Effect of lactoferrin in combination with penicillin on the morphology and the physiology of Staphylococcus aureus isolated from bovine mastitis

The objective of the present study was to evaluate the therapeutic potential of bovine lactoferrin or lactoferricin in combination with penicillin G against Staphylococcus aureus. Minimal inhibitory concentrations of lactoferrin, lactoferricin, penicillin, and combinations of lactoferrin or lactoferricin with penicillin were determined for 15 S. aureus strains including several strains resistant to beta-lactam antibiotics. The fractional inhibitory concentration index indicated a synergistic effect between lactoferrin and penicillin. Combination of lactoferrin with penicillin increased the inhibitory activity of penicillin by two- to fourfold and reduced the growth rate in S. aureus strains tested, whereas the increase in the inhibitory activity of lactoferrin by penicillin was 16- to 64-fold. The addition of iron to the medium containing a combination of penicillin and lactoferrin had no effect on growth inhibition. Electron microscopy revealed that concentration below the minimal inhibitory concentrations of penicillin induced important ultrastructure alterations, which were further enhanced by the presence of lactoferrin. When S. aureus cells were grown in the presence of a combination of penicillin and lactoferrin, changes in the protein profile of the bacteria, including the disappearance of several protein bands due to the presence of lactoferrin, were observed. These data suggest that bovine lactoferrin or lactoferricin in combination with beta-lactam antibiotics can increase the antibacterial activity of these antibiotics against S. aureus resistant to antibiotics.