A single-strand DNA fragment used to detect the complementary fragment. DNA probes are used widely in bacteriology. Recombinant DNA techniques are used to isolate, reproduce, and label a portion of the genetic material, DNA, from the nucleus of a microorganism that is specific for it. This fragment can be added to a specimen containing the organisms. The specimen and known DNA are treated so that the DNA strands from the organisms in the specimen are separated into single strands. The DNA from the specimen rejoins (is annealed to) the known labeled DNA and is thereby labeled. This permits the identification of a single pathogenic organism in a specimen that contains many different microorganisms.