Proposal of Epidemiological Cutoff Values for Apramycin 15 μg and Florfenicol 30 μg Disks Applicable to Staphylococcus aureus

Sofia Santos Costa, Carolina Ferreira, Rute Ribeiro, Andrea T Feßler, Anne-Kathrin Schink, Kristina Kadlec, Heike Kaspar, Ana Amaro, Teresa Albuquerque, Patrícia Abrantes, Catarina Morais, Constança Pomba, Stefan Schwarz, Isabel Couto

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Apramycin and florfenicol are two antimicrobial agents exclusively used in veterinary medicine. Resistance determinants to these antimicrobial agents have been described in several staphylococci, yet no inhibition zone-based epidemiological cutoff (ECOFF) values are available to detect populations harboring resistance mechanisms. In this study, we propose disk diffusion inhibition zone ECOFF values of Staphylococcus aureus for apramycin and florfenicol. The susceptibility to apramycin and florfenicol was evaluated by disk diffusion of five S. aureus collections, comprising 352 isolates of animal (n = 265) and human (n = 87) origin. The aggregated distributions of inhibition zone diameters were analyzed by the normalized resistance interpretation method to obtain normalized wild-type (WT) population distributions and corresponding ECOFF values. The putative WT populations of S. aureus were characterized by an inhibition zone ≥15 mm (ECOFF = 15 mm) for apramycin and ≥21 mm for florfenicol (ECOFF = 21 mm). Five nonwild-type (NWT) isolates were detected for apramycin, all without inhibition zone and harboring the apmA gene, whereas five NWT isolates were identified for florfenicol, all carrying the fexA gene. The proposed ECOFF values for apramycin and florfenicol may be a valuable tool in future antimicrobial resistance monitoring and surveillance studies to identify S. aureus NWT populations toward these antimicrobial agents.

Original languageEnglish
Pages (from-to)1555-1559
Number of pages5
JournalMicrobial Drug Resistance
Issue number11
Early online date5 May 2021
Publication statusPublished - 11 Nov 2021


  • Staphylococcus aureus
  • epidemiological cutoff value
  • apramycin
  • florfenicol


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