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What Is Moraxella Catarrhalis?

By Jillian O Keeffe
Updated May 21, 2024
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Moraxella catarrhalis is a species of bacteria. Historically known as a species that lived relatively harmlessly in the respiratory tract, it is now known as a significant cause of human illnesses like chest infections, pneumonia and ear infections. This bacterium is spherical and needs oxygen to grow, and is capable of developing antibiotic resistance.

This species used to be known as Branhamella catarrhalis. Before that, it went by the name Neisseria catarrhalis or Micrococcus catarrhalis. The names changed rapidly when microbiologists analyzed the traits of the species, and they found that it fit into different groups than previously thought. The advent of genetic testing prompted the newest name change, as typically a bacterium is classified into groups according to the similarity of their genetic material. The species is a close relative of the bacterium Neisseria gonorrheaethat causes the sexually transmitted disease, gonorrhea.

For a long time after its discovery, scientists and medical professionals thought that the bacterium was relatively harmless to humans, because it lived naturally in the upper respiratory tract. In the 1980s, however, scientists discovered that this species was, in fact, an important human pathogen. A significant proportion of adults, up to about 10%, carry Moraxella catarrhalis in the upper respiratory tract, which encompasses all of the respiratory tract apart from the lungs. The species attempts to colonize humans from birth onward, but most people manage to clear the colonization.

Under the microscope, Moraxella catarrhalis is shaped like a sphere. It tends to be present as two individual bacteria stuck together, which is known as a diplococcus. Moraxella catarrhalis needs oxygen to grow, and has a few features that help it infect people. Although different strains of Moraxella catarrhalis have different characteristics, some of the most important abilities of the bacterium include an ability to source iron from the host, proteins that protect it from the host's immune defenses, and a toxin that appears to help it infect cells.

Various respiratory infections can be due to Moraxella catarrhalis. These include nonlethal upper respiratory tract infections in kids and old people, and it may also cause more severe conditions like pneumonia and bronchitis. Some cases of meningitis or blood poisoning can also be caused by the bacterium. Ear infections and infections of the eye are also possible results of infection. As the species lives on a significant proportion of the youth and adult population, and because some strains can have some resistance to common antibiotics like penicillin, Moraxella catarrhalis is also an important source of infections in hospitals.

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