Performance of a New Microbial Test for Quinolone Residues in Muscle

Concerns regarding the presence of drug residues in foods include allergic reactions, toxicity, technological problems in fermented products and the development of antibiotic resistance in human pathogens. The analysis of antimicrobial residues in foods is generally carried out, in a first step, through microbiological screening tests. These tests commonly use Geobacillus stearothermophilus as target specie but show a low ability to detect quinolones. The goal of our study was to evaluate the performance of a new microbiological test (Equinox) for detection of quinolone residues in muscle. The kit contains an ampoule with a standardized number of freeze-dried Escherichia coli and must be diluted with a specific detection medium containing a redox indicator. Microbial growth will modify the redox potential of the medium being observed through a colour change (from blue to brown/orange). Equinox limits of detection for most of tested quinolones (enrofloxacin, norfloxacin, sarafloxacin, marbofloxacin, ciprofloxacin, danofloxacin and difloxacin) were below or around maximum residue limit (MRL) UE and CCβ values obtained corresponded with the determined sensitivities. In contrast, flumequine could not be detected at MRL UE levels. Moreover, Equinox displayed a low sensitivity to other antimicrobials. Sensitivity data obtained in vitro were consistent when testing incurred muscle samples. Matrix constituents, test batch and animal species did not affect the performance of the test. Equinox could be easily automated enabling a large numbers of simultaneous analysis, and a photometric reading can be applied for a precise interpretation. The results obtained in this study prove that Equinox is a useful tool when screening for quinolone residues or can be combined with other methods for screening of unknown samples.