Penicillins and cephalosporins

Changed on: 10.05.2019

Beta-lactam antibiotics (ß-lactam antibiotics) are substances which have an anti-bacterial effect and a four-membered beta-lactam ring in their structural formula (lactams are cyclic amides). They are mostly produced from fungus, some are then semi-synthetically derived in order to improve their pharmacokinetic qualities and their resistance to beta-lactamases.

beta-lactam antibiotics can be subdivided into two major groups:

penicillins (basic structure: 6-aminopenicillanic acid) and
cephalosporins (basic structure: 7-aminocephalosporanic acid).

The first beta-lactam antibiotic to be discovered was Penicillin, found by the English bacteriologist Alexander Fleming in 1928 in cultures taken from the mould Penicillium notatum and extracted from the same. Around 1940, Howard Walter Florey and Ernst Boris Chain developed medicines based on penicillin, which were used therapeutically for the first time a year later. For this they received the 1945 Nobel Prize for Medicine together with Fleming. Cephalosporins were discovered in 1953. Nowadays, beta-lactam antibiotics are mostly produced semi-synthetically.

Beta-lactam antibiotics (ß-lactam antibiotics) are substances which have an anti-bacterial effect and a four-membered beta-lactam ring in their structural formula (lactams are cyclic amides). They are mostly produced from fungus, some are then semi-synthetically derived in order to improve their pharmacokinetic qualities and their resistance to beta-lactamases.

beta-lactam antibiotics can be subdivided into two major groups:

penicillins (basic structure: 6-aminopenicillanic acid) and
cephalosporins (basic structure: 7-aminocephalosporanic acid).

The first beta-lactam antibiotic to be discovered was Penicillin, found by the English bacteriologist Alexander Fleming in 1928 in cultures taken from the mould Penicillium notatum and extracted from the same. Around 1940, Howard Walter Florey and Ernst Boris Chain developed medicines based on penicillin, which were used therapeutically for the first time a year later. For this they received the 1945 Nobel Prize for Medicine together with Fleming. Cephalosporins were discovered in 1953. Nowadays, beta-lactam antibiotics are mostly produced semi-synthetically.

Mode of action and resistance

Mode of action and resistance

Beta-lactam antibiotics are bactericidal and act by irreversibly inhibiting the synthesis of the peptidoglycan layer in bacterial enzymes. The cell wall of a bacterium consists of peptidoglycan (also known as murein). They need this murein layer in order to be able to to withstand the high intracellular osmotic pressure. If the synthesis of the peptidoglycan is disturbed, the cell bursts leading to lysis and no further cell division can take place. This metabolic process does not occur in humans nor animals and consequently beta-lactam antibiotics are generally well tolerated by humans.

Bacteria defend themselves against beta-lactam antibiotics. They form resistance types that can be allocated to three different strategies:
Beta-lactamases: there are more than 300 known varieties of these bacterial enzymes, which break the beta-lactam ring thus deactivating the beta-lactam antibiotic. There are penicillinases and cephalosporinases.
Reduction of external cell membrane permeability so that the peptidoglycan synthesis occurring within cannot be inhibited.
Alteration of bacterial enzymes for peptidoglycan synthesis thus reducing their sensitivity toward beta-lactam antibiotics.

In order to make beta-lactam antibiotics resistant to the resistance mechanisms of the bacteria, the antibiotics are modified chemically thus preventing attack from beta-lactamases or they are administered together with beta-lactamase inhibitors such as clavulanic acid.

Statutory provisions

Statutory provisions

For animal tissue (muscle, fat, liver and kidneys) and for milk, limit values are listed in Commission Regulation (EU) No. 37/2010 dated 22 December in which the classification of pharmacologically active substances regarding maximum residue limits in foodstuffs of animal origin is set out: For milk, these are - as is the case with most other medicines - considerably lower than for animal tissue.

Metabolism

Metabolism

Some foreign substances (e.g. medicines) are modified chemically in the organism by enzymes. If this occurs quickly, not only the original medications (parent substances), but also their metabolites are defined as marker residues in Regulation (EU) 37/2010 and cumulative limits are set for these.
Where beta-lactams are concerned, this is the case for both cephalosporins cephapirin (marker residues: sum of cephapirin and desacetyl cephapirin) and ceftiofur (marker residues: sum of all residues containing the beta-lactam ring and measured as the metabolite desfuroylceftiofur) as well as for the penicillin penethamat (is an ester of benzylpenicillin; marker residue: benzylpenicillin).

Analyses

Analyses

The Department of Veterinary Drugs, Hormones and Contaminants at the Institute for Food Safety in Vienna analyses beta-lactam antibiotics in foodstuffs of animal origin and is also the National Reference Laboratory for these tests.
Regular participation in international inter-laboratory tests and in workshops held by the EU Reference Laboratory (CRL) responsible for antibiotics in Fougères (France) ensure the quality of the result and a constant further development of the analytical methods keeping them on the cutting edge of technology and research.

Screening

Beta-lactams are screened sensitively, quickly and cheaply using microbiological inhibitor tests. In the case of milk this is done using the Delvo-Test and for animal tissue (muscle, kidney, liver) using the STAR protocol, five-plate test (Screening Test for Antibiotic Residues).

Since the five-plate test (STAR protocol) is used to not only test beta-lactams but antibiotics from a whole variety of groups, a positive result generally leads to post-screening using the Charm II-Test and/or HPLC, in order to be able to allocate the antibacterial activity to one of the following groups of antibiotics: beta-lactams, macrolides, aminoglycosides, tetracyclines, quinolones or sulphonamides.

If in the case of milk, the Delvo-Test or in the case of animal tissue the Charm II-Test return a positive result for beta-lactams, a confirmation method using LC/MS is employed for the purpose of identification and quantification. This encompasses the following substances:

Penicillins:
Amoxicillin
Ampicillin
Cloxacillin
Dicloxacillin
Nafcillin
Oxacillin
Penicillin G (Benzylpenicillin)
Penicillin V (Phenoxymethylpenicillin)

Cephalosporins:
Cephalexin (Cefalexin)
Cephalonium (Cefalonium)
Cephapirin (Cefapirin) and its metabolite desacetyl cephapirin
Cefazolin (Cefazoline, Cephazoline)
Cefoperazone (Cefabid)
Cefquinome
Ceftiofur and its metabolite Desfuroylceftiofur

Screening test in animal tissue are conducted at the AGES Institutes for Veterinary Disease Control in Graz, Innsbruck, Linz and Mödling and at the State Institute for Veterinary Examinations in Klagenfurt.

Screening of milk, post-screening of animal tissue and confirmatory analyses in all matrices are carried out at the Department of Veterinary Drugs, Hormones and Contaminants at the Institute for Food Safety in Vienna.

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