Epidemiological cut‐off values for Flavobacterium psychrophilum MIC data generated by a standard test protocol

Abstract

Epidemiological cut-off values were developed for application to antibiotic susceptibility data for Flavobacterium psychrophilum generated by standard CLSI test protocols. The MIC values for ten antibiotic agents against Flavobacterium psychrophilum were determined in two laboratories. For five antibiotics, the data sets were of sufficient quality and quantity to allow the setting of valid epidemiological cut-off values. For these agents, the cut-off values, calculated by the application of the statistically based normalized resistance interpretation method, were ≤16 mg L(-1) for erythromycin, ≤2 mg L(-1) for florfenicol, ≤0.025 mg L(-1) for oxolinic acid (OXO), ≤0.125 mg L(-1) for oxytetracycline and ≤20 (1/19) mg L(-1) for trimethoprim/sulphamethoxazole. For ampicillin and amoxicillin, the majority of putative wild-type observations were 'off scale', and therefore, statistically valid cut-off values could not be calculated. For ormetoprim/sulphadimethoxine, the data were excessively diverse and a valid cut-off could not be determined. For flumequine, the putative wild-type data were extremely skewed, and for enrofloxacin, there was inadequate separation in the MIC values for putative wild-type and non-wild-type strains. It is argued that the adoption of OXO as a class representative for the quinolone group would be a valid method of determining susceptibilities to these agents.

Antimicrobial Susceptibility of Flavobacterium psychrophilum from Chilean Salmon Farms and Their Epidemiological Cut-Off Values Using Agar Dilution and Disk Diffusion Methods

“…In four of the five MIC studies compared (this work, Bruun et al, 2000; Michel et al, 2003; Smith et al, 2016) there was a general agreement in the overall patterns of reduced susceptibility observed ( Table 3 ). All reported <2% NWT frequencies for florfenicol.…”

“…Smith et al (2016) calculated CO WT values by NRI analysis of the MIC values of Danish and UK isolates obtained using a broth micro-dilution method. They employed the CLSI standard protocol for this method (CLSI, 2014a) which recommends the use of diluted MH broth (4 g L -1 ).…”

“…We follow Smith et al (2016) in using the term CO WT to indicate epidemiological cut-off values that have not been set by either of these international agencies. Silley (2012) has argued that the terms resistant and sensitive should not be used to refer to the categories identified by epidemiological cut-off values.…”

“…Some studies have employed disk diffusion (Dalsgaard and Madsen, 2000; Kum et al, 2008; Durmaz et al, 2012; Henríquez-Núñez et al, 2012) whilst others determined MIC values. The MIC studies have variously employed agar dilution (Hawke and Thune, 1992; Bruun et al, 2000; Schmidt et al, 2000; Michel et al, 2003; Izumi and Aranishi, 2004; Kum et al, 2008), broth macro-dilution (Del Cerro et al, 2010), or broth micro-dilution (Rangdale et al, 1997; Hesami et al, 2010; Henríquez-Núñez et al, 2012: Smith et al, 2016). Although most studies used diluted Mueller-Hinton (MH) media there has been variation in the extent of the dilution and the use of additives.…”

“…These species-specific cut-off values allow the categorisation of isolates either as wild-type (WT) fully susceptible members of their species or as manifesting reduced susceptibility (NWT; Silley, 2012). Only two studies (Henríquez-Núñez et al, 2012; Smith et al, 2016) have, so far, taken this approach and both used normalized resistance interpretation (NRI) method developed by Kronvall (2003, 2010). …”