Determination of epidemiological cut-off values for Narasin, Salinomycin, Lasalocid and Monensin in Enterococcus faecium.

Objectives: To establish epidemiological cut-off values (ECOFFs) for the polyether ionophores narasin, salinomycin, lasalocid and monensin in Enterococcus faecium.

Methods: MICs were measured using the broth microdilution method according to ISO 20776-1 (2019). Method validation involved ≥10 replicates of antimicrobial susceptibility testing of Enterococcus faecalis ATCC 29212. A total of 182 E. faecium isolates from various sources were tested in five European laboratories. The ECOFFinder tool from EUCAST was used to establish the ECOFFs for 122 WT isolates, verified by PCR or WGS.

Results: Method validation showed consistency, with acceptable variation within ±1 2-fold dilution. The ECOFF for narasin was 0.5 mg/L, considerably below the current EUCAST ECOFF for E. faecium (ECOFF = 2 mg/L). Salinomycin and lasalocid ECOFFs were 1 and 2 mg/L, respectively. Strains carrying the previously identified narAB resistance genes clearly manifested a separate MIC distribution for narasin and salinomycin, but not for lasalocid, although a clear bias to the higher MIC values within the normal distribution could be observed. Monensin apparently displayed a broader MIC range (0.5-64 mg/L) with multiple modes, which precluded the establishment of an ECOFF for monensin.

Conclusions: The study yielded novel ECOFFs for distinguishing WT E. faecium strains for the key veterinary ionophores, providing a mainstay for a better understanding of ionophore resistance in enterococci.