Identification and whole-genome sequencing of blood-brain barrier-penetrating Riemerella anatipestifer isolated from chickens and ducks.

Riemerella anatipestifer (RA) is a major cause of bacterial meningitis, which poses a serious threat to the poultry industry. To identify RA strains with a strong ability to cross the blood-brain barrier (BBB) and analyze their genomic characteristics, a co-infection model was established using multiple RA strains with different serotypes in this study. We found that during co-infection with multiple RA strains, certain RA strains rapidly penetrated the BBB and were recovered from brain tissue as early as 0.25 h post-infection. The pioneer strains were selected to further evaluate their BBB-penetrating capacity and pathogenicity in vivo and in vitro. Whole-genome sequencing of three pioneer strains and one RA strain that couldn't penetrate the BBB revealed that these strains had circular chromosomes of 2.14-2.28 Mb, encoding 1,982-2,137 protein-coding genes. All strains carried 40 tRNA genes and 9 rRNA genes, and contained 9-15 genomic islands. Comparative genomic analysis revealed variation in orthogroup composition among the four strains, with strain-specific distributions of resistance genes (24-29 per strain) and virulence genes (4-57 strain-specific genes), alongside variable transposon and IS element profiles. Comparative genomic analysis further uncovered strain-specific distributions of virulence and antibiotic resistance genes, underscoring the genomic plasticity among these isolates. In summary, these findings provide a comprehensive phenotypic and genomic characterization of RA strains with strong BBB-penetrating ability, providing a foundation for future mechanistic studies on RA BBB penetration.