An indirect conductimetric screening method using three test bacterium–medium combinations was developed for rapid detection of antibiotic residues in bovine carcasses. The detection time (DT), i.e. the point when the growth of the test bacterium was detected, was determined by observing the rate of change in the conductance plotted against time. This detection time averaged half of the reference time recorded by the instrument software. Total change in conductance (TC) was used as a further measure of growth. Threshold values for DT and TC were determined with inhibitor-free kidney samples. The presence of a residue was indicated if the DT exceeded the respective threshold value and was confirmed if the TC remained below the TC threshold value. The limits of detection (LODs) determined with fortified samples were at about or below the MRLs for cephalexin, chlortetracycline, ciprofloxacin, dihydrostreptomycin, enrofloxacin, oxytetracycline and penicillin G. The
LODs for penicillin G, oxytetracycline and the sum of enrofloxacin and ciprofloxacin were also estimated with incurred samples; these samples were also analysed using liquid chromatography. The LODs determined with fortified and incurred samples were in close agreement. Given its rapid detection, good sensitivity to a wide range of antibiotics and ease of performance, the indirect conductimetric method developed here would seem to offer an appealing alternative to agar diffusion tests.
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