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Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Pseudomonadales
Family: Moraxellaceae
Genus: Moraxella

M. catarrhalis
M. bovis

Moraxella is a genus of Gram negative bacteria in the Moraxellaceae family. Moraxella catarrhalis is associated with human disease, being linked with respiratory tract infections. In the past, M. catarrhalis, which appears as pairs of cocci and is a gram negative bacterium that morphologically resembles Neisseria by Gram stain, was called Neisseria catarrhalis. However, unlike Neisseria, which grows on chocolate agar, M. catarrhalis grows on blood agar. (Note that up until ten years ago, M. catarrhalis was called Branhamella catarrhalis.)

Epidemiology and Disease

M. catarrhalis resides in the human nasopharynx. This bacterium causes similar symptoms to nontypable–Haemophilus influenzae (NTHI), although it is much less virulent. M. catarrhalis is the third most common bacterial cause of otitis media and sinusitis after Streptococcus pneumoniae and Haemophilus, respectively. In patients with pre-existing chronic lung disease, tracheobronchitis and pneumonia can ensue. Of further note, M. catarrhalis hardly ever causes bacteremia or meningitis, unlike Neisseria meningitidis, which is a morphologic cousin of M. catarrhalis (see above).

The species Moraxella bovis causes infectious bovine keratoconjunctivitis in cattle.

Host Response

The immune response to M. catarrhalis is likely mediated by an antibody to the outer membrane proteins of this organism. Any other immune mechanisms are unknown at this time.


The antibiotic susceptibility of M. catarrhalis is similar to NTHI. The majority of these organisms make beta-lactamase, an enzyme that renders beta-lactam antibiotics useless; thus, they are resistant to beta lactams such as penicillin. There currently exists no vaccine to M. catarrhalis, but number of outer membrane proteins are currently being studied for the development of such a vaccine.


Lecture notes of Dr. Daniel Musher, Baylor College of Medicine, Houston, Texas

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