Genetic comparison of Brucella spp. and Ochrobactrum spp. erroneously included into the genus Brucella confirms separate genera

The facultative intracellular pathogen Brucella and the free-living bacteria Ochrobactrum are both α-proteobacteria and very close to each other. A group of researchers recently clustered Ochrobactrum strains into the genus Brucella according to a BLAST distance approach. Thus, we performed a deeper comparative genetic analysis for eleven Ochrobactrum strains and twelve different Brucella isolates to demonstrate important differences between these bacteria. In addition to the clear differences between Brucella and Ochrobactrum, like the differences in genes contents, and different genome sizes, the Brucella-specific gene bscp31 was not found in Ochrobactrum, as well as other important Brucella-specific proteins and virulence factors. Differences in antimicrobial resistance genes content and the presence or absence of plasmids were obvious between Brucella and Ochrobactrum spp. Genome alignment of Brucella spp. and Ochrobactrum spp. revealed a genome similarity of 85.7% maximum, whereas all analyzed Brucella spp. in this study had a similarity of 97.6-99.9%, and all compared Ochrobactrum spp. 82.6-98.0%. Because of these facts mentioned in this work, Brucella and Ochrobactrum should be considered separate genera.