Alexander Fleming — "The mere fact that a substance has bactericidal powers does not mean that it can…"
The mere fact that a substance has bactericidal powers does not mean that it can be used for the treatment of septic infections.
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More from Alexander Fleming
"The greatest reward for a scientist is the advancement of knowledge."
"The public looks for miracles. We scientists look for facts."
"I found penicillin and have given it free for the benefit of humanity. Why should it become a profit-making monopoly of manufacturers in another country?"
"The mold was there, I just saw it."
"We must be careful not to create a race of penicillin-resistant superbugs."
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Understanding this quote
What it means
Killing bacteria in a test tube is not the same as curing an infection in a living patient. A substance may destroy bacteria under controlled conditions yet fail clinically because it breaks down inside the body, cannot reach the infection site, damages healthy tissue, or loses potency in blood serum. Laboratory results must be validated through rigorous biological testing before any compound can be trusted as a real treatment.
Relevance to Alexander Fleming
Fleming discovered lysozyme in 1922 — an enzyme in tears and mucus that kills bacteria — but it couldn't penetrate deep tissues and proved clinically useless against most infections. That experience hardened his skepticism. When penicillin mold killed bacteria on his plate in 1928, he again stopped short of declaring a cure. His caution proved correct: it took Florey and Chain another decade to solve stability and dosing before penicillin became medicine.
The era
In the 1920s–40s, bacteriologists were testing hundreds of compounds — coal tar dyes, heavy metals, antiseptics — that killed bacteria in vitro but poisoned patients or degraded inside the body. Paul Ehrlich's 'magic bullet' concept drove frenzied, often reckless experimentation. Septic infections from war wounds and routine surgery remained a leading cause of death. Fleming's warning was a direct corrective to widespread premature clinical optimism that had repeatedly wasted resources and harmed patients.
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