A total of 30 isolates of Francisella noatunensis subsp. noatunensis isolated from Atlantic cod Gadus morhua L. were tested for susceptibility, in the form of minimal inhibitory concentration (MIC) values, against the following antibacterial agents: flumequine, oxolinic acid, ciprofloxacin, florfenicol, oxytetracycline, erythromycin, streptomycin sulphate, trimetoprim/ sulphadiazine and rifampin. All the isolates had a low susceptibility to oxytetracycline, trimetoprim/sulphadiazine (Tribrissen ® ), erythromycin, ciprofloxacin and streptomycin with MIC values of 64, 64 to 128, 16, 8 and 32 to 128 µg ml Dis Aquat Org 98: [57][58][59][60][61][62] 2012 markedly more active against oxolinic-acid-resistant isolates of Aeromonas salmonicida (Lewin & Hastings 1990). F. noatunensis subsp. noatunensis is a slowgrowing bacterium, and rifampin, erythromycin and streptomycin sulphate are all antibacterial agents that are used in human medicine in the treatment of Mycobacterium tuberculosis, another slow-growing bacterium. Erythromycin is also occasionally used against bacterial kidney disease (BKD) in salmonids.The aim of the present study was to test the susceptibility, in the form of MIC values, to a number of isolates of Francisella noatunensis subsp. noatunensis against the selected group of antibacterial agents.
MATERIALS AND METHODS
Bacterial strainsA total of 30 isolates of Francisella noatunensis subsp. noatunensis from Atlantic cod Gadus morhua L. were analysed. Of these, 28 isolates were provided by the Norwegian Veterinary Institute (Norway) and 2 isolates, GM2212/LMG 24256 (Nylund et al. 2006, Ottem et al. 2007) and EK-4b, were provided by the Institute of Marine Research (Norway). All isolates originate from farmed cod, except EK-4b, which was isolated from a wild cod. The strains were previously identified as F. noatunensis subsp. noatunensis using the methods described by Ottem et al. (2008).
ChemicalsFlumequine, oxolinic acid, oxytetracycline, florfenicol, Tribrissen (trimethoprim/sulfadiazine), erythromycin, ciprofloxacin, streptomycin sulphate and rifampin were all obtained from Norwegian Medical Depot (Bergen, Norway). Stock solutions of antibacterial agents were prepared at a concentration of 1.0 mg ml −1 in methanol (florfenicol, Tribrissen, rifampin), in water (oxytetracycline, erythromycin, streptomycin sulphate), 0.03 M NaOH (flumequine, oxolic acid) and 0.03 M NaOH/methanol (1:1) (ciprofloxacin).
MIC determinationIn order to meet the demands of Francisella noatunensis subsp. noatunensis for specific growth factors, the method of choice in the present work was a modified agar dilution test for the determinations of MIC values (Alderman & Smith 2001). Briefly, F. noatunensis subsp. noatunensis isolates were grown on modified Mueller-Hinton agar (MMHA) supplemented with 3% foetal bovine serum (FBS) (PAA Laboratories), 0.8% glucose (Merck), 0.4% L-cystein (Sigma) and 4% Yeastolate Ultrafiltrate (Gibco) and containing 2-fold dilutions of the antibacterial agents tested. All supplements and a...
