Submitted to: Journal of Association of Official Analytical Chemists International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 7, 1998
Publication Date: N/A
Interpretive Summary: Animal tissues are routinely screened for the presence of antimicrobial compounds using various test microorganisms. Identification of the antimicrobials found is a lengthy and difficult procedure. In many cases, identification is not possible and samples are classified as containing unknown microbial inhibitors (UMIs). This paper describes a combination of a new screening test and HPLC analysis to identify which of UMIs are B- Lactam antibiotics. The practical use of this approach with UMI samples from regulatory agencies is demonstrated. Several of the UMI samples were found by HPLC analysis to contain penicillin G. A ceftiofur metabolite was also found in some of the samples. The preliminary screening test was effective in identifying samples containing B-Lactam antibiotics, thus substantially reducing the number of samples requiring HPLC analysis.
Antibiotic residues can be detected in animal tissues by a variety of screening tests based on microbial inhibition. In the seven-plate assay used by USDA's Food Safety and Inspection Service (FSIS), penicillinase is incorporated into all but one plate in order to distinguish the B-lactam group of antibiotics from other types. However, B-lactams such as cloxacillin and the cephalosporins are resistant to degradation by penicillinase and may not be identified as B-lactams by this procedure. They thus may be identified as unidentified microbial inhibitors (UMIs). These penicillinase-resistant compounds can be degraded by other B- lactamases. The present study describes the use of an improved screening test protocol to identify any B-lactam antibiotics classified as UMIs. A multiresidue HPLC procedure based on a method previously described for B- lactams in milk was also used to identify and quantitate residues. These approaches were tested with 24 tissue samples from FSIS classified as containing UMIs. Of these, three contained penicillin G, one at a violative level, and five contained a metabolite of ceftiofur. The others were negative for B-lactam antibiotics.