title: Bicarbonate as an interference in B. stearothermophilus assays.
abstract: The screening analysis of antibiotic residues in meat products is readily accomplished by a suite of bacterial inhibition assays. Inhibition of B. stearothermophilus is a standard assay noted for its sensitivity to penicillins. In New Zealand, a test kidney from slaughtered animals is accompanied by a urine sample. Generally, penicillins and tetracyclines are both richer in the urine sample, which is applied directly to wells cut in bacterially seeded agar plates. An interference in the assay for B. stearothermophilus has 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. Removal of the bicarbonate was by mild acetic acid treatment to pH 5.5 and vortex 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.
authors.affiliation: National Chemical Residue Laboratory, Ministry of Agriculture and Forestry, Upper Hutt, New Zealand.
authors.forename: A F
authors.initials: AF
authors.lastname: Erasmuson
authors.forename: E R
authors.initials: ER
authors.lastname: Cairns
authors.forename: B
authors.initials: B
authors.lastname: O'Kane
mesh_terms: D000818:Animals; D001639:Bicarbonates; D002417:Cattle / Q000378:metabolism*; D003670:Deer / Q000378:metabolism*; D004350:Drug Residues / Q000032:analysis*; D001411:Geobacillus stearothermophilus; D006863:Hydrogen-Ion Concentration; D007668:Kidney / Q000737:chemistry; D008826:Microbial Sensitivity Tests / Q000379:methods; D010406:Penicillins / Q000032:analysis* / Q000652:urine; D012756:Sheep / Q000378:metabolism*; D019155:Veterinary Drugs / Q000032:analysis* / Q000652:urine
chemical_list: D001639:Bicarbonates; D010406:Penicillins; D019155:Veterinary Drugs