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Moraxella catarrhalis
Scientific classification
Kingdom: Bacteria Phylum: Proteobacteria Class: Gamma Proteobacteria Order: Pseudomonadales Family: Moraxellaceae Genus: Moraxella Species: M. catarrhalis
Binomial name
Moraxella catarrhalis

Moraxella catarrhalis is a gram-negative, aerobic, oxidase-positive diplococcus which may both colonise and cause respiratory tract-associated infection in humans.

M. catarrhalis was previously placed in a separate genus named Branhamella. The rationale for this was that other members of the genus Moraxella are rod-shaped and rarely caused infections in humans. However results from DNA hybridization studies and 16S rRNA sequence comparisons were used to justify inclusion of the species catarrhalis in the genus Moraxella. Consequently, the name Moraxella catarrhalis is currently preferred for these bacteria. Nevertheless, some in the medical field continue to call these bacteria Branhamella catarrhalis.

Clinically, these bacteria are known to cause bronchitis, sinusitis, laryngitis and otitis media. Elderly patients and long-term heavy smokers with chronic pulmonary disease should be aware that M. catarrhalis is associated with bronchopneumonia, as well as exacerbations of existing chronic obstructive pulmonary disease (COPD).

Treatment options include antibiotic therapy or a so-called "watchful waiting" approach. The great majority of clinical isolates of this organism produce beta-lactamases and are resistant to penicillin, with some suggestion of trimethoprim resistance in certain regions of the world. It is susceptible to trimethoprim-sulfamethoxazole (TMP-SMZ), fluoroquinolones, and most second and third generation cephalosporins. In addition, M. catarrhalis infection may be treated with erythromycin and amoxicillin-clavulanate (Augmentin®).

The peak rate of colonisation by M. catarrhalis appears to occur at approximately 2 years of age, with a striking difference in colonisation rates between children and adults (very high to very low).

Current research priorities involve trying to find a suitable vaccine for this genotypically diverse organism, as well as determining factors involved with virulence e.g. complement resistance.

• Murray, P; Rosenthal, K, Pfaller, M. (2005). Medical Microbiology, 5th ed., Mosby, p. 365. ISBN 0-323-03303-2. 

• Tierney, L. et al. (October 19, 2005). Current Medical Diagnosis and Treatment, 45th ed., McGraw Hill Medical, p. 1501. ISBN 0-07-145410-1.