penicillin
Penicillin is the original beta-lactam antibiotic and the drug that launched the modern antibiotic era. Discovered by Alexander Fleming in 1928 from a Penicillium mold, it was the first medicine that could reliably cure deadly bacterial infections, and it still treats illnesses like strep throat and syphilis today.
Chemically, penicillin is a beta-lactam ring fused to a five-membered thiazolidine ring, with a side chain that tunes its spectrum. It works by binding penicillin-binding proteins — the transpeptidase enzymes that cross-link the bacterial cell wall — and acylating their active-site serine. With cross-linking blocked, growing bacteria can no longer hold their wall together and lyse, which makes penicillin bactericidal against susceptible bugs.
Penicillin's enormous selective toxicity comes from a simple fact: human cells have no peptidoglycan wall, so the target is absent in us. Its real limits are honest ones — many people are allergic, the narrow-spectrum forms miss Gram-negative bacteria, and widespread beta-lactamase production has made plain penicillin useless against many strains it once cured.
Penicillin G remains a first-line, highly effective treatment for syphilis, an infection that has never developed meaningful resistance to it.
A nearly century-old drug still indispensable for one infection.
Penicillin allergy is over-reported: many people labelled allergic are not truly so, and unnecessary avoidance pushes prescribers toward broader, less ideal antibiotics.