Two Nordic multicentre studies evaluate susceptibility testing for last-resort antibiotics in enterococci
Members of the research group, together with the NordicAST 2023 LRE-TRE Study Group, have published two companion studies examining how reliably routine laboratory methods detect resistance to two reserve antibiotics, linezolid and tigecycline, in Enterococcus faecalis and Enterococcus faecium. Both studies asked Nordic clinical microbiology laboratories to test blinded collections of 20 well-characterised, whole-genome-sequenced strains using the EUCAST disc diffusion method and two commercial MIC gradient tests (Etest and MTS), comparing results against reference broth microdilution and known resistance genotypes.
The linezolid study (Haldorsen et al., Journal of Antimicrobial Chemotherapy) found that disc diffusion and the Etest aligned most closely with reference MICs, while the Liofilchem MTS test tended to overestimate resistance but reliably flagged strains carrying known resistance genes. Disc diffusion consistently detected strains with confirmed resistance mechanisms, including borderline isolates (MIC 4–8 mg/L) carrying transferable optrA or poxtA genes. Notably, strains with a baseline MIC of 4 mg/L and these genes developed resistance above the clinical breakpoint within four days of linezolid exposure. The authors argue these findings support introducing an area of technical uncertainty or a warning in EUCAST breakpoint tables for linezolid in enterococci.
The tigecycline study (Mathiesen et al., European Journal of Clinical Microbiology & Infectious Diseases) found that both disc diffusion and the gradient tests performed acceptably against international standards, with neither gradient method superior to the other. Errors were concentrated almost entirely in borderline strains near the susceptibility breakpoint. The study also highlights that EUCAST's 2025 decision to raise the tigecycline breakpoint has weakened the correlation between genotype and phenotype in E. faecium, underscoring the need for better methods to categorise borderline strains and a deeper understanding of tigecycline resistance mechanisms.
Together, the studies confirm that current phenotypic methods work well for clearly susceptible and clearly resistant enterococci, but that strains with borderline MICs and known resistance determinants remain a diagnostic challenge with potential clinical consequences.
References
Haldorsen, B.C., et al. (2026). Performance of EUCAST disc diffusion and supplementary methods to detect reduced susceptibility to linezolid in enterococci — the NordicAST LRE-study. Journal of Antimicrobial Chemotherapy, 81(6), dkag174. https://doi.org/10.1093/jac/dkag174
Mathiesen, I., et al. (2026). Performance of disk diffusion and MIC gradient tests in tigecycline susceptibility testing of enterococci: a Nordic multicentre study. European Journal of Clinical Microbiology & Infectious Diseases. https://doi.org/10.1007/s10096-026-05557-7
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