The Antibiotic Disk and the 'Learning' Bacteria
In a lab, students model antibiotic resistance. They spread a non-pathogenic bacterial culture on four agar plates and apply disks: plate 1 has no antibiotic, plates 2-4 have increasing antibiotic concentrations. After 24 h, plate 4 has a few colonies growing near the disk. A student says 'the bacteria learned to resist the antibiotic because they were exposed.' Another says 'the antibiotic caused mutations that made them resistant.' The teacher notes the plates were incubated at 37 °C, but plate 4 was accidentally left at room temperature for 6 h during setup.
1. Identify the misconception in each student's statement and give the correct explanation.
Read the worked answer
Student 1 is using Lamarckian 'learning.' Bacteria do not learn or acquire resistance because they were exposed. Exposure selects; it does not instruct.
Student 2 is claiming directed mutation. The antibiotic did not cause the specific mutations that confer resistance. Mutations arise randomly and largely before exposure.
Correct explanation: the starting population contains variation. Some cells already carry traits that reduce the drug's effect. The antibiotic kills susceptible cells, and the resistant variants survive and reproduce. Resistance is inherited, so the next generation is enriched in resistant cells.
The classic test is the Luria-Delbrück fluctuation experiment, which showed that resistance arises in unselected cultures before the antibiotic is applied.
2. How could the class test whether resistance was present before antibiotic exposure?
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Replica plating: grow a lawn of bacteria on a non-selective plate, press a sterile velveteen pad onto it, then press the pad onto an antibiotic-containing plate. If resistant colonies appear at positions corresponding to colonies on the original plate, the resistance was present before exposure.
Fluctuation test: grow many independent small cultures without antibiotic, then plate each on antibiotic and on non-selective medium. If mutations are random and pre-existing, the number of resistant colonies will fluctuate widely among cultures. If the antibiotic induces resistance, the counts will be more uniform.
Controls: a no-antibiotic plate to measure total viable cells, and a plate with the same solvent but no drug. The judgment is that the experiment must separate selection from induction, not merely show that survivors exist.
3. The plate left at room temperature for 6 h has fewer colonies. How does this affect interpretation of resistance?
Read the worked answer
It is a confound: fewer colonies may reflect slower growth, not less resistance.
Incubation temperature and time strongly affect growth rate and colony formation. Plate 4 spent 6 h at room temperature, so its cells had less time at the optimal 37 °C. A count of colonies is therefore not comparable to the other plates.
To interpret resistance, the class needs a control plate with the same antibiotic concentration and the same room-temperature treatment, plus a non-selective plate to measure total viable cells under both conditions. They should standardize incubation and compare resistance frequencies, not raw colony counts.
The surviving colonies near the disk may still be genuinely resistant, but the quantification is invalid until the temperature confound is removed.