Bactericidal effect of lactoferrin on Legionella pneumophila

Lactoferrin, an iron-binding protein found in mucosal secretions and in specific granules of polymorphonuclear leukocytes, has been shown to be bactericidal for a variety of organisms. In this study, the effect of lactoferrin on Legionella pneumophila was investigated. Purified human apolactoferrin was bactericidal for the Knoxville 1 strain (serogroup 1), with a 4-log decrease in viability within 2 h at 37 degrees C. Killing was dependent on the iron-free state since iron-saturated lactoferrin had no activity. Guinea pig passage of this strain did not affect its sensitivity to lactoferrin. Treatment of the cells with dilutions of the lactoferrin resulted in correspondingly reduced killing. Activity was temperature dependent; there was no loss of viability at 1 or 22 degrees C and slightly enhanced killing at 41 degrees C. Addition of Mg2+ blocked bactericidal activity. In addition, mature human milk, a lactoferrin-containing mucosal secretion, was also bactericidal for L. pneumophila. As demonstrated with the purified lactoferrin, bactericidal activity was lost when the milk was iron saturated.