Inhibition of mastitic bacteria by bovine milk apo-lactoferrin evaluated by in vitro microassay of bacterial growth.

An in vitro microassay was developed to evaluate antimicrobial properties of bovine apo-lactoferrin. The growth of coliform, staphylococcal, and streptococcal bacterial strains in a defined synthetic medium was inhibited by bovine apo-lactoferrin (.5 to 30.0 mg/ml). Addition of iron-saturated lactoferrin to the synthetic medium did not inhibit growth of test strains. Inhibition by apo-lactoferrin was greater for coliform than Gram-positive strains for all concentrations of apo-lactoferrin evaluated. No concentration of apo-lactoferrin proved bactericidal for either coliform or Gram-positive strains. Inhibition of two coliform strains by apo-lactoferrin (10 mg/ml) was abolished by addition of ferric iron to the assay system, indicating an iron-dependent nature of apo-lactoferrin induced inhibition of bacteria. Bicarbonate supplementation of the growth system containing apo-lactoferrin (1 mg/ml) increased inhibition of three coliform strains by apo-lactoferrin. Addition of increasing concentrations of citrate (2.0 mg/ml) to an assay system containing apo-lactoferrin (5 mg/ml) resulted in a concomitant reduction of growth inhibition of three coliform strains. These data indicate a potential relationship between the molar ratio of citrate to lactoferrin of the lacteal secretion and its capacity to inhibit coliform strains associated with mastitis.