Enzyme immunoassay systems and a reagent kit for the determination of bacitracin in foods

Abstract

Two test-systems for a direct and an indirect enzyme-linked immunosorbent assay (ELISA) of peptide antibiotic bacitracin (BC) were developed and studied. For the both systems, polyclonal antibodies were obtained by immunizing rabbits with a conjugate of BC with keyhole limpet hemocyanine synthesized using reaction between the peptide and the high molecular weight protein in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). The product of BC linking to thyroglobulin which was activated with EDC and N-hydroxysulfosuccinimide served as conjugated antigen on a solid phase in the indirect ELISA. For the direct ELISA, the antibodies against BC were immunochemically immobilized onto microplate surface, while the liquid phase contained a conjugate of BC with horseradish peroxidase. This conjugate was obtained by successive reactions of antibiotic amino groups coupling to periodate oxidized carbohydrate chains of enzyme and the reducting of formed Shiff’s base with sodium borohydride. Conjugated antigens binding to anti-BC antibodies provided maximum colorimetric signals of 2.0 and 1.2 optical units for the direct and indirect ELISA, respectively, and depended on BC content in the liquid phase. Antibiotic concentration that caused the inhibition of binding by 50 % was 2.6 ng/ml in the direct ELISA and 10.0 ng/ml in the indirect ELISA. The simple and sensitive direct ELISA system was used as a prototype of the finished reagent kit and a method for measurements with technical-analytical parameters and metrological characteristics allowing the determination of BC residues in a variety of foods including 14 items in a concentration range of 9.0 to 405.0 pg/kg with proper accuracy and precision.