A chance discovery and a lost decade
In September 1928 Alexander Fleming returned from holiday to his laboratory at St Mary's Hospital, London, and noticed that a mould had grown on a dish of staphylococci — and that the bacteria around it had died. He named the substance penicillin and published in 1929, but could not purify it well enough to use as a medicine. A decade later Howard Florey, Ernst Chain and Norman Heatley at Oxford developed a purification method, cured infected mice in 1940 and in 1941 treated the first patient, police constable Albert Alexander. He improved but died when supplies ran out, showing the need for mass production. At the US government laboratory in Peoria, Illinois, a mould from a market cantaloupe boosted yields dramatically.
| Year | Milestone |
|---|---|
| 1928 | Fleming notices mould killing bacteria |
| 1941 | Florey and Chain treat the first patient |
| 1944 | US mass production supplies D-Day |
| 1945 | Nobel Prize to Fleming, Florey and Chain |
| 1945 | Dorothy Hodgkin solves its structure by X-ray |
How it works and its family
Bacteria rely on a tough cell wall of peptidoglycan to withstand osmotic pressure. Penicillin's four-membered beta-lactam ring binds to the enzymes that cross-link the wall (penicillin-binding proteins) and disables them, so the bacterium bursts. Human cells have no cell wall, which makes the drug relatively safe. After Dorothy Hodgkin determined its structure in 1945, chemists developed semi-synthetic penicillins. The family includes penicillin G and V, amoxicillin and ampicillin; cephalosporins are fellow beta-lactams. Penicillin remains first-line treatment for syphilis and strep throat.
Allergy and resistance
Penicillin allergy is commonly reported, yet many studies find that more than 90% of people with the label are not truly allergic when tested. A wrong label leads to broader-spectrum antibiotics, more side effects and more resistance, so allergy testing is worth discussing with a doctor. In his 1945 Nobel lecture Fleming warned that under-dosing would breed resistant microbes. Bacteria producing penicillin-destroying enzymes (penicillinases) duly spread, and strains such as methicillin-resistant Staphylococcus aureus (MRSA) are part of today's antibiotic resistance crisis. Take antibiotics exactly as prescribed and never use leftovers on your own.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- Is penicillin the same as an antibiotic?
- No. Penicillin is one kind of antibiotic — the first to be used widely — while "antibiotic" covers all drugs that kill or inhibit bacteria.
- If I'm allergic to penicillin, should I avoid amoxicillin?
- Yes, if the allergy is genuine, because amoxicillin is a penicillin. Old allergy labels are often inaccurate, though, so ask your doctor about testing.
Sources reviewed: 2026-10-26
References
- Fleming, A. (1929). On the antibacterial action of cultures of a penicillium, with special reference to their use in the isolation of B. influenzæ. British Journal of Experimental Pathology, 10(3), 226–236.
- Chain, E., Florey, H. W., et al. (1940). Penicillin as a chemotherapeutic agent. The Lancet, 236(6104), 226–228.
- Abraham, E. P., Chain, E., et al. (1941). Further observations on penicillin. The Lancet, 238(6155), 177–189.
- Abraham, E. P., & Chain, E. (1940). An enzyme from bacteria able to destroy penicillin. Nature, 146, 837.
- Crowfoot Hodgkin, D. (1949). The X-ray crystallographic investigation of the structure of penicillin. In Chemistry of Penicillin (pp. 310–367). Princeton University Press.
- Fleming, A. (1945). Penicillin. Nobel Lecture, 11 December 1945.
- Blumenthal, K. G., Peter, J. G., Trubiano, J. A., & Phillips, E. J. (2019). Antibiotic allergy. The Lancet, 393(10167), 183–198.
- Lobanovska, M., & Pilla, G. (2017). Penicillin's discovery and antibiotic resistance: Lessons for the future? Yale Journal of Biology and Medicine, 90(1), 135–145.