Tetracyclines are antibiotics with an extraordinarily broad spectrum of activity. They are either obtained naturally from bacteria of the genus Streptomyces or produced semisynthetically. Tetracyclines block bacterial protein biosynthesis and thus have a growth-inhibiting (bacteriostatic) effect on numerous bacteria. They were the first major group of antibiotics for which the term "broad-spectrum antibiotics" was used.

If tetracyclines are used, they can enter the organism and thus animal foods.

Legal residue regulation

The substances chlortetracycline, oxytetracycline, tetracycline and doxycycline are approved for the treatment of food-producing animals. Their use requires compliance with withdrawal periods such that, at the end of the withdrawal period, concentrations in food are below the maximum residue limits. These are specified in the Annex to Regulation (EU) 37/2010 (Table 1) and range, depending on the matrix, from 100 µg/kg (muscle, milk) to 600 µg/kg (kidney). The use of doxycycline in animals from which milk or eggs are obtained for human consumption is explicitly not allowed. Consequently, no residues may be present in these foods. For honey, the so-called "zero tolerance" also applies, since tetracyclines, like all other antibiotics, are not permitted for the treatment of honey bees.

Due to the widespread use of tetracyclines, the control within the framework of the National Residue Control Plan (NRKP) includes practically all food-producing animal species as well as the primary products milk, eggs and honey (a total of about 2800 samples per year). In the interests of preventive consumer:inside protection, sampling is carried out from the production chain so that products contaminated with residues can be traced back to the farm of origin.

Samples from the trade are examined as part of official food control campaigns and in cases of suspicion, whereby the control also extends to products from third countries.

Bacterial resistance mechanisms

Bacterial strains that have developed resistance to one tetracycline are also resistant to all other members of the tetracycline family (complete cross-reaction between all tetracyclines). Since they exert their effect at the ribosomes by inhibiting protein biosynthesis there, one bacterial resistance mechanism is the production of a protein that protects the ribosomes. The second major means of defense is the synthesis of a cytoplasmic membrane protein that pumps tetracyclines out of the bacterial cell. The rarest resistance mechanism is chemical modification (acetylation) and thus inactivation of tetracyclines.

Control of animal foods for tetracycline residues.

Sample types studied:

• Animal tissues (meat, liver, kidney).
• Aquaculture (fish, shrimps, prawns)
• Milk
• Eggs
• Honey

Spectrum of investigation:

• Chlortetracycline
• Doxycycline
• Oxytetracycline
• Tetracycline
• 4-Epimers of Chlortetracycline, Oxytetracycline and Tetracycline (The epimers can be formed to varying degrees in the organism on the one hand, and also during analytical work-up on the other. Together with the respective parent substance they are marker residues according to Regulation (EU) 37/2010).

Investigation strategy and methods

In the Department of Veterinary Medicinal Products, Hormones and Contaminants of the Institute of Food Safety Vienna we analyze tetracyclines in food of animal origin and are also the National Reference Laboratory for these investigations.

Regular participation in international interlaboratory comparisons and in workshops of the EU Reference Laboratory (EURL) in Fougères (France), which is responsible for antibiotics, ensure the quality of the results and a continuous development of the analytical methods to the latest state of the art and research.

For rapid and cost-effective screening, tetracyclines are tested together with 8 other antibiotic groups (aminoglycosides, quinolones, ß-lactams, lincosamides, macrolides, phenicols, pleuromutilins and sulfonamides) by immunochemical methods in muscle and milk samples, and a tetracycline ELISA is used for egg samples. In the case of non-negative results, the samples in question are analyzed using a confirmatory method to identify and quantify the substances.

For honey samples, tetracyclines are analyzed together with 50 other antibiotics by LC-MSMS.

Legal basis

Commission Regulation (EU) No. 37/2010 of 22 December 2009 on pharmacologically active substances and their classification with regard to maximum residue limits in foodstuffs of animal origin (OJ EU No. L 15, 20.1.2010).