Moraxella catarrhalis

Moraxella catarrhalis
Name	Moraxella catarrhalis
Synonyms	Branhamella catarrhalis, Neisseria catarrhalis
Caption	Gram stain of Moraxella catarrhalis showing diplococci.
Field	Infectious disease, Microbiology

Moraxella catarrhalis. It is a fastidious, aerobic, Gram-negative bacterium that commonly colonizes the human respiratory tract. Historically regarded as a commensal, it is now a recognized pathogen responsible for otitis media in children and exacerbations of chronic obstructive pulmonary disease in adults. Its clinical importance has grown alongside increasing reports of beta-lactamase production, complicating treatment.

Microbiology and classification

This organism is a non-motile, oxidase-positive diplococcus, often appearing in pairs resembling Neisseria gonorrhoeae. It grows well on blood agar and chocolate agar, forming non-hemolytic, "hockey puck" colonies. Taxonomically, it belongs to the family Moraxellaceae within the class Gammaproteobacteria. The bacterium was reclassified from the genus Branhamella to Moraxella following 16S ribosomal RNA sequencing analyses. Key biochemical tests for differentiation include its inability to ferment glucose, maltose, or sucrose, and a positive result for DNase production.

Pathogenesis and virulence factors

The transition from commensal to pathogen involves several virulence determinants. Most clinical isolates produce beta-lactamase, primarily BRO-1 and BRO-2, conferring resistance to penicillin. The bacterium adheres to epithelial cells via surface structures like outer membrane proteins and pili. It also evades host immunity through the formation of biofilms and resistance to complement-mediated killing, partly due to its lipooligosaccharide structure. Other factors include UspA1 and UspA2 proteins, which facilitate attachment and serum resistance, and the secretion of IgA1 protease.

Clinical significance and infections

It is a leading cause of otitis media and acute sinusitis in the pediatric population. In adults with underlying lung disease, particularly chronic obstructive pulmonary disease, it is a major etiologic agent in bronchitis and pneumonia. It is also implicated in less common but serious conditions such as conjunctivitis, bacteremia, and, rarely, meningitis or endocarditis, often in immunocompromised hosts or the elderly. Its role in nosocomial infections, especially in intensive care unit settings, is increasingly noted.

Diagnosis and identification

Diagnosis typically begins with Gram stain and culture from clinical specimens like sputum, middle ear fluid, or blood cultures. Its characteristic Gram-negative diplococcal morphology provides an initial clue. Definitive identification in the clinical microbiology laboratory relies on biochemical profiling, including positive oxidase test, catalase test, and nitrate reduction test. Automated systems like VITEK or MALDI-TOF mass spectrometry are now commonly used for rapid and accurate speciation, differentiating it from other Neisseria species and Haemophilus influenzae.

Treatment and antibiotic resistance

Empiric therapy has shifted due to widespread production of beta-lactamase, rendering amoxicillin ineffective in many cases. First-line agents now include amoxicillin-clavulanate, second-generation cephalosporins like cefuroxime, and macrolides such as azithromycin. For more severe infections, respiratory fluoroquinolones (e.g., levofloxacin) or third-generation cephalosporins (e.g., ceftriaxone) may be used. Surveillance programs like the SENTRY Antimicrobial Surveillance Program monitor emerging resistance patterns, including rare reports of resistance to trimethoprim-sulfamethoxazole and tetracyclines.

Epidemiology and risk factors

Colonization rates are highest in early childhood, with studies showing carriage in up to 75% of infants, declining with age. Key risk factors for infection include daycare attendance, passive smoke exposure, and anatomical abnormalities like eustachian tube dysfunction. In adults, the primary risk is pre-existing chronic obstructive pulmonary disease, followed by advanced age, immunosuppression, and HIV infection. Seasonal variation is observed, with higher incidence of otitis media in winter and spring. Global distribution is universal, though antibiotic resistance rates vary by region.

Category:Bacteria Category:Pathogenic bacteria