Monitoring of food products from animal origin for the presence of antimicrobial residues is preferably done using microbial screening methods because of their high cost-effectiveness. Traditionally applied methods fail to detect the maximum residue limits which were established when EU Council Regulation 2377/90 came into effect. Consequently, during the last decade this has led to the development of improved microbial screening methods. This review provides an overview of the efforts expended to bring antibiotic screening methods into compliance with EU legislation. It can be concluded that the current situation is still far from satisfactory. Microbial inhibition assay for muscle samples

Microbial growth inhibition tests are widely used as the primary screening approach for the detection of antibiotic residues in slaughter animals. In this study we evaluated and compared the performance of the European Union Four-Plate Test (EU4pt), the Nouws Antibiotic Test (NAT), and a commercial ampoule test, the Premi®Test (applied to both muscle and kidney), by parallel analysis of 735 slaughter animals. The EU4pt only showed significant inhibition with two muscle samples containing 305 µg kg⁻¹ doxycycline and 648 µg kg⁻¹ tulathromycin, while an maximum residue limit (MRL) violation of 1100 µg kg⁻¹ sulfamethazine remained unnoticed. Premi®Test-muscle only detected the sulfamethazine containing sample, all other (1.1%) suspect samples appeared false-positive results. The same test applied to kidney yielded 4.1% suspect samples, while the NAT screening (based on analysis of renal pelvis fluid) showed 4.9% suspect results. The vast majority of these samples contained tetracycline and/or aminoglycoside residues. Premi®Test-kidney appeared to be more sensitive to aminoglycosides than the NAT screening, which failed to detect an MRL violation of 870 µg kg⁻¹ gentamicin in kidney. Detection of less than MRL levels of tetracycline residues by the NAT proved its suitability for this residue group. Whether Premi®Test is sufficiently sensitive for accurate tetracycline detection in kidney remains doubtful, although changing over to kidney definitely improved the suitability of Premi®Test for the detection of residues in slaughter animals.

An improved microbiological screening assay is reported for the detection of quinolone residues in poultry muscle and eggs. The method was validated using fortified tissue samples and is the first microbial assay to effectively detect enrofloxacin, difloxacin, danofloxacin, as well as flumequine and oxolinic acid, at or below their EU maximum residue limits (MRL). The accuracy of the assay was shown by analysing incurred tissue samples containing residue levels around the MRL. Liquid chromatography–tandem mass spectrometry (LC–MS/MS) quantification of the quinolone concentration in these samples showed that the test plate can be used semi-quantitatively, allowing the definition of an “action level” as an inhibition zone above which a sample can be considered “suspect”. The presented assay is a useful improvement or addition to existing screening systems.

Tetracyclines are extensively used in veterinary medicine. For the detection of tetracycline residues in animal products, a broad array of methods is available. Luminescent bacterial biosensors represent an attractive inexpensive, simple and fast method for screening large numbers of samples. A previously developed cell-biosensor method was subjected to an evaluation study using over 300 routine poultry samples and the results were compared with a microbial inhibition test. The cell-biosensor assay yielded many more suspect samples, 10.2% versus 2% with the inhibition test, which all could be confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Only one sample contained a concentration above the maximum residue limit (MRL) of 100 µg kg⁻¹, while residue levels in most of the suspect samples were very low (<10 µg kg⁻¹). The method appeared to be specific and robust. Using an experimental set-up comprising the analysis of a series of three sample dilutions allowed an appropriate cut-off for confirmatory analysis, limiting the number of samples and requiring further analysis to a minimum.