Novel Immunofluorescent Technique for Identification of Ovine Immunoglobulins and Other Potential Opsonins Binding to Live Staphylococcus aureus

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Abstract
A qualitative in vitro technique was developed for identification of potential opsonins (IgG1, IgG2, IgA, IgM; the 3rd component of ovine complement, C3, and fibronectin) of sheep origin, binding to cell walls of viable S. aureus. The technique uses monospecific antisera to these ovine proteins. The antisera are conjugated to fluorescein isothiocyanate (FITC)-protein A such that the protein A-binding sites in the Fc region of the Ig molecules are occupied with FITC-protein A complexes and are prevented, therefore, from binding to protein A in the staphylococcal cell wall. The technique is highly specific and sensitive, and, once conjugated monospecific antisera are prepared, many tests can be done in a short time. S. aureus incubated in samples of milk whey showed variable binding of IgG1, IgG2 and IgM; IgA was bound from only 1 sample of milk whey, and 3rd C3 and fibronectin binding were not detected. Most samples of blood serum and colostral whey produced binding of IgG1, IgG2, IgA and IgM and C3 to the S. aureus cell surface; fibronectin binding was found in serum, but not in colostral whey. Washings from involuted mammary glands invariably produced binding of all Ig, but not C3 or fibronectin.