Bicarbonate as an interference in B-stearothermophilus assays

The screening analysis of antibiotic residues in meat products is readily a ccomplished by a suite of bacterial inhibition assays. Inhibition of B. ste arothermophilus is a standard assay noted for its sensitivity to penicillin s. Ln New Zealand, a test kidney from slaughtered animals is accompanied by a urine sample. Generally, penicillins and tetracyclines are both richer i n the urine sample, which is applied directly to wells cut in bacterially s eeded agar plates. An interference in the assay for B. stearothermophilus h as been traced to excessive amounts of bicarbonate in the urine. It is the bicarbonate, not the pH nor the ionic strength that causes the problem. Rem oval of the bicarbonate was by mild acetic acid treatment to pH 5.5 and vor tex mixing to de-gas carbon dioxide. This resulted in complete eradication of the interference without affecting the ability to detect penicillin. The pH change was readily monitored by the indicator bromcresol purple and no complex equipment was needed.