Alexander Fleming returned from a two-week holiday in September 1928 to find that a stray mold had contaminated one of his laboratory petri dishes. What observation about that dish led him to discover penicillin?
- A ring of dead staphylococcus bacteria surrounded the mold, showing that something the mold produced was killing nearby bacteria
- The bacteria appeared to grow only where the mold was absent, suggesting the mold's presence had simply made the growing conditions worse everywhere else
- The mold changed the color of the culture medium as it fed on its nutrients, and Fleming used that color shift, not the bacteria's behavior, to flag the dish for study
- The mold caused the dish's gelatin-based growth medium to liquefy, and Fleming inferred from the liquefied texture that a new enzyme must be present
Why A? And why not the others?
Correct answer: A. A ring of dead staphylococcus bacteria surrounded the mold, showing that something the mold produced was killing nearby bacteria
Fleming noticed a clear, bacteria-free zone in the ring immediately surrounding the mold, meaning something the mold itself was releasing actively killed the staphylococcus colonies nearby; he isolated the mold, identified it as Penicillium, and this became the basis for the first antibiotic. The option describing bacteria simply failing to grow near the mold because conditions were generally worse is a tempting near-miss because it gets the spatial pattern right but misses the key point Fleming drew from it: the effect was a specific antibacterial substance being produced, not a generic decline in growing conditions. The option describing a color change used to flag the dish is wrong because it was the bacteria's death around the mold, not any color shift, that caught Fleming's attention. The option describing a liquefied growth medium is wrong because no such texture change is part of the discovery; it invents a detail that never happened.
Source: Britannica: Alexander Fleming