greater than that of HBD-1. Each point represents mean ± SE for triplicate samples. (B) Salt sensitivity of HBD-1 and HBD-2. Increasing concentrations of NaCl inhibit the activity of both peptides against E. coli. The degree of inhibition by salt is also dependent on the concentration of peptide. For HBD-1, ○ = 12 μg/ml, □ = 6 μg/ml, and ⋄ = 1.2 μg/ml. For HBD-2, ○ = 0.5 μg/ml, ⋄ = 0.1 μg/ml, □ = 0.05 μg/ml, and ▵, 0.01 μg/ml. Results were replicated twice. (C) Colony-forming unit (CFU) assay for killing of P. aeruginosa by HBD-1 and HBD-2. Each point represents mean ± SE for triplicate samples.