Q.75 The minimal inhibitory concentration (MIC) of an antibiotic X against Clostridium tetani,
Staphylococcus sp., Shigella sp., and Streptococcus sp. is 𝟐𝟓, 𝟏𝟓, 𝟐 and 𝟏𝝁 𝐠/𝐦𝐥, respectively. Assuming
that the bioavailable concentration of X in an animal model is 𝟐𝟎𝝁 𝐠/𝐦𝐥, which one of these bacteria
may develop resistance against X in the animal model?
(A) Clostridium tetani
(B) Staphylococcus sp.
(C) Shigella sp.
(D) Streptococcus sp.

The Minimal Inhibitory Concentration (MIC) determines the lowest antibiotic dose needed to inhibit bacterial growth, guiding treatment efficacy. In this scenario, antibiotic X’s bioavailable concentration of 20 μg/ml reveals which bacterium—Clostridium tetani, Staphylococcus sp., Shigella sp., or Streptococcus sp.—may develop resistance due to sub-optimal exposure.​

Key Concept: MIC vs Bioavailable Concentration

MIC values show antibiotic potency: higher MIC means lower sensitivity. A bioavailable concentration below MIC fails to fully inhibit growth, creating selective pressure for resistance mutants. Here, 20 μg/ml exceeds MICs for Shigella sp. (2 μg/ml) and Streptococcus sp. (1 μg/ml), but falls short for Clostridium tetani (25 μg/ml) and Staphylococcus sp. (15 μg/ml).​

Correct Answer and Explanation

Answer: (A) Clostridium tetani

Clostridium tetani’s MIC of 25 μg/ml exceeds the 20 μg/ml bioavailable level, allowing partial growth and favoring resistant strains. This sub-MIC exposure promotes resistance evolution, unlike fully inhibited bacteria.​

Analysis of All Options

• (A) Clostridium tetani (MIC 25 μg/ml): Bioavailable concentration (20 μg/ml) is below MIC, enabling survival and resistance selection—correct choice.

• (B) Staphylococcus sp. (MIC 15 μg/ml): 20 μg/ml surpasses MIC, inhibiting growth effectively and minimizing resistance risk.

• (C) Shigella sp. (MIC 2 μg/ml): Well above MIC at 20 μg/ml, ensuring strong inhibition with low resistance potential.

• (D) Streptococcus sp. (MIC 1 μg/ml): Highest margin above MIC (20 μg/ml), fully suppressing growth and resistance development.​