title: Identification of beta-lactam antibiotics in tissue samples containing unknown microbial inhibitors.
abstract: Antibiotic residues in animal tissues can be detected by various screening tests based on microbial inhibition. In the 7-plate assay used by the U.S. Department of Agriculture's Food Safety and Inspection Service (FSIS), penicillinase is incorporated into all but one plate to distinguish beta-lactam antibiotics from other types. However, beta-lactams such as cloxacillin and the cephalosporins are resistant to degradation by penicillinase. They may not be identified as beta-lactams by this procedure, and thus, they may be identified as unidentified microbial inhibitors (UMIs). However, these penicillinase-resistant compounds can be degraded by other beta-lactamases. The present study describes an improved screening protocol to identify beta-lactam antibiotics classified as UMIs. A multiresidue liquid chromatographic procedure based on a method for determining beta-lactams in milk was also used to identify and quantitate residues. The 2 methods were tested with 24 tissue FSIS samples classified as containing UMIs. Of these, 3 contained penicillin G, including one at a violative level, and 5 contained a metabolite of ceftiofur. The others were negative for beta-lactam antibiotics.
authors.affiliation: U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705, USA.
authors.forename: W A
authors.initials: WA
authors.lastname: Moats
authors.forename: R D
authors.initials: RD
authors.lastname: Romanowski
authors.forename: M B
authors.initials: MB
authors.lastname: Medina
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000032:analysis*; D002417:Cattle; D002511:Cephalosporins / Q000032:analysis / Q000378:metabolism; D002853:Chromatography, Liquid; D003023:Cloxacillin / Q000032:analysis / Q000378:metabolism; D004350:Drug Residues / Q000032:analysis*; D007668:Kidney / Q000737:chemistry; D008099:Liver / Q000737:chemistry; D016982:Micrococcus luteus; D008892:Milk / Q000737:chemistry; D010405:Penicillinase; D001618:beta-Lactamases
chemical_list: D000900:Anti-Bacterial Agents; D002511:Cephalosporins; C053503:ceftiofur; D010405:Penicillinase; D001618:beta-Lactamases; D003023:Cloxacillin