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The results of an in-house laboratory validation of a microbiological method for the screening of antibiotic residues in milk are presented. The sensitivity of this five-plate test, called Screening Test for Antibiotic Residues (STAR), was established by the analysis of milk samples spiked with 66 antibiotics at eight different concentrations. Ten different groups of antibiotics were studied: macrolides, aminoglycosides, cephalosporins, penicillins, quinolones, tetracyclines, sulphonamides, lincosamides, phenicolated and miscellaneous drugs. It was shown that 21 antibiotics were detected by the STAR protocol at or below the maximum residue limit (MRL), and that a further 27 drugs could be detected at levels from the MRL up to four times the MRL. The sensitivity of the STAR protocol was at or below the MRL for three macrolides, one tetracycline, two aminoglycosides, some sulphonamides, half of the beta-lactams, quinolones, lincosamides, trimethoprim and baquiloprim. Moreover, the STAR protocol was at least twice as sensitive as conventional methods for macrolides, quinolones and tetracyclines. The other antibiotics had limits of detection between four and 150 times the MRL. Each plate was preferentially sensitive for one or two families of antibacterials: the plate Bacillus cereus for tetracyclines, the plate Escherichia coli for quinolones, the plate Basillus subtilis for aminoglycosides, the plate Kocuria varians for macrolides, and the plate Bacillus stearothermophilus for sulphonamides and beta-lactams. This method has been used routinely on a day-to-day basis to direct the physicochemical confirmation towards one or two families of antibiotics. Considering the high cost of liquid chromatography coupled with tandem mass spectrometry detection analyses, the reduction of the range of antibiotics to test for confirmation is a significant gain in time and money.

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Aims:  To determine the susceptibility to disinfectants and cross‐resistance to antibiotics in Listeria monocytogenes strains isolated from fish products and the fish‐processing environment. Methods and Results:  Minimal inhibitory concentration assessment, using the agar dilution method, showed 108 of 255 L. monocytogenes isolates with low susceptibility to benzalkonium chloride (BC), commonly used in food industries. Most of them are from raw products of farmed fish during processing, while the remaining resistant isolates were mainly from the environment and finished products irrespective of the fish species. Two BC‐resistant isolates were resistant to ethidium bromide (EB). The conservation of resistance after plasmid curing suggested that the resistance genes are not plasmid associated. EB accumulation assays demonstrated that the two BCR EBR isolates used an efflux pump to expel these substrates whereas a different mechanism was probably used by the majority of the strains with BCR EBS pattern. No cross‐resistance was found with antibiotics. Conclusions:  This study highlights the difference in susceptibilities to BC for L. monocytogenes strains isolated from fish‐processing plants and in resistance mechanisms to BC developed by these bacteria. Significance and Impact of the Study:  The presence of BC resistant L. monocytogenes strains could contribute to their adaptation and so explained their survival and persistence in the fish‐processing environment.

Biosensor immunoassays were developed recently for antibiotics with an established maximum residue limit (MRL). In this study, according to the regulatory banning of chloramphenicol (CAP) use for food producing animals, the main objectives were: the specificity of the biosensor assay and the lowest detection limit possible. The assay was based on the inhibition of the binding of polyclonal antibodies against CAP to immobilized CAP on a sensor chip by CAP in solution. The response varied inversely with the antibiotic concentration in the sample. Two different antibodies and two immobilization protocols were tested. As in ELISA tests the antibody influenced the assay performances. Moreover, we showed that particular care should be concentrated on the immobilization step because it is a critical point in the assay development. Three different protocols were developed in milk. The best assay was obtained with antibody 1 in milk on the CAP base surface because of its very low detection limit (0.1 m g l -1 ) and the decreased consumption of antibody (four times less than on the CAP surface). This assay is rapid (3 min/run), sensitive, and specific for CAP and CAP glucuronide. It could be integrated in a multi-residue screening test and applied to other matrices (bile, urine, meat).

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Extensive use of DDAC at subinhibitory concentrations may lead to the development of antibiotic-resistant bacteria and may represent a public health issue.