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; 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 gonorrhea and syphilis. However, they don’t work well against most gram negative bacteria.