Enhancement of the activity of novobiocin against Escherichia coli by lactoferrin

The activity of novobiocin against Escherichia coli ATCC 25922 and three E. coli strains that were isolated from cases of bovine mastitis was determin ed in timekill studies in the presence of bovine lactoferrin. Lactoferrin a lone did not affect the growth of any of the strains of E. coli. A combinat ion of 1.0 mg/ml of lactoferrin and novobiocin at 1/16x minimum inhibitory concentration (MIC) was bactericidal for E. coli ATCC 25922. When the conce ntration was increased to 3.0 mg/ml of lactoferrin, novobiocin was bacteric idal at 1/64x MIG. Among the mastitis strains tested, 6789 and 6806 were mo re susceptible to killing by novobiocin than was strain 6800. Strains 6789 and 6806 were killed when treated with novobiocin concentrations of 2, 1/2, and 1/4x MIG. When these strains were also treated with lactoferrin at 3.0 mg/ml, there was a bacteriostatic effect at novobiocin concentrations of 1 /8 and 1/16x MIC for strains 6789 and 6800. Strain 6806 appeared to be more susceptible to the combination of lactoferrin and novobiocin as was eviden ced by a bactericidal effect over the 24-h testing period. The combination treatment with cephapirin and lactoferrin showed that there was a synergist ic bactericidal effect against all of the E. coli strains tested. These stu dies indicate that lactoferrin can potentiate the activity of antibiotics a gainst Gram-negative bacteria.