Protein X of Streptococcus agalactiae induces opsonic antibodies in cows.

Protein X of Streptococcus agalactiae is a surface protein frequently associated with strains isolated from cases of mastitis of dairy cows. By immunizing cows with purified protein X, we obtained an antibody response which was restricted to X-bearing strains of S. agalactiae in a whole-cell enzyme-linked immunosorbent assay. This response resulted in an increase in the opsonic activity of serum for strains bearing protein X, as assessed through the augmentation of the chemiluminescence response of phagocytosing polymorphonuclear cells and through an increased ingestion of bacteria, although the proportion of ingested bacteria which were killed (about 60%) remained unchanged. Protein X behaved as a target of opsonins and, as such, could be a protective antigen worth incorporating in a vaccine against S. agalactiae mastitis.