Moraxella catarrhalis is a gram-negative, aerobic, oxidase-positive diplococcus that was first described in 1896. The organism has also been known as Micrococcus catarrhalis, Neisseria catarrhalis, and Branhamella catarrhalis; currently, it is considered to belong to the subgenus Branhamella of the genus Moraxella. For most of the 20th century, M catarrhalis was considered a saprophyte of the upper respiratory tract that was associated with no significant pathogenic consequences.
Various diagnostic studies and procedures may be warranted, depending on the site of the infection and underlying conditions. Confirmation of the diagnosis of M catarrhalis infection is based on culture. Any of a number of antimicrobial drugs may be used to treat M catarrhalis infection, depending on the need for use of oral or parenteral medication, the age of the patient, any underlying conditions present, the sensitivity of the organism, and the desired spectrum of coverage.
Pathophysiology and Etiology
Studies have shown that M catarrhalis colonizes the upper respiratory tract in 28-100% of humans in the first year of life. In adults, the colonization rate is 1-10.4%. Colonization appears to be an ongoing process with an elimination-colonization turnover of various strains. Transmission is believed to be due to direct contact with contaminated secretions by droplets.
The endotoxin of M catarrhalis, a lipopolysaccharide similar to those found in Neisseria species, may play a role in the disease process. Some strains of M catarrhalis have pili or fimbriae, which may facilitate adherence to the respiratory epithelium. Some strains produce a protein that confers resistance to complement by interfering with the formation of the membrane attack complex. M catarrhalis also expresses specific proteins for iron uptake that act as receptors for transferrin and lactoferrin.
M catarrhalis has been shown to have increased cell adhesion and proinflammatory responses when cold shock (26°C for 3 hours) occurs. Physiologically, this may occur with prolonged exposure to cold air temperatures, resulting in coldlike symptoms. 
Humoral responses against M catarrhalis appear to be age-dependent, with the titer of immunoglobulin G (IgG) gradually increasing in children. Antibody responses to outer-membrane proteins have been obtained, predominantly in the IgG3 subclass.
Although the commensal status of M catarrhalis in the nasopharynx is still accepted, the organism is a common cause of otitis media and sinusitis and an occasional cause of laryngitis. M catarrhalis causes bronchitis and pneumonia in children and adults with underlying chronic lung disease and is occasionally a cause of bacteremia and meningitis, especially in immunocompromised persons. Bacteremia can be complicated by local infections, such as osteomyelitis or septic arthritis. M catarrhalis is also associated with nosocomial infections.
United States statistics
M catarrhalis is the third most common cause of otitis media and sinusitis in children (after Streptococcus pneumoniae and Haemophilus influenzae). M catarrhalis is estimated to be responsible for 3-4 million cases of otitis media annually, with an associated health care cost (direct and indirect) of $2 billion each year.
M catarrhalis infections may occur at any age. Although colonization is more common in children, only a small percentage of positive cultures findings have clinical significance in the pediatric population. In one study, 9% of cultures positive for M catarrhalis in children younger than 5 years and 33% of isolates from children aged 6-10 years were found to be clinically significant. However, all cultures positive for M catarrhalis had clinical importance in adults.
In one study involving adult patients, the male-to-female ratio was 1.6:1.
The prognosis of M catarrhalis infection is poor in hospitalized patients with underlying conditions, especially the following:
Patients hospitalized for prolonged periods
Patients in pulmonary units or pediatric intensive care units
Patients of advanced age
The most significant infections caused by M catarrhalis are upper respiratory tract infections (URTIs) such as otitis media and sinusitis in children and lower respiratory tract infections (LRTIs) in adults. Infections with M catarrhalis in adults are more common if underlying conditions are present, especially if the patient is elderly. In a study of 42 cases of pneumonia with M catarrhalis isolated as the single agent in sputum cultures, the mortality rate attributable to the underlying problems within 3 months of pneumonia was 45%. 
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