Penicillins and cephalosporins are antibacterial substances that belong to the class of ß-lactam antibiotics. They have a four-membered ß-lactam ring in their structural formula (lactams are cyclic amides). These antibiotics are mostly produced by fungi. Some substances are still chemically modified (derivatized) to improve their properties as drugs and their resistance to defense mechanisms of bacteria. ß-lactam antibiotics inhibit the cell wall synthesis of bacteria, causing the cell to burst. In humans and animals, the target protein does not occur, so ß-lactam antibiotics are usually well tolerated by humans.

Penicillin was the first ß-lactam antibiotic to be discovered and extracted from cultures of the mold Penicillium notatum by the English bacteriologist Alexander Fleming in 1928. Around 1940, Howard Walter Florey and Ernst Boris Chain developed drugs based on penicillin, which were used therapeutically for the first time a year later. For this, together with Fleming, they received the Nobel Prize for Medicine in 1945. Cephalosporins were discovered in 1953. Today, ß-lactam antibiotics are mainly produced semisynthetically.

If ß-lactams are applied, they can enter the organism and thus animal foods.

For animal tissues (muscle, fat, liver and kidney) and milk, limits are listed in Commission Regulation (EU) No. 37/2010 of December 22, which establishes the classification of pharmacologically active substances with regard to maximum residue limits in food of animal origin. These are - as for most other medicinal products - significantly lower for milk than for animal tissues.

Over 2600 samples of carcasses and food are tested annually for residues of penicillins and cephalosporins under the National Residue Control Plan.

Mode of action and resistance

ß-lactam antibiotics act bactericidally by irreversibly inhibiting bacterial enzymes for peptidoglycan synthesis. The cell wall of bacteria consists of peptidoglycan (also called murein). They need this murein layer to withstand the high intracellular osmotic pressure. If peptidoglycan synthesis is disturbed, the cell bursts and lyses, and cell division can no longer take place. This metabolic process does not occur in animals or humans, so ß-lactam antibiotics are generally well tolerated by humans.

Bacteria defend themselves against ß-lactam antibiotics. They develop resistance, which can be assigned to three different strategies:

• ß-lactamases: there are more than 300 known variants of these bacterial enzymes that cleave the ß-lactam ring and thus inactivate ß-lactam antibiotics. There are penicillinases and cephalosporinases.
• Decrease in permeability of the outer cell membrane so that peptidoglycan synthesis occurring within cannot be inhibited.
• Alteration of bacterial enzymes for peptidoglycan synthesis, thereby reducing their sensitivity to ß-lactam antibiotics.

To make ß-lactam antibiotics resistant to bacterial resistance mechanisms, the antibiotics are chemically altered to prevent attack by ß-lactamases or co-administered with ß-lactamase inhibitors such as clavulanic acid.

Metabolism

Some foreign substances (e.g., also drugs) are chemically modified in the organism by enzymes. If this occurs rapidly, not only the original drugs (parent substances) but also their metabolites are defined as marker residues in Regulation (EU) 37/2010 and maximum residue limits are set for them.

In the case of ß-lactams, this is the case for the two cephalosporins cefapirin (marker residues: sum of cefapirin and desacetylcefapirin) and ceftiofur (marker residues: sum of all residues containing the ß-lactam ring and measured as desfuroylceftiofur) as well as for the penicillin penethamate (is an ester of benzylpenicillin; marker residue: benzylpenicillin).

Analyses

In the Department of Veterinary Drugs, Hormones and Contaminants of the Institute of Food Safety Vienna, we analyze ß-lactam antibiotics 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.

Screening

ß-Lactams are screened together with 8 other antibiotic groups (aminoglycosides, quinolones, lincosamides, macrolides, phenicols, pleuromutilins, sulfonamides and tetracyclines) by immunochemical methods in muscle and milk samples.

In case of non-negative results, the samples in question are analyzed using a confirmatory method (HPLC with mass spectrometric detection; LC-MSMS) to identify and quantify the substances.