Moraxella catarrhalis (M. catarrhalis), also known as Branhamella catarrhalis (B. catarrhalis), is a type of bacteria that causes upper respiratory (nose, sinuses, and throat) and lower respiratory (lungs, bronchi, and bronchioles) infections.

M. catarrhalis is typically found in the nasopharynx of young children and is one cause of childhood infections such as otitis media (middle ear infection), sinusitis (inflammation of the sinuses), and conjunctivitis (pink eye).

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It can also cause respiratory disease in adults, such as bronchopneumonia. In older people with chronic obstructive pulmonary disorder (COPD) (a long-term, inflammatory lung disease that blocks airflow to the lungs) or chronic bronchitis, it causes lower respiratory tract infections. Although rare, systemic (widespread) infections may occur, including infective endocarditis and meningitis.

First reported in 1896, M. catarrhalis was originally named Micrococcus catarrhalis followed by Neisseria catarrhalis. M. catarrhalis was later reclassified to the genus Moraxella due to evolutionary differences with Neisseria (a group of bacteria that cause serious infections including endocarditis, meningitis, septicemia, and gonorrhea).

For this reason, the name of the bacteria has caused much confusion. While M. catarrhalis is sometimes mistaken for species of Neisseria, they are distinguishable through lab tests.

This article will discuss the features, common infections, and treatments of M. catarrhalis.

The Emergence of a Pathogen

For several decades, M. catarrhalis was believed to be a harmless inhabitant of the nasopharynx. Over time, the variety of M. catarrhalis infections has become more apparent, and it is now known to cause respiratory infections in healthy children and adults.

M. Catarrhalis Bacteria Features

Features of M. catarrhalis include the morphology (size, shape, and arrangement of cells) and optimal temperature, which include:

  • Transparent, pink/brown in color
  • Circular-shaped, occurring in pairs (diplococcus)
  • Approximately 0.6 to 1 micrometer in size
  • Optimal temperature growth of 33 to 35 degrees C

Evolution Over Time

There are two distinct species of M. catarrhalis: type 1 seroresistant (SR) and type 2 serosensitive (SS). Type 1 SR represents the younger strain of the two, and is believed to have appeared approximately 4 million years ago while co-existing with Homo sapiens.

Type 1 SR is also pathogenic and can readily attach itself to epithelial cells. By comparison, type 2 SS is believed to have existed about 50 million years ago and possesses less virulent (disease-causing) properties.

M. catarrhalis has become more resistant to antibiotics since the 1980s. Now, 90% to 100% of cultures of this bacteria produce β-lactamase, which makes it resistant to ampicillin. Still, amoxicillin (a similar derivative of penicillin) is the first-line therapy for acute otitis media, with clavulanate added if the infection shows treatment resistance.

Susceptibility in Children

Children are more likely than adults to carry M. catarrhalis in their upper respiratory tracts. Studies have shown that M. catarrhalis colonizes the nose and throat of 22% to 55% of infants by age 6 months. It is the third-leading bacterial cause of otitis media (middle ear infection) in children.

M. catarrhalis is also the third-leading bacterial cause of respiratory infections in children. It is preceded by Streptococcus pneumoniae (S. pneumoniae) and Haemophilus influenzae (H. influenzae).

However, invasive diseases (like bacteremia) that are caused by M. catarrhalis are rare in children, even those who have underlying health conditions or are immunocompromised (weakened immune system).

List of Associated Infections

Otitis media (OM) is a common bacterial infection in children that involves the inflammation of the middle ear (air-filled space between the eardrum and oval window of the inner ear). OM can also occur in adults, but is uncommon.

Common risk factors for OM include a weakened immune system; having a cold, flu, or allergies; exposure to cigarette smoke or pollution; being in crowded settings; having a family history of OM; and drinking fluids while laying down (infants).

When to Seek Medical Attention

Common symptoms of OM include ear pain, fever, and difficulty hearing or sleeping. Seek medical attention if you or your child experience high fever (typically 102 degrees F or above), hearing loss in one or both ears, symptoms that last more than two to three days, worsening symptoms, or the draining of pus (fluid) from the ear(s).

Sinusitis, or sinus infection, is an upper respiratory infection that involves inflammation of the sinuses (hollow regions near facial bones). It usually occurs as a complication of a cold or seasonal allergies and can affect both children and adults. Common symptoms include nasal congestion, runny nose, facial tension, pressure, or pain, sore throat, and headaches.

Conjunctivitis (pink eye) is an infection that occurs when the conjunctiva (lining of the eye) becomes inflamed. Bacterial conjunctivitis occurs far less than viral conjunctivitis in adults. But if bacterial, it typically produces pus (discharge) from the eyes that may cause the eyelids to stick together.

Conjunctivitis caused by bacteria is more common in children than adults and accounts for 50% to 75% of cases in children. Of these cases, approximately 19% are caused by M. catarrhalis, and the numbers are increasing.

Meningitis is a life-threatening disease that occurs when the meninges (protective membranes that cover the brain and spinal cord) are inflamed. Although very rare, meningitis caused by M. catarrhalis has been reported in newborns. Common symptoms include fever, headache, irritability, seizures, and a bulging fontanelle (swelling in the brain).

Although vaccines can prevent some types of meningitis, there are currently no vaccines for M. catarrhalis meningitis.

When It’s an Emergency

Meningitis is a life-threatening illness with high mortality and morbidity in newborns and infants. If your child has symptoms of meningitis, seek immediate medical attention.

Chronic obstructive pulmonary disease, which includes emphysema and chronic bronchitis, refers to a group of diseases that cause major obstruction of the airways in adults. Common symptoms of COPD include wheezing, difficulty breathing, coughing, and excess mucus in the lungs.

COPD can go through periods of stable but progressive disease and acute exacerbations associated with respiratory infections. M. catarrhalis is found in 10% to 20% of bacterial cultures in acute exacerbations. It is thought to contribute to inflammation and progression of COPD.

Bronchopneumonia is a type of pneumonia that occurs when the alveoli become inflamed, affecting patches throughout both lungs. Although M. catarrhalis is a common pathogen in community-acquired pneumonia (people infected outside of group living or healthcare settings), understanding the role of the pathogen in community-acquired bronchopneumonia is limited.

However, some studies have suggested that cases of bronchopneumonia by M. catarrhalis appear to affect older adults during the winter months.

Infective endocarditis (IE) is a life-threatening infection that causes inflammation of the endocardium (heart's inner lining). IE caused by M. catarrhalis is rare, perhaps because only about 5% of adults have the bacteria harmlessly colonizing the upper respiratory tract.

However, a weakened immune system can cause M. catarrhalis to become opportunistic and invade the bloodstream. This is especially true for individuals with artificial heart valves or a pacemaker. Common symptoms of IE include shortness of breath, fever, fatigue, and muscle aches.

Treatment Effectiveness With Antibiotics

M. catarrhalis infections are commonly treated with antibiotics (drugs that inhibit or kill bacteria). They include amoxicillin-clavulanate, doxycycline, clarithromycin, and trimethoprim-sulfamethoxazole (TMP/SMX), which show strong effectiveness against M. catarrhalis.

However, increasing antibiotic resistance is a concern. This is due to the presence of an enzyme called beta-lactamase, which primarily inhibits beta-lactam antibiotics (penicillin and cephalosporins).

Studies demonstrate that M. catarrhalis show strong resistance to beta-lactams and some resistance to macrolides (azithromycin), quinolines (levofloxacin), and fluoroquinolones (ciprofloxacin).

M. Catarrhalis Vaccine Status

Currently, there is no licensed vaccine for M. catarrhalis. However, due to the significance of M. catarrhalis as a respiratory pathogen and the strain that it poses on global healthcare systems, researchers are working to develop effective vaccine candidates.

Immune System Health

The immune system comprises two parts: innate (general, first-line of defense) and adaptive (specific, second-line of defense). M. catarrhalis has the ability to evade the innate immune system, which functions to remove any non-specific pathogen from the body.

M. catarrhalis is also adept at attaching itself to cell surfaces due to the presence of several types of adhesins (proteins). One such adhesin, known as the Hag protein, plays an important role in anchoring to cells in the middle ear and lung, which promotes associated infections such as otitis media and lower respiratory tract infections.


Moraxella catarrhalis (M. catarrhalis) is bacteria that cause upper and lower respiratory infections. M. catarrhalis is often found in the nasopharynx of young children and causes common childhood infections such as otitis media, sinusitis, and conjunctivitis.

It can also cause respiratory disease in adults, such a bronchopneumonia and lower respiratory tract infections believed to trigger acute COPD exacerbations. In rare cases, M. catarrhalis can cause systemic diseases, including infective endocarditis and meningitis.

M. catarrhalis is the third-leading bacterial cause of respiratory infections in children. However, invasive diseases that are caused by M. catarrhalis are rare in children, even those who have underlying health conditions or are immunocompromised.

M. catarrhalis infections are commonly treated with antibiotics. These include amoxicillin-clavulanate, doxycycline, clarithromycin, and trimethoprim-sulfamethoxazole (TMP/SMX), which show strong effectiveness against M. catarrhalis.

A Word From Verywell

M. catarrhalis infections are preventable. In children, reduce exposure to cigarette smoke and avoid crowded areas.

As there is no current licensed vaccine for M. catarrhalis infections, speak to a healthcare provider about the potential use of pneumococcal and influenza vaccines to prevent recurrent otitis media (OM). This is because S. pneumoniae and H. influenzae can also cause OM.

To prevent M. catarrhalis infections in adults, practice good hygiene practices (handwashing) and sterilization techniques. If infected, do not hesitate to seek medical attention, especially if you have underlying health issues or are immunocompromised. Complications from M. catarrhalis are rare, but can happen if your infection is left untreated.

Frequently Asked Questions

  • Do antibiotics kill Moraxella catarrahlis?

    Infections caused by Moraxella catarrhalis are commonly treated with antibiotics, including amoxicillin-clavulanate, doxycycline, clarithromycin, and trimethoprim-sulfamethoxazole (TMP/SMX).

  • Is Moraxella catarrhalis contagious?

    Although Moraxella catarrhalis is part of the normal flora of up to 100% of infants, the colonization decreases to about 10% in adults, with a cycle of elimination and then re-colonization. In addition, there are different variants of Moraxella catarrhalis that permit different pathogenicities (tendency to cause illness).

    Moraxella catarrhalis is contagious and can be spread through respiratory droplets (coughing and sneezing) and mucus secretions. It can also be transmitted in hospitals, especially during the winter months.

  • Is Moraxella catarrhalis the same type of bacteria that causes UTIs and STDs?

    Moraxella catarrhalis belongs to the family Moraxellacea. Despite similarities between Neisseria (Neisseria gonorrheae causes sexually transmitted infections) and Moraxella, they are distinct. M. catarrhalis is an uncommon cause of urinary tract infections (UTIs), and there has been a case of sexually transmitted M. catarrhalis that produced urethritis (inflammation of the urethra).

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