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

Funding Information This study was supported by Project BIOSAFE funded by FEDER through the Programa Operacional Factores de Competitividade–COMPETE and by Fundação para a Ciência e a Tecnologia (FCT, Portugal)—Grant LISBOA-01-0145-FEDER-030713, PTDC/CAL-EST/30713/2017 and by FCT through funds to GHTM (UID/04413/2020), CIISA Project (UID/CVT/00276/2020), and Project PTDC/CVT-CVT/28469/2017. The contributions of Andrea T. Feßler and Stefan Schwarz were financially supported by the German Federal Ministry of Education and Research (BMBF) under project numbers 01KI1727D and 01KI2009D as part of the Research Network Zoonotic Infectious Diseases. Part of this research was supported by Cost Action CA18217: European Network for Optimization of Veterinary Antimicrobial Treatment (ENOVAT). 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. publishersversion published