Merck Manual

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Moraxella catarrhalis Infection


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University;

Maria T. Vazquez-Pertejo

, MD, FACP, Wellington Regional Medical Center

Reviewed/Revised Sep 2022
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Moraxella catarrhalis is a gram-negative diplococcus that causes ear and upper and lower respiratory infections.

M. catarrhalis (formerly known as Branhamella catarrhalis) is a frequent cause of

The prevalence of M. catarrhalis colonization depends on age. About 1 to 5% of healthy adults have upper respiratory tract colonization. Nasopharyngeal colonization with M. catarrhalis is common throughout infancy, may be increased during winter months, and is a risk factor for acute otitis media; early colonization is a risk factor for recurrent otitis media. Substantial regional differences in colonization rates occur. Living conditions, hygiene, environmental factors (eg, household smoking), genetic characteristics of the populations, host factors, and other factors may contribute to these differences.

The organism appears to spread contiguously from its colonizing position in the respiratory tract to the infection site.

There is no pathognomonic feature of M. catarrhalis otitis media, acute or chronic sinusitis, or pneumonia. In lower respiratory disease, patients have increased cough, purulent sputum production, and increased dyspnea.

These gram-negative diplococci resemble Neisseria species but can be readily distinguished by routine biochemical tests after culture isolation from infected fluids or tissues.

All strains now produce beta-lactamase. The organism is generally susceptible to beta-lactam/beta-lactamase inhibitors, sulfamethoxazole, tetracyclines, extended-spectrum oral cephalosporins, aminoglycosides, macrolides, and fluoroquinolones.

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