Genomic Characterization Provides New Insights for Detailed Phage- Resistant Mechanism for Brucella abortus

As the causative agent of cattle brucellosis, Brucella abortus commonly exhibits smooth phenotype (by virtue of colony morphology) that is characteristically sensitive to specific Brucella phages, playing until recently a major role in taxonomical classification of the Brucella species by the phage typing approach. We previously reported the discrepancy between traditional phenotypic typing and MLVA results of a smooth phage-resistant (SPR) strain Bab8416 isolated from a 45-year-old custodial worker with brucellosis in a cattle farm. Here, we performed whole genome sequencing and further obtained a complete genome sequence of strain Bab8416 by a combination of multiple NGS technologies and routine PCR sequencing. The detailed genetic differences between B. abortus SPR Bab8416 and large smooth phage-sensitive (SPS) strains were investigated in a comprehensively comparative genomic study. The large indels between B. abortus SPS strains and Bab8416 showed possible divergence between two evolutionary branches at a far phylogenetic node. Compared to B. abortus SPS strain 9-941 (Bab9-941), the specific re-arrangement event in Bab8416 displaying a closer linear relationship with B. melitensis 16M than other B. abortus strains resulted in the truncation of c-di-GMP synthesis, and 3 c-di-GMP-metabolizing genes, were present in Bab8416 and B. melitensis 16M, but absent in Bab9-941 and other B. abortus strains, indicating potential SPR-associated key determinants and novel molecular mechanisms. Moreover, despite almost completely intact smooth LPS related genes, only one mutated OmpA family protein of Bab8416, functionally related to flagellar and efflux pump, was newly identified. Several point mutations were identified to be Bab8416 specific while a majority of them were verified to be B. abortus ST2 characteristic. In conclusion, our study therefore identifies new SPR-associated factors that could play a role in refining and updating Brucella taxonomic schemes and provides resources for further detailed analysis of mechanism for Brucella phage resistance.