**Question:** The following are true for Bordetella pertussis except?

A. It causes whooping cough
B. It is highly contagious
C. It produces a toxin
D. It is a Gram-positive bacterium

**Correct Answer:** D. It is a Gram-positive bacterium

**Core Concept:** Bordetella pertussis is a Gram-negative bacterium, which is a key feature that distinguishes it from Gram-positive bacteria like Staphylococcus and Streptococcus species. Understanding the classification of microorganisms based on their Gram staining characteristics is crucial for identifying infections and selecting appropriate antimicrobial treatments.

**Why the Correct Answer is Right:** Bordetella pertussis is a Gram-negative bacterium, meaning it possesses a lipid-rich cell wall that prevents the penetration of the primary stain (safranin) and allows the secondary stain (crystal violet) to diffuse into the cell, resulting in the characteristic "reddish-purple" (Gram-negative bacteria) appearance. In this case, the correct answer (Gram-positive) is wrong because B. pertussis is not Gram-positive.

**Why Each Wrong Option is Incorrect:**

A. It causes whooping cough: B. pertussis is indeed known to cause whooping cough, a respiratory illness characterised by paroxysms of coughing and the "whoop" sound heard when the affected person takes a deep breath after coughing fits.

B. It is highly contagious: B. pertussis is a highly contagious bacterium, transmitted from person to person through respiratory droplets, making it an important consideration in outbreaks and epidemics.

C. It produces a toxin: While B. pertussis does produce several virulence factors, it does not produce a specific toxin. Its pathogenesis involves the production of adhesins, iron acquisition systems, and evasion of host immune responses, but not a toxin.

**Clinical Pearl:**

Understanding the classification of microorganisms based on their cellular staining characteristics is essential for recognizing bacterial infections, understanding pathogenesis, and selecting appropriate antimicrobial treatments. Gram-negative bacteria are typically associated with severe infections and may require different antibiotics compared to Gram-positive bacteria.