Antimicrobial susceptibility of Flavobacterium psychrophilum isolates

16 Routine application of antimicrobials is the current treatment of choice for rainbow trout 17 fry syndrome (RTFS) or bacterial coldwater disease (BCWD) caused by Flavobacterium 18 psychrophilum. In this study, the antimicrobial susceptibilities of 133 F. psychrophilum 19 isolates, 118 of which were from the UK, were evaluated by broth microdilution and disc 20 diffusion methods following VET04-A2 and VET03-A guidelines of Clinical and 21 Laboratory Standards Institute (CLSI), respectively. Isolates were categorised as wild 22 type (fully susceptible, WT) or non-wild type (NWT) using normalised resistance 23 interpretation (NRI) determined cut-off values (COWT). Broth microdilution testing 24 showed that only 12% of UK isolates were WT to oxolinic acid (MIC COWT 0.25 mg L 25 ) and 42% were WT for oxytetracycline (MIC COWT 0.25 mg L). In contrast, all the 26 isolates tested were WT (MIC COWT 2 mg L) for florfenicol, the main antimicrobial 27 for RTFS control in the UK. Disc diffusion-based COWT values were ≥51 mm for 10 μg 28 amoxicillin, ≥44 mm for 30 μg florfenicol, ≥30 mm for 2 μg oxolinic acid and ≥51 mm 29 for 30 μg oxytetracycline. There was a high categorical agreement between the 30 classifications of the isolates by two testing methods for florfenicol (100%), 31 oxytetracycline (93%), and oxolinic acid (99%). 32 33