Endemic circulation without acquired antimicrobial resistance: Genomic and phenotypic insights into human Brucella melitensis isolates in Bosnia and Herzegovina.

BACKGROUND

Human brucellosis remains endemic in Bosnia and Herzegovina (B&H), yet integrative studies explaining transmission patterns, population structure, and susceptibility profiles in an endemic setting are lacking. This study aimed to characterise human Brucella melitensis (B. melitensis) isolates from B&H using a combined epidemiological, phenotypic, and genomic approach.

METHODS

A total of 60 B. melitensis blood isolates obtained from patients with confirmed brucellosis between 2015 and 2021 were selected from a multicentre collection for phenotypic antimicrobial susceptibility testing (AST) and whole-genome sequencing (WGS). Broth microdilution (BMD) in three different broth media was used and interpreted according to the Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria. Genomic analyses included phylogenomic contextualisation using publicly available genomes, and screening for acquired antimicrobial resistance determinants and virulence-associated genes.

RESULTS

Human brucellosis predominantly affected working-age males and was strongly associated with occupational and household exposure. While phenotypic AST showed that first-line antimicrobials remain effective in vitro, the results were significantly impacted by both the choice of culture media and the interpretive framework, with major discrepancies observed between CLSI and EUCAST classification, particularly for streptomycin and trimethoprim-sulfamethoxazole. WGS revealed a high degree of clustered genomic homogeneity within the East Mediterranean lineage. No acquired antimicrobial resistance (AMR) genes were detected, while a conserved virulence gene repertoire was observed across isolates, including genes associated with membrane adaptation such as mprF.

CONCLUSIONS

Human brucellosis in B&H is characterised by sustained endemic transmission within a genetically conserved B. melitensis population. Integration of epidemiological, phenotypic AST, and genomic data highlights the importance of interpreting susceptibility patterns in the light of Brucella's slow evolutionary dynamics and local epidemiological context. The genomic baseline established in this study supports future One Health surveillance integrating human and animal reservoirs.