Discovery
| Parameter | Value |
|---|---|
| Date | September 1928 |
| Location | St. Mary's Hospital, London |
| Discoverer | Alexander Fleming, Scottish bacteriologist |
| Prior art | Sulfonamides (1935, limited spectrum) |
| Mold | Penicillium notatum |
| Target bacterium | Staphylococcus aureus |
| Inhibition zone | ~2–3 cm diameter |
| Publication | British Journal of Experimental Pathology, 1929 |
| Key innovation | First antibacterial substance produced by a living organism |
Technical Explanation
Alexander Fleming, a Scottish bacteriologist at St. Mary's Hospital in London, returned from vacation in September 1928 to find that a Petri dish of staphylococci had been contaminated by a mold. Around the mold colony, a clear inhibition zone: the bacteria had vanished.
Before penicillin, bacterial infections killed on a massive scale: pneumonia, septicemia, wound infections. Sulfonamides (1935) offered limited protection, but no natural antibiotic was known.
1. Transpeptidase inhibition — Penicillin binds covalently to the enzyme transpeptidase (PBP, Penicillin-Binding Protein) that catalyzes the cross-links of peptidoglycan — the structural component of the bacterial cell wall.
2. Cell wall disintegration — Without cross-links, the peptidoglycan wall cannot withstand the internal osmotic pressure (~5–20 atm in bacteria). The cell swells and bursts (osmotic lysis).
3. Selectivity — Human cells have no peptidoglycan wall (they rely on a flexible plasma membrane). Penicillin is therefore toxic to bacteria but harmless to human cells — a remarkably selective mechanism of action. This exploits a fundamental structural difference between prokaryotic and eukaryotic cells.
4. Industrial production (1940–1943) — Howard Florey and Ernst Boris Chain at Oxford purified penicillin and demonstrated its efficacy in vivo. Mass production began in the US using deep-tank fermentation, scaling from a few milligrams to tons per month in time for D-Day (1944).
Why It Worked
The genius of penicillin is its selectivity: it exploits a fundamental structural difference between bacteria (peptidoglycan cell wall) and animal cells (no cell wall). This same principle — targeting a structure unique to the pathogen — still guides the development of all modern antibiotics. Fleming did not invent penicillin: he observed it and had the insight to recognize its significance.
Many researchers before him had likely seen inhibition zones on contaminated plates and discarded them. Fleming's contribution was asking "why?" instead of throwing the dish away.
Causal Chain
Fleming's discovery (1928) → Florey/Chain purification (1940) → Mass production for D-Day (1944) → Nobel Prize 1945 → Streptomycin (1943, tuberculosis) → All antibiotic classes → Modern surgery made possible → Antibiotic resistance (MRSA, 1960s) → Current AMR crisis
Anecdote
In his 1945 Nobel lecture, Fleming warned: "The time may come when penicillin can be bought by anyone. Then there is the danger that the ignorant man may easily underdose himself and, by exposing his microbes to non-lethal quantities of the drug, make them resistant." — a prophecy realized less than 80 years later. Today, the WHO lists antimicrobial resistance as one of the top 10 global health threats.
Legacy and Current Data
Penicillin paved the way for over 15 classes of antibiotics. But massive overuse (70% of global antibiotics are used in livestock farming) has driven resistance. The WHO estimates that antimicrobial resistance (AMR) causes ~1.27 million deaths per year (2019 data).
Sources
References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.