Which of the following is a gram-negative bacterium?

• A. Bacillus anthracis
• B. Cornyebacterium diptheriae
• C. Haemophilus influenzae
• D. Clostridium botulinum

Answer: (C)

The key idea is the Gram stain result based on cell wall structure. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet dye, so they stay purple after the decolorization step. Gram-negative bacteria have a thinner peptidoglycan layer plus an outer membrane, so they do not retain the dye and take up the counterstain, appearing pink or red.

Among the listed organisms, Haemophilus influenzae is a Gram-negative bacterium; it typically appears as a small Gram-negative coccobacillus. The other organisms are all Gram-positive rods: Bacillus anthracis and Clostridium botulinum are spore-forming Gram-positive rods, and Corynebacterium diphtheriae is a Gram-positive rod as well.