from Bruno W. Sobral and Alice R. Wattam writing in Brucella: Molecular Microbiology and Genomics:
Brucella species are characterized by extremely high levels of nucleotide similarity and yet vary in microbial and disease phenotypes, as well as in pathogenicity and host preference. These variations initially resulted in classification of six species; B. abortus, B. canis, B. melitensis, B. neotomae, B. ovis and B. suis. The lack of sequence diversity has inhibited molecular studies, but the development of new techniques and the recent availability of genome sequences have revealed interesting differences, including the expansion of the known Brucella species.
Further reading: Brucella: Molecular Microbiology and Genomics