What is the most common bacterial cause of conjunctivitis in adults?

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

Answer: (D)

Staphylococcus aureus is the most common bacterial cause of conjunctivitis in adults because it’s a frequent inhabitant of the skin and eyelid margins and readily contaminates the ocular surface through eye rubbing, finger contact, or contact lens handling. This high rate of eyelid colonization makes it the leading culprit behind acute bacterial conjunctivitis, presenting with mucopurulent discharge and conjunctival injection that respond well to topical antibiotics that cover staph species.

Moraxella catarrhalis is a recognized cause of conjunctivitis as well, but it occurs less often overall. It tends to be seen more in older adults or in cases associated with blepharitis or angular conjunctivitis, rather than representing the most common pathogen across the adult population. Other common bacteria, such as Streptococcus pneumoniae and Haemophilus influenzae, also contribute to adult cases, but Staphylococcus aureus remains the predominant agent.