β-lactams: penicillins

β-lactams: penicillins

Penicillins have an antibacterial action on dividing bacteria. By binding to the penicillin binding-protein they inhibit the transpeptidation of peptidoglycans. Autolytic enzymes cut the bounds between the peptidoglycans. This causes disruption of the bacterial cell wall leading to an influx of water.

Resorption of penicillins after oral administration displays huge differences. Most of the penicillins are distributed over the extracellular fluid; only in high concentrations penicillins can penetrate the blood-brain-barrier (e.g. necessary to treat meningitis). Penicillins have only few side effects: bone marrow depression, granulocytopenia, hepatitis. Some patients are allergic to penicillins. Adverse effects such as neurotoxicity and nephrotoxicity are displayed when high doses are applied. Penicillins (penicillin G, benzylpenicillin) are commonly used for treatment Gram-positive cocci and rods and Gram-negative cocci. Streptococcal, pneumococcal, and meningococcal infections are sensitive to penicillins. Penicillins are excreted via the kidneys.

Aminopenicillins (amoxicillin) are destroyed by β-lactamases. They are used in upper respiratory tract infections, urinary tract infections, meningitis, and salmonella infections. Their main adverse effects include hypersensitivity reactions (rash, bronchospasm, fever). In order to overcome the action of β-lactamases, the combination amoxicillin-clavulanic acidcan be applied. Clavulanic acid is an inhibitor of β-lactamases and thus prevents breakdown of amoxicillin.

Flucloxacillin is insensitive to β-lactamases and is able to act throughout acid conditions. It has a small spectrum, mainly covering S. aureus.


Benzylpenicillin is the drug of choice for infections with Gram-negative bacteria. 


The release of autolytic enzymes by penicillins is an important step in bacterial cell death. 


Resistance to benzylpenicillin from pneumococci is caused by β-lactamases.


Resistance to benzylpenicillin from Staphylococcus aureus is caused by β-lactamases.