Genomic phylogenetic characterization of Brucella abortus biovar 1 from a human case in Yunnan Province, China.

Brucella abortus (B. abortus) is an important zoonotic pathogen, and genomic profiles of circulating B. abortus strains in Yunnan Province remain poorly characterized. This study reports the first human infection case caused by B. abortus biovar 1 in Yunnan, China, highlighting an emerging public health threat. Epidemiological investigation, bacterial isolation, serological testing, conventional bio-typing, and whole-genome sequencing-based single-nucleotide polymorphism (WGS-SNP) analysis were conducted to elucidate molecular characteristics and phylogenetic origins of the isolate. Strain YN24001 was recovered from a febrile patient with documented livestock exposure. The patient yielded positive results in both the rose Bengal plate test (RBPT) and standard tube agglutination test (SAT), with a SAT titer of 1:200 (++). The isolate was confirmed as B. abortus via Brucella cell surface protein 31 (BCSP-31, 223 bp) and AMOS-PCR (489 bp), showing 99.98% average nucleotide identity (ANI) to B. abortus 544; conventional biotyping further assigned it to B. abortus biovar 1. Genomic annotation classified strain YN24001 as sequence type 1 (ST1), harboring 72 virulence-associated genes and the defensin resistance gene mprF. Phylogenetic analyses based on multilocus variable-number tandem repeat analysis (MLVA)-16 and core genome single nucleotide polymorphism (cgSNP) demonstrated that YN24001 is genetically more closely related to South American isolates than to domestic Chinese strains, representing a novel B. abortus lineage in Yunnan. This study expands known regional diversity of Brucella species and highlights the emerging public health risk of B. abortus, providing evidence for targeted brucellosis surveillance and prevention.