An automated turbidimetric method for the identification of certain antibiotic groups in incurred kidney samples

An automated turbidimetric method was developed for group level identification of penicillinase sensitive penicillins, tetracyclines and fluoroquinolones in kidney samples. A sample pretreatment procedure was elaborated for the extraction of incurred residues from kidney tissue in a translucent solution to enable the measurement of changes in optical density. The method was comprised of three pairs, one for each antibiotic group: a sensitive test bacterium strain and a resistant strain for the identification of fluoroquinolones and tetracyclines, and a sensitive strain with and without penicillinase for the identification of penicillinase sensitive penicillins. The algorithm employed compared the areas under the OD vs. time curves; threshold values and experimentally observed intra-test criteria were also included in the algorithm. Antibiotics were reliably identified to group level, and no false identifications were obtained with antibiotics belonging to groups not included in the reference panel. Incurred penicillin G, oxytetracycline and enrofloxacin-ciprofloxacin residues were identified at or below the MRL levels for kidney tissue. The graphically determined shortest possible identification times varied between 2 and 7 h. The method developed could furthermore easily be diversified to include other antibiotic groups by adding new "sensitive-resistant" bacterium and medium combinations.