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Determination of epidemiological cut-off values for Narasin, Salinomycin, Lasalocid and Monensin in Enterococcus
Maria Laura Ferrando1, Francesca Saluzzo1, Rikki Franklin Frederiksen2
1Division Immunology, Transplant and Infectious Diseases, IRCCS Ospedale San Raffaele (OSR), Via Olgettina 60, Milan 20123, Italy.
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.
Insights
New epidemiological cut-off values (ECOFFs) were established for veterinary ionophores in Enterococcus faecium. These findings aid in understanding and monitoring antimicrobial resistance in bacteria.
Area of Science:
- Veterinary Microbiology
- Antimicrobial Resistance
- Molecular Epidemiology
Background:
- Polyether ionophores are crucial veterinary antimicrobials.
- Monitoring antimicrobial resistance in Enterococcus faecium is essential for animal health.
- Establishing epidemiological cut-off values (ECOFFs) is key for resistance surveillance.
Purpose of the Study:
- To determine ECOFFs for narasin, salinomycin, lasalocid, and monensin in Enterococcus faecium.
- To differentiate wild-type (WT) from non-wild-type strains.
- To provide a basis for antimicrobial resistance monitoring.
Main Methods:
- Broth microdilution method (ISO 20776-1) for Minimum Inhibitory Concentrations (MICs).
- Testing 182 E. faecium isolates across five European laboratories.
- Utilizing the EUCAST ECOFFinder tool and PCR/WGS for verification.
Main Results:
- Established ECOFFs: narasin (0.5 mg/L), salinomycin (1 mg/L), lasalocid (2 mg/L).
- Narasin and salinomycin ECOFFs are lower than current EUCAST values.
- Monensin did not yield a distinct ECOFF due to a broad MIC distribution.
Conclusions:
- Novel ECOFFs for key veterinary ionophores in E. faecium were successfully established.
- These ECOFFs facilitate the identification of resistant strains.
- The findings contribute to a better understanding of ionophore resistance mechanisms.
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