Moraxella catarrhalis

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Kingdom: Bacteria Phylum: Proteobacteria Class: Gammaproteobacteria Order: Pseudomonadales Family: Moraxellaceae Genus: Moraxella Species: M. catarrhalis

Description and Significance

Moraxella catarrhalis is known to cause otitis media, which is inflammation of the inner ear. This is most frequently seen in children and older adults; it does not infect middle-age adults nearly as much. Currently there is no licensed vaccine available for M. catarrhalis, but this is an area of active research due to the high prevalence of pathogenesis and colonization.


  • non-motile
  • gram-positive
  • diplococcus
  • aerobic
  • oxidase-positive
  • sticks to host cell using a trimeric autotransported adhesion (TAA)
  • commonly resistant to penicillin, ampicillin, and amoxicillin

Structure, Metabolism, and Life Cycle

M. catarrhalis displays an endotoxin that is similar to many found in the Neisseria species, which play a role in the disease process. Some strains of M. catarrhalis exhibit fimbriae or pili, which help the cells adhere to the respiratory epithelium. Also, the cells express specific proteins that allow uptake for iron which act as receptors. M. Catarrhalis forms round opaque colonies on blood and chocolate agar, and the colonies can be slid around agar surfaces without being disrupted; this is called the "hockey puck sign". One interesting feature of the cellular structure of M. catarrhalis is the presence of trimeric autotransporter adhesins, which are essentially a type of virulence factor. These are structures of gram-negative bacteria that allow the cells to infect a host through a process called cell adhesion. Another term for trimeric autotransporter adhesins is oligomeric coiled-coil adhesins (OCAs).

Ecology and Pathogenesis

Moraxella catarrhalis is specifically a human pathogen and it can cause infection in immunocompromised hosts, such as HIV/AIDS patients. Also, it can colonize the upper respiratory tract in children and infants more easily than adults and cause pneumonia and sinusitis. Moraxella catarrhalis enters the nasopharynx and can invade numerous cell types, including bronchial epithelium, small airway epithelium, and type II alveolar pneumocytes [4]. It can migrate to the middle ear after it enters the nasopharnyx. It forms a biofilm in vitro, but it is not clear what the function of this biofilm is. The patient will experience symptoms of acute sinusitis, urethritis, septiciema, meningitis, maxillary sinusitis, conjunctivitis, and septic arthritis, and exacerbation of chronic obstructive pulmonary disease [3].

The process of infection includes:

  1. Adhesion to the host epithelium
  2. Invasion of the host epithelium
  3. Biofilm formation
  4. Evasion of the host immune system
  5. Nutrient acquisition
    1. M. catarrhalis can utilize human transferrin, human lactoferrin, and to some extent human hemoglobin as iron sources, which is mediated by many cell surface iron-binding proteins. There are two specific lactoferrin-binding proteins, LbpA and LpbB, two specific transferrin-binding proteins, TbpA and TbpB, hemoglobin utilization protein, mHuA. M. catarrhalis relies on acetate, lactate, and fatty acids for growth, and it is considered an arginine auxotroph.


  • [1] Bakri F, Brauer AL, Sethi S, Murphy TF. Systemic and mucosal antibody response to Moraxella catarrhalis following exacerbations of chronic obstructive pulmonary disease. J Infect Dis 2002;185:632-40.
  • [2] Nicotra B, Rivera M, Luman JI, Wallace RJ.Branhamella catarrhalis as a lower respiratory tract pathogen in patients with chronic lung disease. Arch Intern Med1986;146:890-3.
  • [3] Tolentino LF. Causes of Moraxella catarrhalis pathogenicity: review of literature and hospital epidemiology. Laboratory Medicine 2007;38:420-1.
  • [4] Dirk Linke, Tanja Riess, Ingo B. Autenrieth, Andrei Lupas, Volkhard A.J. Kempf, Trimeric autotransporter adhesins: variable structure, common function, Trends in Microbiology, Volume 14, Issue 6, June 2006: 264-270.(


Page authored by Aaron Yeshe, student of Mandy Brosnahan, Instructor at the University of Minnesota-Twin Cities, MICB 3301/3303: Biology of Microorganisms.