Antimicrobial anionic peptide binds in vivo to Mannheimia (Pasteurella) haemolytica attached to ovine alveolar epithelium.

Endogenous antimicrobial peptide activity in vivo has rarely been demonstrated. To assess this, Mannheimia haemolytica (log(10) 10.20 cfu) was deposited into the lungs of adult sheep, which were killed at 0, 5, 10 and 20 min for necropsy. At 0 min, M. haemolytica appeared normal and monoclonal antibody to antimicrobial anionic peptide (AP) and Protein A-colloidal gold identified AP already bound to the bacterial surface. At 5-20 min, many organisms were distorted with flocculated intracellular constituents characteristic of AP cellular damage indicating that AP can bind to and presumably help inactivate organisms in vivo.