Moraxella catarrhalis is a bacterium recognized as a significant cause of respiratory infections, particularly in vulnerable populations. It often resides harmlessly in the human upper respiratory tract but can transition into an opportunistic pathogen. While it can colonize without causing illness, it is responsible for common infections like middle ear infections and more serious lower respiratory tract diseases. Understanding its characteristics and required treatment is important for managing its public health impact.
The Organism and Its Role
The bacterium Moraxella catarrhalis is a nonmotile, Gram-negative, aerobic diplococcus, appearing as spherical pairs under a microscope. Historically, it was considered a relatively harmless commensal residing in the mucous membranes of the nose and throat. Colonization rates are extremely high in children; up to 100% of infants carry the organism in their upper respiratory tract during their first year of life.
The prevalence of colonization decreases sharply in healthy adults, who show carriage rates between 1% and 10%. M. catarrhalis transitions to a pathogen when local defenses are compromised, often following a viral infection. It uses virulence factors, such as adhesins, to stick to host cells and forms biofilms that protect it from the immune system. This opportunistic behavior means it primarily causes disease in young children and adults with underlying chronic respiratory conditions.
Common Manifestations and Symptoms
In pediatric populations, M. catarrhalis is a leading bacterial cause of upper respiratory tract infections, particularly acute otitis media (AOM) and sinusitis. It is the third most common bacterial cause of AOM in children, accounting for an estimated 15% to 20% of cases. Symptoms of AOM include ear pain, fever, and sometimes fluid drainage from the ear. The infection causes inflammation and pressure in the middle ear space.
The organism also frequently causes acute bacterial rhinosinusitis, commonly known as a sinus infection. Sinusitis symptoms in both children and adults include headache, facial pain or pressure, and a persistent cough. In younger children, this often presents as a persistent nasal discharge lasting longer than two weeks.
In adults, M. catarrhalis is primarily associated with lower respiratory tract infections (LRTIs), especially in those with pre-existing conditions. It is the second most common bacterial cause of acute exacerbations of chronic obstructive pulmonary disease (COPD). Exacerbations are marked by increased cough, changes in the color or amount of sputum, and worsening shortness of breath (dyspnea). The bacterium can also cause pneumonia and bronchitis in adults who are immunocompromised or have chronic lung diseases like cystic fibrosis.
Diagnosis and Management
Diagnosis typically begins with clinical suspicion based on the patient’s symptoms and history, such as AOM in a child or a COPD exacerbation in an adult. Confirmation relies on microbiological methods, often involving culturing a sample from the site of infection (e.g., middle ear fluid or sputum). The organism is a Gram-negative diplococcus that grows well on standard media, though it can be overlooked due to its resemblance to non-pathogenic Neisseria species.
Treatment is complicated by the widespread ability of M. catarrhalis to resist common antibiotics. Almost all clinical strains (approximately 95%) produce the enzyme beta-lactamase. This enzyme breaks down the beta-lactam ring structure found in antibiotics like penicillin and amoxicillin, rendering them ineffective.
Due to this high resistance rate, standard treatment avoids using penicillin and amoxicillin alone. First-line therapy involves antibiotics protected from the beta-lactamase enzyme, such as amoxicillin-clavulanate, which includes a beta-lactamase inhibitor. Other effective options include second- or third-generation cephalosporins, macrolides like azithromycin, and trimethoprim-sulfamethoxazole. Supportive care, such as pain relievers for fever or decongestants for congestion, is also used to alleviate symptoms.