Antibiotic Plates and a Claim About Learning
A class inoculates four agar plates with 0.1 mL of an E. coli culture containing about 1 × 10^7 cells. Plate 1 has no antibiotic and grows a continuous lawn. Plate 2 contains 1 µg/mL ampicillin and grows 12 colonies. Plate 3 contains 5 µg/mL and grows 3 colonies. Plate 4 contains 10 µg/mL and grows no colonies. The students then take cells from one colony on plate 3, streak them onto a new plate containing 10 µg/mL ampicillin, and incubate. After 24 hours, 8 colonies appear. A student concludes: 'The bacteria learned to survive the antibiotic and passed that ability to their offspring.' Another says: 'The ampicillin caused the mutations that let the bacteria grow on the new plate.' The teacher asks the class to explain the results using variation and selection, and to design a test of whether resistant cells existed before exposure.
1. Explain why 8 colonies grew on the 10 µg/mL plate after transfer, without saying the bacteria 'learned.'
Read the worked answer
The colony on plate 3 already grew at 5 µg/mL, so its cells were not killed by that concentration. When a sample of that colony is spread on 10 µg/mL, most cells may still die, but a few cells with higher tolerance survive and form the 8 colonies. The population already contained variation in resistance; the antibiotic did not teach the cells. The transfer simply enriched for the cells that could grow at the higher dose. 'Learned' implies a directed response acquired during exposure, but the surviving cells pass on existing genes, not a memory of the drug.
2. Evaluate the claim that ampicillin caused the mutations. What experiment would distinguish selection from induction?
Read the worked answer
The claim that ampicillin caused the mutations is not supported by this design. The antibiotic acts as a selective filter: cells that already carry resistance mutations survive, while susceptible cells die. To distinguish selection from induction, grow many independent cultures in the absence of antibiotic, then plate equal cell numbers on antibiotic and on plain agar. If resistant colonies appear on antibiotic plates without any prior exposure, the mutations existed before selection. A replica-plating or fluctuation test can also show that resistance arises randomly and at different times in different cultures.
3. Why does plate 1 grow a lawn while plate 4 grows nothing, and what does that tell you about the relationship between dose and resistance in this population?
Read the worked answer
Plate 1 has no antibiotic, so nearly all cells grow and form a lawn. Plate 4 has 10 µg/mL, which exceeds the resistance of every cell in the 10^7-cell sample, so no colonies appear. Plates 2 and 3 show that some cells tolerate 1 and 5 µg/mL, but tolerance is not uniform. The zero on plate 4 does not prove the species cannot evolve resistance; it means resistant cells were rarer than about 1 in 10^7 in that sample. A larger sample or repeated plating would be needed to estimate the frequency, and the 8 colonies from the transferred colony show that higher-level resistance can be selected.