Antibiotic Disk Diffusion and the Misconception of Induced Resistance
Students perform a disk-diffusion test with a nonpathogenic strain of E. coli on Mueller-Hinton agar. They spread a lawn, place disks soaked in 10 µL of antibiotic A at 5 µg/mL, 10 µg/mL, and 20 µg/mL, plus a disk soaked in sterile water. After 24 h at 37°C, zones of inhibition are 8 mm, 13 mm, and 19 mm; the water disk has no zone. A second plate is inoculated with bacteria taken from the edge of the 20 µg/mL zone; after regrowth and a repeat test, the zone around 20 µg/mL is 11 mm. A student writes, 'The antibiotic made the bacteria mutate to become resistant; the higher dose made more mutations, which is why the zone grew back smaller.' The class has not measured mutation rates and did not sequence the before and after populations.
1. Critique the student's claim. Explain the zone sizes and the regrowth using selection rather than induced mutation.
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The claim confuses selection with induced mutation. The antibiotic does not create resistance mutations at a higher rate simply because the dose is higher; it kills susceptible cells and allows pre-existing resistant variants to survive and multiply. The disk creates a diffusion gradient, so a higher concentration extends farther above the minimum inhibitory concentration and produces a larger zone. When bacteria from the edge of the 20 µg/mL zone are regrown and retested, the smaller zone (11 mm versus 19 mm) indicates that a subpopulation with reduced susceptibility was selected. The water disk controls for the disk and handling, showing no inhibition by the vehicle. To demonstrate induction, one would need evidence of increased mutation frequency or a reversible physiological change, not merely survival after exposure.
2. Design a control to test whether resistance was present before exposure or induced by the antibiotic.
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Sample the original culture before any antibiotic exposure. Plate it on antibiotic-free medium, isolate individual colonies, and test a random sample against 20 µg/mL. If resistant isolates are already present at some frequency, resistance is pre-existing and the antibiotic only selected it. Include replicates and use the same inoculum density and growth conditions. A stronger design is a fluctuation test or a mutation-reporter assay comparing mutation frequency with and without sublethal antibiotic exposure; that directly tests whether the drug increases mutation rate. Also retain the before-exposure isolates for later comparison with the regrown resistant population. The key control is the unexposed ancestor population, not another antibiotic disk.
3. Why did the 20 µg/mL disk produce a larger zone than the 5 µg/mL disk? What limitation does that create for inferring resistance mechanism?
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Zone size depends on the amount of drug loaded, its diffusion coefficient and solubility, the inoculum density, and the organism's susceptibility. A 20 µg/mL disk creates a steeper and farther-reaching concentration gradient above the MIC, so inhibition extends farther than around the 5 µg/mL disk. That does not identify the resistance mechanism. A larger zone could reflect greater drug potency, better diffusion, or lower resistance, and the relationship between dose and zone diameter is not linear; it can plateau when solubility limits diffusion. Zone size cannot distinguish an efflux pump, a modified target, or reduced uptake. MIC testing, gene expression analysis, or biochemical assays would be needed to infer mechanism.