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Chapter 188 Moraxella catarrhalis
Timothy F. Murphy
Moraxella catarrhalis, an unencapsulated gram-negative diplococcus, is a human-specific pathogen that colonizes the respiratory tract beginning in infancy. Colonization and infection with M. catarrhalis are increasing in countries in which pneumococcal conjugate vaccines are used widely. The most important clinical manifestation of M. catarrhalis infection in children is otitis media.
M. catarrhalis has long been considered to be an upper respiratory tract commensal. Initially named Micrococcus catarrhalis, its name was changed to Neisseria catarrhalis in 1970 because of its phenotypic similarities and similar ecological niche with commensal Neisseria species. On the basis of more modern analyses of genetic relatedness, Moraxella catarrhalis is now the accepted name.
Substantial genetic heterogeneity exists among strains of M. catarrhalis. Several outer membrane proteins demonstrate sequence differences among strains, particularly in regions of the proteins that are exposed on the bacterial surface. M. catarrhalis endotoxin lacks repeating polysaccharide side chains and is thus a lipo-oligosaccharide. In contrast to other gram-negative respiratory pathogens, such as Haemophilus influenzae and Neisseria meningitidis, the lipo-oligosaccharide of M. catarrhalis is relatively conserved among strains; only 3 serotypes (A, B, and C) that are based on oligosaccharide structure have been identified. Genetic and antigenic differences among strains account for the observation that resolving an infection by one strain does not induce protective immunity to other strains. M. catarrhalis causes recurrent infections, which generally represent re-infection by new strains.
The ecological niche of M. catarrhalis is the human respiratory tract. The bacterium has not been recovered from animals or environmental sources. Age is the most important determinant of the prevalence of upper respiratory tract colonization. Common throughout infancy, nasopharyngeal colonization is a dynamic process with active turnover due to acquisition and clearance of strains of M. catarrhalis. Some geographic variation in rates of colonization is observed. On the basis of monthly or bimonthly cultures, colonization during the 1st year of life may range from 33% to 100%. Several factors likely account for this variability among studies, including living conditions, daycare attendance, hygiene, environmental factors (e.g., household smoking), and genetics of the population. The prevalence of colonization steadily decreases with age. Understanding nasopharyngeal colonization patterns is important, because the pathogenesis of otitis media involves migration of the bacterium from the nasopharynx to the middle ear via the eustachian tube.
The widespread use of pneumococcal polysaccharide vaccines in some countries has resulted in alteration of patterns of nasopharyngeal colonization in the population. A relative increase in colonization by non-vaccine pneumococcal serotypes, nontypable H. influenzae, and M. catarrhalis has occurred. These changes in colonization patterns may account for the increased rates of otitis media due to nontypable H. influenzae and M. catarrhalis. Similar shifts in etiology are being observed in children with sinusitis as well.
Strains of M. catarrhalis differ in their virulence properties. The species is composed of complement-resistant and complement-sensitive genetic lineages, the complement-resistant strains being more strongly associated with virulence. Strains that cause infection in children differ in several phenotypic characteristics from strains that cause infection in adults, in whom the most common clinical manifestation is lower respiratory tract infection in the setting of chronic obstructive pulmonary disease.
The presence of several adhesin molecules with differing specificities for various host cell receptors reflects the importance of adherence to the human respiratory epithelial surface in the pathogenesis of infection. M. catarrhalis has long been viewed as an exclusively extracellular pathogen. However, the bacterium is now known to invade multiple cell types, including bronchial epithelial cells, small airway cells, and type 2 alveolar cells. In addition, M. catarrhalis resides intracellularly in lymphoid tissue, providing a potential reservoir for persistence in the human respiratory tract.
M. catarrhalis
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