Antibiotics - Penicillins

Penicillins are antibiotics that got their name from the Penicillium mold, from which they were originally extracted. They belong to the pharmacological group of beta-lactam antibiotics, because they have a beta-lactam ring in their structure. Penicillins are used to treat a wide range of infections, including streptococcal infections, like pharyngitis, tonsillitis, scarlet fever, and endocarditis; as well as pneumococcal infections; staphylococcal infections; diphtheria; anthrax; and syphilis.

Now, to build their cell walls, bacteria need an enzyme called DD-transpeptidase, or penicillin binding protein, or PBP for short. Penicillins, like all beta lactam antibiotics, bind to this enzyme thanks to their beta-lactam ring, and prevent it from working.

Now, some bacteria have developed resistance to beta lactam antibiotics. The most notable is Staphylococcus aureus, which has developed an enzyme called beta-lactamase or penicillinase, that breaks down the beta-lactam ring within the antibiotic, rendering it ineffective.

So, penicillins are further classified into four groups: basic penicillins, broad-spectrum or aminopenicillins, penicillinase-resistant or antistaphylococcal penicillins, and extended-spectrum or antipseudomonal penicillins.

Basic penicillins include penicillin V, which is given orally, and penicillin G, which is administered intramuscularly or intravenously. In addition, there’s a specific penicillin G called penicillin G benzathine, which is a long-acting penicillin that’s only administered intramuscularly. These are quite effective against common gram-positive bacteria, so they’re used to treat upper respiratory infections, otitis media, pneumonia, rheumatic fever, erysipelas, skin and soft-tissue infections, and STIs like syphilis. However, they don’t work well against most gram-negative bacteria.