Forms of lactoferrin: their antibacterial effect on enterotoxigenic Escherichia coli

The antibacterial effects of various forms of lactoferrin on enterotoxigenic strains of Escherichia coli were tested in vitro using a microassay for bacterial growth. Native and apo-lactoferrin exhibited variable activity against 19 strains, whereas holo-lactoferrin had no effect. At a concentration of .2 mg/ml of apo-lactoferrin, strains could be distinguished as either sensitive or resistant to inhibition. Zinc-saturated lactoferrin was as inhibitory as apo-lactoferrin when sensitive and resistant strains were tested over the concentration range .04 to 1.0 mg/ml of lactoferrin. A bactericidal effect was observed for native, apo-, and Zn-saturated lactoferrin against some sensitive strains. The antibacterial activity of apo-lactoferrin depended on bacterial inoculum size and was not enhanced by the addition of lysozyme. Addition of holo-lactoferrin or cytochrome c diminished the antibacterial effect of apo-lactoferrin, whereas addition of BSA had no effect. Resistance to inhibition by lactoferrin was not related to the production of bacterial siderophores.