A bacterial strain is tested for susceptibility to antibiotic X. In a broth‑dilution series the starting concentration is $128$ µg·mL⁻¹ and two‑fold serial dilutions are prepared down to $0.5$ µg·mL⁻¹. After incubation, turbidity (growth) is observed in tubes containing $\le 4$ µg·mL⁻¹, while tubes with $4$ µg·mL⁻¹ are clear. In a disc‑diffusion assay the following zone diameters are recorded: 8 mm at 2 µg disc, 12 mm at 4 µg, 18 mm at 8 µg, 22 mm at 16 µg. (a) State the minimum inhibitory concentration (MIC) from the broth test. (b) Assuming the relationship $D = a - b\log_{10}C$ (where $D$ is zone diameter in mm and $C$ is disc concentration in µg), determine the constants $a$ and $b$ using the data for 2 µg and 8 µg discs. (c) Predict the zone diameter for a 32 µg disc. (d) Comment on the consistency between the two methods.

Question

A bacterial strain is tested for susceptibility to antibiotic X. In a broth‑dilution series the starting concentration is $128$ µg·mL⁻¹ and two‑fold serial dilutions are prepared down to $0.5$ µg·mL⁻¹. After incubation, turbidity (growth) is observed in tubes containing $\le 4$ µg·mL⁻¹, while tubes with >$4$ µg·mL⁻¹ are clear. In a disc‑diffusion assay the following zone diameters are recorded: 8 mm at 2 µg disc, 12 mm at 4 µg, 18 mm at 8 µg, 22 mm at 16 µg. (a) State the minimum inhibitory concentration (MIC) from the broth test. (b) Assuming the relationship $D = a - b\log_{10}C$ (where $D$ is zone diameter in mm and $C$ is disc concentration in µg), determine the constants $a$ and $b$ using the data for 2 µg and 8 µg discs. (c) Predict the zone diameter for a 32 µg disc. (d) Comment on the consistency between the two methods.

Accepted Answer

MIC = 4 µg·mL⁻¹; a = 3.00, b = -16.61; predicted zone for 32 µg = 28.0 mm; the two methods give consistent results.

Neco Model Questions Vol1 2025 Biology Question 34

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