Moraxella catarrhalis

Etiology

Moraxella catarrhalis has long been considered to be an upper respiratory tract commensal. 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 lipooligosaccharide (LOS). In contrast to other gram-negative respiratory pathogens, such as Haemophilus influenzae and Neisseria meningitidis, the LOS of M. catarrhalis is relatively conserved among strains; only 3 serotypes (A, B, and C) 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 reinfection by new strains.

Epidemiology

The ecologic 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 as a result of 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 yr of life may range from 33–100%. 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 many countries has resulted in alteration of patterns of nasopharyngeal colonization in the population. A relative increase in colonization by nonvaccine pneumococcal serotypes, nontypeable H. influenzae, and M. catarrhalis has occurred. Whether changes in colonization patterns will result in a true increase in new episodes of otitis media and sinusitis caused by nontypeable H. influenzae and M. catarrhalis requires continuous surveillance.

Pathogenesis of Infection

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 reservoir for persistence in the human respiratory tract. As with many gram-negative bacteria, M. catarrhalis sheds vesicles from its surface during growth. These vesicles are internalized by respiratory epithelial cells and mediate several virulence mechanisms, including B-cell activation, induction of inflammation, and delivery of β-lactamases. Analysis of genomes reveals modest genetic heterogeneity among strains.

M. catarrhalis forms biofilms in vitro and in the middle ears of children with chronic and recurrent otitis media. Biofilms are communities of bacteria encased in a matrix attached to a surface. Bacteria in biofilms are more resistant to antibiotics and to host immune responses than bacteria growing individually in planktonic form.