Neisseria gonorrhoeae has been subdivided into several classes of serological distinct groups. The serotyping system is based upon the antigenic specificity of a protein serotype antigen. This protein is the major polypeptide of the outer membrane of the gonococcus and accounts for over 60% of that membrane's total protein. The serotype antigen complex was isolated by mild extraction of intact organisms in 200 mM lithium acetate buffer, pH 6.0 with 10 mM EDTA for 2 h at 45 degrees C. The extract was fractionated on Sepharose 6B and partially purified by precipitation at pH 4.2 by addition of 10% (vol/vol) acetic acid. Each serotype antigen has a unique subunit molecular size as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Preliminary typing of a gonococcal strain may be performed by comparative SDA-PAGE. To date, 16 different serotypes, representing a diverse distribution, have been isolated.

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