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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 β-lactam antibiotics from other types. However, β-lactams such as cloxacillin and the cephalosporins are resistant to degradation by penicillinase. They may not be identified as β-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 β-lactamases. The present study describes an improved screening protocol to identify β-lactam antibiotics classified as UMIs. A multiresidue liquid chromatographic procedure based on a method for determining β-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 β-lactam antibiotics.

Identification of β-Lactam Antibiotics in Tissue Samples Containing Unknown Microbial Inhibitors