Abstract

Flavobacteriosis, caused by Flavobacterium psychrophilum is considered the most important bacterial disease in Chilean freshwater salmonid farming, and is currently mainly controlled by the use of antimicrobial agents. The aim of this study was to correlate the results obtained by disk diffusion and minimum inhibitory concentration (MIC) assays of antimicrobials used in fish farming using F. psychrophilum isolates recovered from Chilean farmed salmonids. Antimicrobial susceptibility patterns and MIC values of six antibacterial agents against 125 Chilean isolates of F. psychrophilum were tested on Diluted (1:5) Cation-Adjusted Mueller-Hinton (DCAMH) agar using a disk diffusion method and an agar dilution method, respectively. A moderate correlation for antimicrobials amoxicillin and florfenicol was observed (Pearson correlation indices of -0.6894 and -0.5857, respectively), whereas highly significantly high (p .001) correlations were detected for oxytetracycline, oxolinic acid, flumequine and enrofloxacin (Pearson correlation indices of -0.7901, -0.8386, -0.8151 and -0.7818, respectively), demonstrating that MIC and disk values were tightly associated (R-2 >= 0.61). When isolates were categorized as fully susceptible Wild-Type or Non Wild-Type, the number of isolates differently categorized using each method varied between 8 and 23 for assayed antibacterials. The use of DCAMH without biological supplements was efficient to perform agar dilution tests with F. psychrophilum but its use to perform disk diffusion assays is questionable, due to some isolates exhibiting poor growth produced inhibition zones that are difficult to read. This study demonstrates that the agar dilution method is a good alternative to determine MIC values when a high number of isolates must be assayed. In contrast, further research is required to advance in the standardization of a disk diffusion procedure for this pathogen.