Moraxella is a genus of gram-negative diplococci, with M. catarrhalis being the most clinically relevant species. M. catarrhalis is part of the normal flora of the upper respiratory tract, but it can cause infection in susceptible individuals. The infection is transmitted through respiratory droplets and can lead to chronic obstructive pulmonary disease (COPD) exacerbations in adults and otitis media in children. Moraxella species are catalase- and oxidase-positive, allowing them to resist damage from reactive oxygen species in the highly oxygenated environment of the respiratory tract.

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Gram negative bacteria classification flowchart

Gram-negative bacteria:
Most bacteria can be classified according to a lab procedure called Gram staining.
Bacteria with cell walls that have a thin layer of peptidoglycan do not retain the crystal violet stain utilized in Gram staining. These bacteria do, however, retain the safranin counterstain and thus appear as pinkish-red on the stain, making them gram negative. These bacteria can be further classified according to morphology (diplococci, curved rods, bacilli, and coccobacilli) and their ability to grow in the presence of oxygen (aerobic versus anaerobic). The bacteria can be more narrowly identified by growing them on specific media (triple sugar iron (TSI) agar) where their enzymes can be identified (urease, oxidase) and their ability to ferment lactose can be tested.
* Stains poorly on Gram stain
** Pleomorphic rod/coccobacillus
*** Require special transport media

Image by Lecturio.



  • Stain: gram-negative
  • Morphology: diplococcus
  • Nonmotile
  • Growth and culture:
    • Microaerophilic
    • Oxidase-positive, catalase-positive
  • Asaccharolytic (does not ferment carbohydrates)

M. catarrhalis and M. lacunata are the most clinically relevant species within this genus.

Moraxella Catarrhalis

Moraxella catarrhalis (diplococcus formation)

Image: “Moraxella Catarrhalis” by CDC/Dr. W.A. Clark. License: Public Domain



  • Humans are the primary reservoir.
  • M. catarrhalis spreads from person to person via respiratory droplets.
  • Nasopharyngeal colonization: 
    • Common in infancy, but declines with age
    • Bacteria can migrate to the middle ear from the nasopharynx via the Eustachian tube.
    • ↑ Rate of colonization noted with increased pneumococcal vaccination (which is believed to cause shifts in respiratory microbiome).

Pathogenic features

  • Opportunistic pathogen: 
    • Infection occurs in the setting of a weakened immune system or owing to existing respiratory disease (i.e., chronic obstructive pulmonary disease (COPD)).
    • Bacterial stress (i.e., cold shock response) causes expression of pathogenic mechanisms within M. catarrhalis.
  • Pathogenic mechanisms:
    • Adhesins allow the bacteria to bind to the mucosa.
    • Inhibits toll-like receptor 2 signaling (responsible for foreign substance recognition), allowing for immune system evasion
    • Aggregates and creates a biofilm, making it difficult for antibiotics to reach the bacteria

Associated Diseases

M. catarrhalis

  • Tracheobronchitis and pneumonia:
    • Noted in susceptible individuals, most commonly manifesting as acute COPD exacerbations
    • M. catarrhalis causes 10%–20% of COPD exacerbations.
  • Otitis media: M. catarrhalis causes 15%–20% of cases of otitis media in children.  
  • Uncommon cause of acute bacterial sinusitis

M. lacunata

M. lacunata is a common cause of chronic angular blepharoconjunctivitis in humans.


  • Clinical features of the diseases are often sufficient in arriving at the diagnosis.
  • Microbiologic studies are often reserved for recurrent, severe conditions or for cases in which therapy has failed.
  • Other tests are determined by clinical presentation (chest X-ray for suspected pneumonia).


  • Moraxella: > 90% produce β-lactamase and thus are resistant to penicillin, ampicillin, and amoxicillin.
  • Susceptible to amoxicillin–clavulanate, macrolides, extended-spectrum cephalosporins, tetracyclines, fluoroquinolones
Community-acquired moraxella catarrhalis

Pneumonia secondary to Moraxella catarrhalis:
Chest X-ray shows a left lower lobe infiltrate secondary to Moraxella catarrhalis, a gram-negative diplococcus that can be an etiologic agent for lung infections in immunocompromised patients and in those with chronic obstructive pulmonary disease (COPD).

Image: “Chest X-ray on admission” by Hospital Universitario Central de Asturias (HUCA), Instituto Nacional de Silicosis (INS), Área del Pulmón, Facultad de Medicina, Universidad de Oviedo, 33005 Oviedo, Spain. License: CC BY 4.0


  1. Aebi, C. (2011). Moraxella catarrhalis—pathogen or commensal? Adv Exp Med Biol 697:107–116.
  2. Goldstein, E. J. C., Murphy T. F. (2009). Moraxella catarrhalis: a human respiratory tract pathogen. Clin Infect Dis 000:000–000.
  3. Heiniger, N., Troller, R., Meier, P.Ss, Aebi, C. (2005). Cold shock response of the UspA1 outer membrane adhesin of Moraxella catarrhalis. Infect Immun 000:000–000.
  4. Murphy, T. F., File, T. M. (2019). Moraxella catarrhalis infections. UpToDate. Retrieved 23 April 2021, from
  5. Pelton, S., Tahtinen, P., Kaplan, S.L., Isaacson, G.C. (2020). Acute otitis media in children: dpidemiology, microbiology, and complications. UpToDate. Retrieved 23 April 2021, from

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