Detection of streptomycin and dihydrostreptomycin residues in milk, honey and meat samples using an optical biosensor
Immuno-biosensor inhibition assays for the detection of streptomycin and dihydrostreptomycin residues in whole cows′ milk, honey, pig kidney and pig muscle are reported. The antibody showed high cross-reactivity with dihydrostreptomycin in various foodstuffs (buffer 103%, milk 96%, honey 84%, kidney extract 129% and muscle extract 98%). There was no significant cross-reaction with other aminoglycosides or commonly used antibiotics. A streptomycin derivative was used to prepare a stable, reusable sensor chip surface. The assay allowed the direct analysis of bovine whole milk (fat content ∼3.5%). Honey samples required dilution with buffer, while kidney and muscle samples from pigs were homogenized in an aqueous extraction buffer and clarified by centrifugation. The limit of detection for each assay was determined from known streptomycin-free samples (n = 20; mean − (3 × standard deviation)) and the results were as follows: milk 30 µg kg−1, honey 15 µg kg−1, kidney 50 µg kg−1 and muscle 70 µg kg−1. Repeatability (or relative standard deviation) between runs were calculated (n = 3) at the respective Community maximum residue limits (MRL) and 0.5 × MRL with the exception of honey since no European MRL exists at present. Results were determined as 4.3% (200 µg kg−1) and 2.8% (100 µg kg−1) in milk, 13.3% (40 µg kg−1) and 9.5% (20 µg kg−1) in honey, 7.1% (1000 µg kg−1) and 7.6% (500 µg kg−1) in kidney and 7.1% (500 µg kg−1) and 11% (250 µg kg−1) in muscle.