Detection of Group A Streptococci in Throat Cultures by Immunofluorescence: Development and Application of a Specific Reagent

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Abstract
A specific fluorescent antibody reagent for group A streptococci and labor-saving techniques for processing large numbers of throat cultures for the presence of these organisms were developed by the laboratory division of the Connecticut State Department of Health. Specificity of fluorescein-labeled group A conjugate is achieved by appropriate sorption and addition of unlabeled serum to inhibit nonspecific staining of groups C and G streptococci and of staphylococci Parallel culture of throat swabs on blood agar and in broth permits use of the blood agar plate as a screening device to reduce the burden of microscopic examinations of broth cultures. Use of the fluorescent antibody reagent in a one-step inhibition technique resulted in bright and distinct fluorescent antibody staining of group A sterptococci without interference from other streptococcal groups or staphylococci, when present. Sensitivity and specificity of the reagent have been routinely evidenced.