Comparative Genomic Analysis of <i>Brucella melitensis</i> Vaccine Strain M5 Provides Insights into Virulence Attenuation

<div><p>The <i>Brucella melitensis</i> vaccine strain M5 is widely used to prevent and control brucellosis in animals. In this study, we determined the whole-genome sequence of M5, and conducted a comprehensive comparative analysis against the whole-genome sequence of the virulent strain 16 M and other reference strains. This analysis revealed 11 regions of deletion (RDs) and 2 regions of insertion (RIs) within the M5 genome. Among these regions, the sequences encompassed in 5 RDs and 1 RI showed consistent variation, with a large deletion between the M5 and the 16 M genomes. RD4 and RD5 showed the large diversity among all <i>Brucella</i> genomes, both in RD length and RD copy number. Thus, RD4 and RD5 are potential sites for typing different <i>Brucella</i> strains. Other RD and RI regions exhibited multiple single nucleotide polymorphisms (SNPs). In addition, a genome fragment with a 56 kb rearrangement was determined to be consistent with previous studies. Comparative genomic analysis indicated that genomic island inversion in <i>Brucella</i> was widely present. With the genetic pattern common among all strains analyzed, these 2 RDs, 1 RI, and one inversion region are potential sites for detection of genomic differences. Several SNPs of important virulence-related genes (<i>motB</i>, <i>dhbC</i>, <i>sfuB</i>, <i>dsbAB, aidA, aroC</i>, and <i>lysR)</i> were also detected, and may be used to determine the mechanism of virulence attenuation. Collectively, this study reveals that comparative analysis between wild-type and vaccine strains can provide resources for the study of virulence and microevolution of <i>Brucella</i>.</p>