Bovine lactoferrin binding to six species of coagulase-negative staphylococci isolated from bovine intramammary infections

Abstract

Bovine lactoferrin (BLf), an acute-phase iron-binding secretory protein present in secretions of the bovine udder, was demonstrated to bind to the following staphylococcal species associated with bovine intramammary infections: S. epidermidis, S. warneri, S. hominis, S. xylosus, S. hyicus, and S. chromogenes. The degree of 125I-labeled BLf uptake significantly varied among the blood agar-grown cells of all six species of coagulase-negative staphylococci tested. Isolates identified as S. xylosus demonstrated the highest (mean, 35.1 x 106 ± 13.3 ± 106 nmol) and S. hyicus the lowest (mean, 10.7 x 106 ± 5.9 x 106 nmol) binding to 125I-BLf. BLf binding was optimum at an acidic pH, with time-dependent binding saturation ranging from 70 min for S. warneri to 240 min for S. hominis. The BLf-binding mechanism was specific, with affinity constants (K(a) values) ranging between 0.96 x 106 and 11.90 x 106 liters/mol. The numbers of BLf-binding sites per cell, as determined by using Scatchard analysis, were as follows: S. epidermidis, 3,600; S. warneri, 1,900; S. hominis, 4,100; S. xylosus, 4,400; S. hyicus, 6,100; and S. chromogenes, 4,700. 125I-BLf binding to all species was inhibited by unlabeled BLf and unlabeled human lactoferrin, whereas none of the various plasma, connective tissue, or mucosal secretory proteins or carbohydrates tested caused significant interference. BLf-binding receptors of the six coagulase-negative staphylococcal species demonstrated marked differences in patterns of susceptibility to proteolytic or glycolytic enzyme digestion and to heat or periodate treatment. These data suggest that the BLf-binding components in S. epidermidis and S. warneri are proteins containing glycosidyl residues. In the remaining four species, the proteinaceous nature of the BLf-binding component was evident, but the involvement of glycosidyl residues was not clear. Results of this study establish the presence of specific binding components for BLf on coagulase-negative staphylococci isolated from bovine intramammary infections.