Our project aims at speeding up while lowering the cost of the method used to detect antibiotic residues in milk. We take β-lactams as the target, and implement a penicillin binding protein, which recognizes all β-lactams, in our testing device. First, we improve the ELISA (enzyme linked immunosorbent assay) method, making the raw materials cheaper by replacing the immune response that occurs on the ELISA plate with a simple competitive binding reaction, in which the penicillin (or other β-lactams) on the plate and in the testing sample compete to bind with GFP-PBP5 (green fluorescent protein - penicillin binding protein 5). We can then measure the concentration of penicillin in the sample by measuring the fluorescence intensity on the plate. To even further reduce the budget and strengthen the sensitivity of the test, we created a testing paper by binding our fusion protein CBD (cellulose binding protein)-PBP5 to filter paper. When the paper is dipped in a milk sample, the presence of β-lactams alters PBP5’s enzymatic activity, which can be measured through electric conductivity test. Its electric conductivity therefore indicates how much of this type of antibiotic exists in our milk.

So far, we have constructed pET-GFP-PBP5 and pET-CBD-PBP5. The plasmids are to be integrated into bacterial genes and express the fusion proteins that will be used in our device.

Our methodology provides new solutions for simplifying and reducing the cost of a standardized antibiotic residue testing procedure, making bovine milk safer to drink in China, and maybe someday throughout the world.