Identification and phylogenetic analysis of seven Brucella abortus strains in Zhejiang, China

Introduction Brucella abortus poses a serious zoonotic threat in China, however its genomic epidemiology in Zhejiang Province remains largely uncharacterized. This study addresses this knowledge gap by conducting the first integrated genomic analysis of seven B. abortus isolates obtained from patients and cows in the region. Methods The phenotypic, genotypic, and phylogenomic characters of the isolates were illustrated by BCSP-31 and AMOS-PCR, MLST, ANI, cgSNP, and Pan-Genome analysis. Results All seven isolates were identified as B. abortus via AMOS-PCR and belonged to a single sequence type (ST2) based on MLST, a classification further corroborated by an average nucleotide identity (ANI) of 99.99% to the reference strain B. abortus 544. Pan-genome analysis disclosed a highly conserved genome comprising 3,084 core genes and a minimal accessory genome of only 10 shell genes, alongside the prediction of 68 virulence-associated factors and one AMR gene, mprF . Core genome SNP phylogeny indicated that the Zhejiang strains form a monophyletic cluster with Russian references and trifurcate into three distinct subclades. Each subclade demonstrates specific phylogenetic connections to strains from separate northern Chinese provinces. Discussion As the first study to integrate phenotypic, genotypic, and phylogenomic data for B. abortus in this region, our work substantially expands the understanding of its local population structure and provides a critical evidence base for proactive genomic surveillance, thereby informing targeted public health interventions against brucellosis.