Moraxella (Branhamella) catarrhalis--clinical and molecular aspects of a rediscovered pathogen

Moraxella catarrhalis is a gram-negative diplococcus that has undergone nomenclatural changes and shifts in clinical perception from harmless commensal to recognized pathogen. Molecular analysis based on 16S rDNA sequences confirms its position within the Moraxellaceae family, more closely related to Acinetobacter than Neisseria, though phenotypic similarity to Neisseria can cause diagnostic confusion. M. catarrhalis is now the third most common respiratory pathogen after Streptococcus pneumoniae and Haemophilus influenzae, causing significant otitis media and sinusitis in children and lower respiratory tract infections in adults with underlying lung disease. Carriage rates are high in children (66-77.5%) and elderly populations (26.5% in those >60 years), though its role as a mere commensal has been overstated due to historical misidentification with N. cinerea. The organism exhibits dramatic antibiotic resistance, with approximately 90% of strains producing β-lactamase by the 1990s—the fastest prevalence increase documented for any β-lactamase. Two β-lactamase types (BRO-1 and BRO-2) are unique to Moraxella and related species. Invasive disease remains uncommon, though 89 bacteraemia cases were documented in England and Wales from 1992-1995, peaking in children aged 1-2 years. Nosocomial spread has been documented in hospital respiratory wards.

Key findings

• M. catarrhalis is the third most common respiratory tract pathogen, after S. pneumoniae and H. influenzae, causing otitis media, sinusitis, and lower respiratory infections especially in elderly and immunocompromised patients
• Approximately 90% of M. catarrhalis strains produce β-lactamase (BRO-1 or BRO-2 types), representing the fastest prevalence increase of any β-lactamase in bacterial species since the first resistant strain appeared in 1976
• Historical misidentification with commensal Neisseria cinerea led to overestimation of M. catarrhalis as a harmless commensal; actual carriage is 66-77.5% in children but only 1-5.8% in healthy adults
• Molecular phylogenetic analysis using 16S rDNA sequences conclusively places M. catarrhalis within Moraxellaceae, more closely related to Acinetobacter than to Neisseria
• Nosocomial spread of M. catarrhalis occurs in hospital respiratory wards with good evidence of strain transmission between patients and environmental persistence