Which antibiotic was discovered in 1928 by Sir Alexander Fleming due to a laboratory contaminant?

• A. Penicillin
• B. Streptomycin
• C. Tetracycline
• D. Erythromycin

Answer: (A)

The main idea here is how an antimicrobial substance was first found by observing an accidental contamination. In 1928, Sir Alexander Fleming saw a mold—Penicillium—growing on one of his Staphylococcus culture plates. The mold prevented bacteria from growing in the surrounding area, creating a clear zone of inhibition. From that observation, he identified the active compound as penicillin, the first true antibiotic. This discovery showed that microbes can produce substances that kill other microbes, and it paved the way for turning penicillin into a mass-produced life-saving drug, with later development by Florey and Chain. The other antibiotics listed were found later from different organisms and at later times, not from Fleming’s 1928 contamination.