Antibacterial activity of lactoferrin and a pepsin-derived lactoferrin peptide fragment

Abstract

Although the antimicrobial activity of lactoferrin has been well described, its mechanism of action has been poorly characterized. Recent work has indicated that in addition to binding iron, human lactoferrin damages the outer membrane of gram-negative bacteria. In this study, we determined whether bovine lactoferrin and a pepsin-derived bovine lactoferrin peptide (lactoferricin) fragment have similar activities. We found that both 20 μM bovine lactoferrin and 20 μM lactoferricin release intrinsically labeled [3H]lipopolysaccharide ([3H]LPS) from three bacterial strains, Escherichia coli CL99 1-2, Salmonella typhimurium SL696, and Salmonella montevideo SL5222. Under most conditions, more LPS is released by the peptide fragment than by whole bovine lactoferrin. In the presence of either lactoferrin or lactoferricin there is increased killing of E. coli CL99 1-2 by lysozyme. Like human lactoferrin, bovine lactoferrin and lactoferricin have the ability to bind to free intrinsically labeled [3H]LPS molecules. In addition to these effects, whereas bovine lactoferrin was at most bacteriostatic, lactoferricin demonstrated consistent bactericidal activity against gram- negative bacteria. This bactericidal effect is modulated by the cations Ca2+, Mg2+, and Fe3+ but is independent of the osmolarity of the medium. Transmission electron microscopy of bacterial cells exposed to lactoferricin show the immediate development of electron-dense 'membrane blisters.' These experiments offer evidence that bovine lactoferrin and lactoferricin damage the outer membrane of gram-negative bacteria. Moreover, the peptide fragment lactoferricin has direct bactericidal activity. As lactoferrin is exposed to proteolytic factors in vivo which could cleave the lactoferricin fragment, the effects of this peptide are of both mechanistic and physiologic relevance.