Which Gram-negative diplobacillus is a known cause of angular conjunctivitis?

• A. Moraxella catarrhalis
• B. Moraxella lacunata
• C. Neisseria meningitidis
• D. Haemophilus influenzae

Answer: (B)

Angular conjunctivitis is classically caused by Moraxella lacunata, a Gram-negative diplobacillus. The term diplobacillus describes two rod-shaped cells arranged in pairs, which is a key feature that helps distinguish this organism from others listed. Moraxella lacunata has long been associated with angular conjunctivitis (sometimes called Morax-Axenfeld diplobacillary conjunctivitis), presenting with redness and crusting at the canthi. The other organisms listed differ in shape: Moraxella catarrhalis and Neisseria meningitidis are Gram-negative diplococci, and Haemophilus influenzae is a small Gram-negative coccobacillus, not a diplobacillus. This morphological distinction, along with the classic site of infection, makes Moraxella lacunata the best answer.