A rapid assay for protein-A in Staph. aureus strains, using immunomagnetic monosized polymer particles

Monosized magnetic polystyrene beads, Dynabeads M-450, coated with sheep IgG were used in an agglutination test for the detection of cell-surface protein-A, based on Fc-protein-A binding. Ninety-three per cent of the bovine Staphylococcus aureus strains isolated from clinical mastitis expressed surface protein-A. The agglutination test was rapid compared to FITC-labelled IgG binding or dog-serum agar precipitation test for protein-A, and was significantly correlated with these methods. Scanning electron micrographs demonstrated binding of protein-A positive bacteria to the particle surface, thus bridging the beads and forming the agglutination lattice. The microsphere agglutination assay described is a quick, simple and reproducible method for detecting surface protein-A in staphylococci.