An evaluation of agglutination and coagglutination techniques for serotyping of Haemophilus pleuropneumoniae isolates.

A comparative evaluation of rapid slide agglutination, tube agglutination, 2-mercaptoethanol tube agglutination, and coagglutination tests was made for serotyping isolates of Haemophilus pleuropneumoniae. The results indicated that a majority of the isolates could be serotyped by any of these tests. But, it was not uncommon to find isolates which were inagglutinable or poorly agglutinable in homologous sera. Heat treatment of whole-cell suspensions of such isolates was essential to unmask the serotype-specific antigenic determinants; however, in the process of heat treatment, cross-reactive common antigens of minor nature were also exposed. The antibodies involved in such cross-reactions were mainly of immunoglobulin M type, because the cross-reactivities were completely abolished in coagglutination and 2-mercaptoethanol agglutination tests. Thus, both these tests were satisfactory for serotyping inagglutinable mucoid strains. For serotyping strains which were either polyagglutinating or autoagglutinating, agglutination tests could not be used, but the coagglutination test proved to be satisfactory. The coagglutination test was serotype-specific, sensitive, simple, rapid, reproducible, and easier to read and interpret than rapid slide or tube agglutination tests. This test could be used to serotype mucoid, smooth, or rough isolates.