ABSTRACT
Antibiotics include naturally-occurring, semi-synthetic and synthetic chemical compounds with antimicrobial activity. Antibiotics used to treat and prevent farm animal diseases may be administered for the purposes of therapy (to animals exhibiting clinical signs of a disease), control (of a disease spread) or prevention (of diseases); depending on their chemical nature, the administration route can be oral, parenteral or topical. Their use to the end of growth promotion involves the administration of an antibiotic agent usually as a feed additive over a period of time and results in an improved physiological performance of livestock, particularly in the increase in average daily weight gain and increase in feed efficiency (Gaskins et al. 2002, Lefebvre et al. 2006). An incorrect use of antibiotics in Veterinary Medicine practice may leave behind their residues in edible tissues that may have direct toxic effects on consumers, e.g. allergic reactions in hypersensitive individuals, or may cause problems indirectly through the nascence of resistant bacterial strains. Antibiotics comprise the following sub-groups: aminoglycosides, tetracyclines, b-lactams, macrolides, peptides, sulphonamides (and trimethoprim), quinolones, and miscellaneous (chloramphenicol and malachite green) (Stolker and Brinkman 2005). Some of the most commonly used antibiotics are benzathine penicillin, procaine penicillin, ampicillin, amoxicillin, cloxacillin, cefuroxime, cephalonium dihydrate, cefuroxime, ceftiofur, erythromycin, oxytetracycline, sulfadiazine, sulphadimidine, sulphadoxine, dihydrostreptomycin, novobiocin, trimethoprim, fluorphenicol, neomycin, tylosin and flunixin (Khan et al. 2008). Codex MRLs in muscle and fat tissue and the Acceptable Daily Intakes (ADIs) of some of the most important antimicrobial drugs are shown in Table 2. Polyether ionophores (lasalocid, monensi, narasin and salinomycin) are used to improve feed metabolism efficiency in farm animals (Page 2003) and are toxic to many bacteria, protozoa, fungi and higher organisms (Khan et al. 2008). The lipolytic nature of polyether ionophores enables easy penetration through cell membranes and an uncontrolled influx and/or efflux of the selected ions such as potassium and sodium from the cell, leading to its death. Tylosin is a macrolide produced by Streptomyces spp. bacteria that is able to penetrate host cells and be therapeutically active, therefore being particularly valuable in the treatment of cell-associated pathogens such as mycoplasmas. Sulphonamides represent broad-spectrum antibiotics active against gram-positive and gram-negative bacteria, acting on specific targets in bacterial DNA synthesis. Aminoglycosides are a large and diverse class of antibiotics that characteristically contain two or more aminosugars
linked by glycosidic bonds to an aminocyclitol component. Well-known aminoglycosides are gentamicin, lincomycin, neomycin and streptomycin (Stolker and Brinkman 2005). Beta-lactams are probably the class of antibiotics most widely used in Veterinary Medicine for the treatment of bacterial infections of livestock bred on farms and used for bovine milk production. The presence of antibiotics or their residues in food of animal origin that goes above the maximum permissible level has been recognized worldwide. When it comes to the presence of antibiotic residues in meat and meat products, the fact that raises special concern is the increasing incidence of resistance to antibiotics among a wide variety of pathogenic bacteria and potential deleterious effects that these compounds may have on natural soil bacterial populations. The meat processing industry also faces some problems when using antibiotic-treated meats as raw materials in the fermented meats’ production, including a lower quality of the resulting products, problems in fermentation arising on the grounds of antibiotic residues’ presence and a lower meat fat content with a subsequent loss of juiciness and a poorer flavour development (Toldrá and Reig 2006). Therefore, careful management of antibiotic residues is necessary in order to minimise the potential public health risks coming from resistant bacterial strains (Khan et al. 2008).
