EUCAST 2026 Breakpoints for Brucella Validated in Large French Reference Laboratory Study

Brucellosis, caused by bacteria of the genus Brucella, remains a significant global health burden in many parts of the world. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) recently established standardized clinical breakpoints, in version 16.1 dated 2026, for Brucella to support global surveillance and evidence-based treatment. A new study from the French National Reference Centre set out to validate whether these breakpoints hold up when applied with the routine laboratory methods that clinical microbiology laboratories actually use.

The researchers retrospectively analyzed 234 Brucella isolates collected between January 2017 and June 2026. Species identification was performed with MALDI-TOF mass spectrometry and multiplex PCR, while antimicrobial susceptibility testing was carried out using E-test strips and disk diffusion according to EUCAST guidelines. Brucella melitensis predominated in the collection, accounting for 210 of the isolates, or 89.7 percent, with smaller numbers of B. suis, B. abortus and B. amazoniensis also represented.

Across all 234 isolates, universal susceptibility was observed for the eight antibiotics tested. The authors report that this validates the EUCAST 2026 breakpoints as clinically appropriate and epidemiologically sound when applied using appropriate routine laboratory methods, and they state that the findings demonstrate that France remains free from antimicrobial resistance in this genus.

One result stood out. Nearly half of the isolates, 49.6 percent, displayed rifampicin minimum inhibitory concentrations within two doubling dilutions of the EUCAST epidemiological cut-off value. The authors highlight this proximity as a signal that continued surveillance of rifampicin susceptibility in Brucella is needed, even though no resistance was detected.

A critical methodological finding emerged around streptomycin. E-test strips indicated elevated streptomycin MICs in a substantial proportion of isolates, with results consistent across different lots. Yet when disk diffusion was performed on the same isolates, all 234 showed susceptibility, with inhibition zone diameters ranging from 21 to 35 millimeters. On this basis, the authors recommend disk diffusion as the reference method for streptomycin susceptibility determination in Brucella species.

The study offers practical guidance for clinical microbiology laboratories in both endemic and non-endemic regions, supporting standardized antimicrobial susceptibility testing for the optimized management of human brucellosis. As a retrospective analysis of a single national reference collection, its scope is limited to the isolates and methods described, but the authors present it as essential evidence that the new EUCAST breakpoints can be applied reliably in routine practice.

Subject of Research: Validation of EUCAST 2026 antimicrobial breakpoints for Brucella species using routine laboratory methods

Article Title: Validation of EUCAST 2026 breakpoints for Brucella species using routine laboratory methods: A comprehensive French National Reference Centre study

Article References: Gaillard, M., Pastre, T., Magnan, C., Sotto, A., O’Callaghan, D., Keriel, A., & Lavigne, J.-P. (2026). Validation of EUCAST 2026 breakpoints for Brucella species using routine laboratory methods: A comprehensive French National Reference Centre study. PLOS Neglected Tropical Diseases, 20(10), e0014817. https://doi.org/10.1371/journal.pntd.0014817

Image Credits: AI Generated

DOI: 10.1371/journal.pntd.0014817

Keywords: brucellosis, Brucella, EUCAST, antimicrobial susceptibility testing, breakpoints, rifampicin, streptomycin, disk diffusion, E-test, MALDI-TOF, French National Reference Centre, drug resistance