Application of synthetic solid culture medium to improve the detection of antimicrobial drug residues in foodstuffs (secondary publication)

Summary Background: Recent laboratory tests being in use in Europe to detect antibiotic resi dues in human food are either based on the so called Four Plate Test (UE4pt) employ ing Bacillus subtilis as test organism, or on direct chemical test procedures. The Four Plate Test is widely utilized as a scanning method due to simplicity and low cost, however, sensitivity of the chemical detection methods has been about a magnitude higher. Objectives: Purpose of this study was to increase the sensitivity of the biological testing, using B. subtilis as test organism, through restricting nitrogen and carbon supply in the culture media. It was hoped that an effective, portable test tool with low lab technology requirement could be developed. Materials and Methods: Growth and antibiotic sensitivity characteristics of B. sub tilis were compared using Mueller-Hinton Agar and our two limited nutrient supply semisolid media. Ammonium sulphate was combined either with glucose or sodium pyruvate in buffered agar. Each composed medium was tested at both pH 6.0 and pH 8.0. Growth inhibition was measured around commercial antibiotic resitance test disks containing 21 respective antibiotics. Results and Discussion: The medium based on ammonium sulphate and sodium pyruvate, pH 6.0 proved to be most suitable as a replacement of Mueller Hinton Agar in the Four Plate Test. Growth inhibition, indicating sensitivity, improved by 1.4 to 4.0-fold, depending on the antibiotic. This allows the detection of levels up to a magnitude smaller amount of antibiotics than the original Four Plate Test. Unlike Mueeler Hinton Agar, our artificial medium does not contain any accidental inhibitor substances. Morover, the medium is superselective for B. subtilis, cheap and highly consistent in composition, as well as results