Antimicrobial susceptibility of Flavobacterium psychrophilum isolates from the United Kingdom

Routine application of antimicrobials is the current treatment of choice for rainbow trout fry syndrome ( RTFS ) or bacterial coldwater disease ( BCWD ) caused by Flavobacterium psychrophilum . In this study, the antimicrobial susceptibilities of 133 F. psychrophilum isolates, 118 of which were from the UK , were evaluated by broth microdilution and disc diffusion methods following VET 04‐A2 and VET 03‐A guidelines of Clinical and Laboratory Standards Institute ( CLSI ), respectively. Isolates were categorized as wild type (fully susceptible, WT ) or non‐wild type ( NWT ) using normalized resistance interpretation ( NRI )‐determined cut‐off values ( CO WT ). Broth microdilution testing showed that only 12% of UK isolates were WT to oxolinic acid ( MIC CO WT  ≤ 0.25 mg/L) and 42% were WT for oxytetracycline ( MIC CO WT  ≤ 0.25 mg/L). In contrast, all the isolates tested were WT ( MIC CO WT  ≤ 2 mg/L) for florfenicol, the main antimicrobial for RTFS control in the UK . Disc diffusion‐based CO WT values were ≥51 mm for 10 μg amoxicillin, ≥44 mm for 30 μg florfenicol, ≥30 mm for 2 μg oxolinic acid and ≥51 mm for 30 μg oxytetracycline. There was a high categorical agreement between the classifications of the isolates by two testing methods for florfenicol (100%), oxytetracycline (93%) and oxolinic acid (99%).