Classification of Isolates by DNA-DNA Hybridization

In routine diagnosis, bacterial field isolates are identified by serological and/or biochemical tests. Occasionally, however, isolates deviate from the generally accepted patterns, i.e., they may react with diagnostic antisera of more than one type strain or fail to react with any of them, or their abilities to ferment certain substrates may be altered. In such a situation, or in the case where no diagnostic antiserum is available for the respective species, the diagnostician can turn to alternative methods of classification based on genotypic rather than phenotypic characteristics, such as DNA-DNA hybridization. Genotypic classification may become even more important in the light of recent findings concerning antigenic variation of mycoplasmas, i.e., the ability of several species to vary size, structure, and composition of major surface antigens with high frequency. Moreover, the investigation of genetic relatedness between bacterial strains in the absence of genomic sequence data, as well as the introduction and definition of new species require information on DNA homology.