Five-assay microbiological system for the screening of antibiotic residues

A novel microbiological system in microtiter plates consisting of five bioassays is presented for the detection and classification of antibiotic residues in milk. The bioassays were optimized for the detection of beta-lactams (Bioassay B: Geobacillus stearothermophilus), macrolides (Bioassay M: Bacillus megaterium with fusidic acid), tetracyclines (Bioassay T: B. megaterium with chloramphenicol), quinolones (Bioassay Q: Bacillus licheniformis) and sulfamides (Bioassay QS: B. licheniformis with trimethoprim) at levels near the maximum residue limits (MRL). The response of each bioassay was interpreted visually (positive or negative) after 4-5.5h of incubation. The system detects and classifies beta-lactams (5μg/l of amoxicillin, 4μg/l of ampicillin, 36μg/l of cloxacillin, 22μg/l of amoxicillin, 3μg/l of penicillin, 114μg/l of cephalexin, 89μg/l of cefoperazone and 116μg/l of ceftiofur), tetracyclines (98μg/l of chlortetracycline, 92μg/l of oxytetracycline and 88μg/l of tetracycline), macrolides (33μg/l of erythromycin, 44μg/l of tilmicosin and 50μg/l of tylosin), sulfonamides (76μg/l of sulfadiazine, 85μg/l of sulfadimethoxine, 77μg/l of sulfamethoxazole and 87μg/l of sulfathiazole) and quinolones (94μg/l of ciprofloxacin, 98μg/l of enrofloxacin and 79μg/l marbofloxacin). In addition, the specificity values were high for B, T, Q (99.4%), M (98.8%) and QS (98.1%) bioassays. The control of antibiotics through this system can contribute to improving the quality and safety of dairy products.