Taxonomy of Brucella

Brucellosis is named after Dr. David Bruce who first isolated the bacterium that caused Malta fever from four fatal cases amongst the British forces on the island. The genus Brucella was subsequently proposed after similar bacteria were isolated from cattle and swine and the zoonotic connection recognized. The close similarities between these isolates were acknowledged but nomen species were subsequently designated on the basis of their specific host preference, phage susceptibility and oxidative metabolism pattern with specific carbohydrate and amino acid substrates. The later isolation of B. suis strains of divergent host preference and of strains of low human pathogenicity, such as B. neotomae and B. ovis, has inspired a debate regarding Brucella taxonomy. On the one hand, the DNA homologies are strikingly similar, justifying inclusion of all members of the genus in a single species with sub-divisions. On the other hand, whole genome analyses such as MLVA, MLST, microarray studies, and SNP have confirmed subtle differences between the species. As a result, a return to a multi-nomen species taxonomy has recently been proposed and accepted by the Sub-committee on Taxonomy of Brucella. Phylogenetic studies have shown four clades in the genus that have possibly evolved from a Brucella - Ochrobactrum-like common soil ancestor. These are: B. melitensis-B. abortus; B. suis-B. canis; B. neotomae; and B. ceti-B. pinnipedialis (B. microti not yet established), with B. ovis standing as a basal lineage of the tree. B. inopinata is a recently identified isolate that slightly diverges from classical Brucella according to the 16S-rRNA sequence and other molecular studies. This review elaborates on the classification of the genus Brucella according to insights that have emerged since it was first described.