ABSTRACT
Recently, Verger et al. 30 examined the DNA homologies of the polynucleotide sequences in 51 strains of Brucella which included representatives of the six species and several strains of biotypes within each of the classical species (there are no reported biotypes within the three new species). In DNA-DNA reassociation experiments using labeled DNA strands from B. melitensis 16M to determine its homology with the other 50 strains, they reported relative binding ratios (percent homology) of from 84 to 100%. In their results on reciprocal DNA-DNA relatedness, they reported percentages ranging 87 to 104%. Even though their 23% range in percentages of binding ratios and 17% range in reciprocal ratios considerably exceeded the reported standard error of 3% in DNA relatedness results,3 1.32 they nonetheless denied the validity of Hoyer and McCullough's finding concerning the 6% difference between B. ovis and the other species. However, by using a different molecular genetic technique, De Ley et al. 33 established with certainty the genetic similarity of the six species. These investigators previously had found that genome sizes (i.e., molecular complexes) are similar among different strains within a single, well-defined species (standard deviation of a group of averages is less than 14.5%). When the same techniques were applied to the six species of Brucella, 34 they found genome molecular complexities of 2.37 x 109 , with standard deviation of 8%, indicating an intimate genetic relationship. Their data on DNA ribosomal RNA hybridization also shows, via a similarity map, there to be but little measurable differences among the species. Thus, by all available molecular genetic techniques for ascertaining relatedness at the genome level, it is clear that the relationship among all brucellae is exquisitely close.
