Bovine NK-lysins, which are functionally and structurally similar to human granulysin and porcine
NK-lysin, are predominantly found in the granules of cytotoxic T-lymphocytes and NK-cells. Although antimicrobial activity of bovine
NK-lysin has been assessed for several bacterial pathogens, not all the important bacterial pathogens that are involved in the
bovine respiratory disease complex have been studied. Therefore the objective of the present study was to evaluate the antimicrobial activity of bovine
NK-lysin-derived
peptides on bovine respiratory pathogen Histophilus somni. Four, 30-mer
peptides corresponding to the functional region of
NK-lysin helices 2 and 3 were synthesized and assessed for antibacterial activity on four bovine pneumonic H. somni isolates. Although there were some differences in the efficiency of bactericidal activity among the
NK-lysin peptides at lower concentrations (2-5 μM), all four
peptides effectively killed most H. somni isolates at higher concentrations (10-30 μM) as determined by a bacterial killing assay. Confocal microscopic and flow cytometric analysis of Live/Dead Baclight stained H. somni (which were preincubated with
NK-lysin peptides) were consistent with the killing assay findings and suggest
NK-lysin peptides are bactericidal for H. somni. Among the four
peptides, NK2A-derived
peptide consistently showed the highest antimicrobial activity against all four H. somni isolates. Electron microscopic examination of H. somni following incubation with
NK-lysin revealed extensive cell membrane damage, protrusions of outer membranes, and cytoplasmic content leakage. Taken together, the findings from this study clearly demonstrate the antimicrobial activity of all four bovine
NK-lysin-derived
peptides against bovine H. somni isolates.