In 1877 Pasteur and Joubert discovered that growth of Bacillus anthracis could be inhibited by the presence of other microorganisms. This discovery led to the isolation of pyacyanase, the first antibiotic. In 1928 penicillin was discovered by Fleming when he observed that the outgrowth of an agar culture of Staphylococcus aureus was inhibited by mold. In 1932 Domagk discovered that the dye prontosil red, the first sulfonamide, demonstrated antimicrobial properties. Since these early discoveries the number of antimicrobials either isolated from natural sources or synthesized in the laboratory has grown tremendously. Antibiotics, in the narrowest sense, are products produced by living organisms that are not toxic to the producing organism but are capable of inhibiting the growth of or terminating other organisms. However, there are many synthetically manufactured compounds that inhibit or kill organisms as effectively as natural antibiotics. The chemistry and mode of action of these synthetic antimicrobial compounds are well documented. While there are virtually thousands of known antimicrobials, only a few are marketed for use in present-day food animal production.
Treatment of food animals with therapeutic and subtherapeutic dosage forms of antibiotic-antimicrobial drugs has increased in the last decade. This has been due to the advent of modern mass production operations that involve the maintenance of thousands of animals simultaneously and requires the utilization of carefully formulated and medicated diets that serve to maximize growth, minimize production costs, and provide an acceptable consumer product that is wholesome and affordable. Animal feeds that contain antibiotics and other antimicrobials as a prophylactic are now used routinely in beef, swine, chicken, and turkey production. In this way the antibiotics— antimicrobials help to maintain the optimal health of the animals so treated. Antibiotics may be coadministered with antimicrobial drugs resulting in a net increase in drug effectiveness compared to individual drug administrations. Strictly speaking, sulfonamides are not antibiotics. However, for purposes of this article sulfonamide antimicrobials will be collectively termed antibiotics.
The aminoglycoside, beta-lactam, ionophore, macrolide, sulfonamide, tetracycline, and other antibiotics are an integral part of food animal production. Regulations governing the use, dosage, and withdrawal times for many of the members of these antibiotic classes in animal production have been established in the United States by federal law, as outlined in the Code of Federal Regulations, Title 21, for each compound (1). The U.S. Food and Drug Administration (FDA) regulates the use of antibiotics in animal production while the U.S. Department of Agriculture (USDA) monitors residue levels by testing animal-derived products for antibiotics. Specific information regarding manufacturers, new animal drug application (NADA) codes, approved dosage forms and directions for proper use can be found in the Code of Federal Regulations, Title 21 and other published sources (2).
Antibiotics may manifest themselves as residues in animal-derived human foods if improperly used or if withdrawal times have not been observed for treated animals. Animal-derived human foods that contain violative antibiotic residues may pose a potential human health hazard. These potential health hazards can be broken down into three broad categories (3): toxicological, microbiological, and immunopathological. Toxicological concerns relate to the direct toxic effect of the compound on the consumer, resulting in physiological abnormalities, an example being sulfamethazine, which has recently been shown to produce cancer in laboratory animals (4). Microbiological concerns relate to the transmittance of antibiotic resistance. Antibiotics have the potential to act as a selective force that favors the emergence of resistant pathogenic bacteria in the natural flora of meat consumers. An example is a reported outbreak of salmonella poisoning in humans believed to be associated with a resistant strain in hamburger obtained from culled cattle that had been treated routinely with chloramphenicol (5). Finally, there are immunopathological mechanisms, whereby the drug serves as an antigen, demonstrates allergenic properties, and may result in hypersensitivity reactions to the drug that has sensitized some individuals. Penicillin is a prime example. Antibiotics are the principal compounds of concern in the federal residue control strategy (6). Such routine monitoring for antibiotic residues in animal-derived foods is intended to minimize the occurrence of violative antibi otic levels in the food supply. Such a capability is dependent on the analytical methods utilized.
Methods for the determination of antibiotics can include but are not limited to bioassay (ba), thin layer chromatography (tic), gas chromatography (gc), liquid chromatography (lc), and various immunoassay (ia) techniques. Each of these methods has found utility in antibiotic determinations and many have been shown to be precise, specific, or sensitive, depending on which analytical technique, detector, visualization method, and compound are used. In this regard, a method utilized to detect an antibiotic residue at low nanogram antibiotic per gram of sample (ng/g) or even picogram antibiotic per gram of sample (pg/g) levels may be confounded by the presence of interferences found in the sample matrix or in the sample extract. Thus analytical capability is governed ultimately by the sample preparation and extraction steps. The isolation of such residues from a complex biological matrix poses unique problems to the analyst and because of the number of antibiotics being utilized in animal production the need for multiresidue determinations exists. An emerging trend is the development of rapid multiresidue—multidrug class isolation techniques that result in clean, interference-free extracts and that minimize cost, time, and expendable material requirements, enabling the analyst to test for multiple drugs isolated from one sample. In conjunction with improved isolation methods the need to screen samples rapidly and accurately for antibiotic residues exists. Such antibiotic screening protocols and analytical capability can be enhanced by rapid, reproducible, efficient, and cost-effective residue isolation techniques.
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