Disk Diffusion and the Language of Resistance
In a 10th-grade biology lab, students run a disk-diffusion assay with nonpathogenic E. coli K-12 on Mueller-Hinton agar. Each pair places a 10-µg ampicillin disk and a 30-µg ampicillin disk on one plate. After 24 hours at 37°C, the class means are 12 mm and 19 mm. One student says the bacteria learned to resist the 10-µg disk because they had to survive. Another says the 30-µg disk is stronger, so concentration is the only variable that matters. The teacher then has students streak growth from the margin of the 10-mm zone into broth containing 10 µg/mL ampicillin; after 48 hours the broth is turbid. A subculture grows on agar with 20 µg/mL ampicillin but not on 40 µg/mL. The teacher candidate must use this sequence to teach natural selection without implying intentional mutation or that the antibiotic created resistance on demand.
1. Explain what the zone-diameter and serial-passage results do and do not demonstrate about resistance. Include the role of pre-existing variation and the limits of these data.
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What the data support: The 12-mm and 19-mm zones show that ampicillin inhibits growth and that the larger dose produces more inhibition under these conditions. They do not show that the bacteria learned, chose to mutate, or were instructed by the antibiotic.
What the passage experiment adds: Streaking from the zone margin recovered cells that survived at the edge of 10 µg/mL inhibition. Growth at 20 µg/mL but not 40 µg/mL indicates a low-level resistant subpopulation was enriched, not that resistance is unlimited. The antibiotic acted as a selective filter.
Limits: There is no replicate count, no MIC, no genetic identification of a resistance allele, and no control for inoculum density. Zone size also depends on diffusion, agar depth, and incubation time. A defensible claim is: the population contained pre-existing variation in susceptibility; exposure reduced susceptible cells and allowed less-susceptible cells to reproduce. The candidate should not say the antibiotic caused a mutation.
2. Design a short instructional move of 5–8 minutes that surfaces and corrects the bacteria learned misconception. Identify the exact student reasoning you want to make public.
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Make the reasoning public: Ask pairs to write a one-sentence prediction for what will happen when the zone-margin cells are regrown. Have two pairs read predictions; label one as instruction/learning and one as selection. Then run a four-minute bead model: a bag contains 40 white beads (susceptible) and 4 red beads (resistant). Students remove all white beads when ampicillin is added, leaving red beads to reproduce. Ask: did the ampicillin turn white beads red? This forces the contrast.
Discussion move: Require students to use three phrases: pre-existing variation, differential survival, and differential reproduction. Reject the phrase the bacteria needed to and ask for a mechanistic replacement. Close by returning to the 20 µg/mL result: the red beads increase in frequency, but the population is not necessarily resistant to 40 µg/mL. This move surfaces the Lamarckian reasoning and replaces it with a selection model.
3. A multiple-choice item asks: Why do bacteria become resistant to antibiotics? The keyed answer is: They mutate in order to survive. Rewrite the stem and key so the item assesses natural selection accurately, and explain what the distractors should diagnose.
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Original flaw: The key is Lamarckian and implies directed mutation.
Rewritten item: A patient is treated repeatedly with ampicillin. Before treatment, a few bacteria in the population have a mutation that reduces drug binding. After several courses, resistant bacteria are common. Which statement best explains this increase?
Key: Ampicillin kills susceptible bacteria, so bacteria with the pre-existing resistance mutation survive and reproduce more.
Distractors: The antibiotic causes bacteria to develop the mutation they need; bacteria learn to resist after exposure and pass the learned trait to offspring; all bacteria become resistant at the same rate because the drug is the same. These distractors diagnose the misconception rather than reward a memorized phrase. The item now requires using the selection mechanism in a context, not recalling that mutations exist.