Conjunctivitis is commonly caused by which bacterium?

• A. Staphylococcus aureus
• B. Moraxella catarrhalis
• C. Streptococcus pneumoniae
• D. Haemophilus influenzae

Answer: (D)

Conjunctivitis from bacteria often comes from organisms that colonize the upper respiratory tract and can spread to the eye. Haemophilus influenzae is a classic, frequently encountered cause, especially in children, and it tends to produce the mucopurulent discharge you’d expect with bacterial conjunctivitis. In the lab, this organism is a small gram-negative coccobacillus that requires both X (hemin) and V (nicotinamide adenine dinucleotide) factors to grow, usually on chocolate agar, which helps distinguish it from others.

While Staphylococcus aureus, Streptococcus pneumoniae, and Moraxella catarrhalis can also cause conjunctivitis, Haemophilus influenzae is commonly highlighted in foundational teaching as a typical bacterial eye pathogen, aligning with the usual clinical presentation in pediatric cases.