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Moraxella catarrhalis is a gram-negative aerobic diplococcus that belongs to the Neisseraceae family. It has been known as Micrococcus catarrhalis, Niesseria catarrhalis, and Branhamella catarrhalis. It commonly inhabits the upper respiratory tract. For many years, it was considered a nonpathogenic member of the resident flora of the nasopharynx. Over the past 25 to 30 years, it has been recognized as a genuine mucosal pathogen and is now considered an important cause of otitis media and sinusitis in healthy children and adults. It also causes lower respiratory tract infections and exacerbation of bronchitis in adults with chronic lung disease. Occasionally, it can cause a variety of severe infections, including septicemia, pneumonia, and meningitis, especially in the immunocompromised hosts.


M catarrhalis is a normal inhabitant of the upper respiratory tract. Nasopharyngeal colonization rate is highest during infancy and early childhood and lowest in adulthood. Colonization rates of as high as 36% to 50% in infants and young children,1 and 1% to 3% in adults,2 have been reported. In one study of a large cohort of infants who were followed prospectively from birth to 2 years of age, 66% became colonized with M catarrhalis by 1 year and 77.5% by 2 years of age. In the same group, nasopharyngeal colonization increased from 27% during healthy visits to 63% on visits associated with otitis media.3 Other studies have shown that colonization of children varies with the season and is more common in fall and winter (46%) than in spring and summer (9%).4 Overall, colonization was higher in children with upper respiratory tract infection (36%) than in children without (18%), and was more common in children ages 24 months (32%) than in children older than 24 months (14%). Colonization with M catarrhalis is reported to be more common in asthmatic children than in normal children.

Factors influencing colonization and elimination are not fully understood, but adhesion to mucosal receptors and immune responses are implicated in addition to bacterial colonization dynamics. In general, potential pathogens are more likely to colonize the nasopharynx of children prone to recurrent otitis media, where impaired local immunity and repeated exposure to respiratory pathogens are additional risk factors.5Colonization appears to be an ongoing process with an elimination-colonization turnover of various strains.

The mode of transmission of the organism is presumed to be direct contact with contaminated respiratory tract secretions and/or droplet spread.


M catarrhalis is an aerobic gram-negative diplococcus that has a striking resemblance to meningococcus and gonococcus, except that it is unencapsulated. It grows well on blood and chocolate agars, forming small, opaque, gray-white nonhemolytic colonies. Recovery of the organism from the mixed flora of mucosal surfaces can be enhanced by using selective culture media such as modified Thayer-Martin or TV broth (Mueller-Hinton broth supplemented with trimethoprim and vancomycin).

After the nasopharynx ...

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