Comparative Genomics Reveals Divergent Adaptive Strategies in Classical Pathogenic and Environmental/Opportunistic Brucella Lineages

Background: Classical pathogenic Brucella species are host-adapted facultative intracellular bacteria, whereas several species formerly classified within Ochrobactrum are predominantly environmental organisms and opportunistic pathogens. The genomic basis of the contrasting ecological and pathogenic phenotypes of these lineages remains incompletely understood. This study compared classical pathogenic and environmental/opportunistic Brucella lineages to identify genomic features associated with their contrasting ecological strategies. Results: We analysed 1,557 publicly available RefSeq assemblies representing four classical pathogenic species (B. abortus, B. melitensis, B. suis and B. canis) and four environmental/opportunistic species reclassified from Ochrobactrum (B. anthropi, B. intermedia, B. pseudogrignonensis and B. haematophila); 1,159 genomes passed quality control. Genome architecture, pangenome structure, VFDB-associated homologues, predicted antimicrobial resistance genes (ARGs), functional annotations and mobile genetic elements were compared. Classical pathogenic Brucella showed compact and comparatively conserved genomes, whereas the environmental/opportunistic group had larger genomes and a substantially broader accessory-gene repertoire. The groups also differed in the distribution of virulence-associated homologues, predicted ARGs, metabolic annotations and mobile genetic elements. Conclusions: The observed genomic contrasts are consistent with host specialization in classical pathogenic Brucella and adaptation to heterogeneous external niches in environmental/opportunistic lineages. Database-derived virulence, resistance and pathway annotations remain sequence-based predictions and should not be interpreted as direct evidence of phenotype. These findings provide a genomic framework for experimentally testable hypotheses on ecological adaptation, host association and the evolution of pathogenicity within the Brucella/Ochrobactrum complex.