Which of these unexpected observations led Alexander Fleming to discover penicillin, one of science's most transformative products?

While many scientific discoveries have changed the world, some products transformed medicine forever. Penicillin revolutionized healthcare by introducing the first effective antibiotic treatment, dramatically reducing deaths from bacterial infections. This discovery marks one of the most significant product developments in scientific history, saving countless lives since its introduction to mainstream medicine.

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Noticing that mold contamination on a culture plate had killed surrounding bacteria
Observing that certain plants prevented infection when applied to wounds
Analyzing blood samples from patients who seemed naturally resistant to bacterial infections
Testing thousands of chemical compounds systematically in a targeted drug development program