What is Moraxella catarrhalis?

Moraxella catarrhalis (M. catarrhalis) is a type of bacteria that’s also known as Neisseria catarrhalis and Branhamella catarrhalis.

It used to be considered a normal part of the human respiratory system, but more shows that can it sometimes causes infections.

Many young children have M. catarrhalis in their respiratory tract in the first few years of life, but it doesn’t always cause infections. When it does, it often results in a simple ear or sinus infection. In children with weakened immune systems, it can cause more serious infections, such as pneumonia or bronchitis.

Adults, on the other hand, usually don’t have M. catarrhalis in their respiratory tract. When they do, they typically have a weakened immune system due to an underlying condition, such as an autoimmune disorder, or from treatment such as chemotherapy.

Adults with lung conditions, especially cystic fibrosis and chronic obstructive pulmonary disease (COPD), are also more likely to develop an M. catarrhalis infection. This is because chronic lung conditions make it harder for your lungs to clear out bacteria.

What does it cause?

Middle ear infection

M. catarrhalis is increasingly recognized as a common cause of acute otitis media, also known as a middle ear infection, in children. Many young children have this bacteria in their noses, and it can sometimes move into the middle ear, causing infection.


Pneumonia is an infection in the lungs that’s often caused by bacteria. While M. catarrhalis typically doesn’t cause pneumonia, it can in adults with weakened immune systems or chronic lung diseases. People with a lung disease who spend a lot of time in hospitals have the highest risk of developing pneumonia due to M. catarrhalis.


Bronchitis is an inflammation of the lungs that’s usually caused by a virus, not bacteria. However, in adults with weakened immune systems or chronic lung conditions, M. catarrhalis can cause bronchitis. Like pneumonia, bronchitis due to M. catarrhalis is most common in adults with lung conditions in hospitals.

Both pneumonia and bronchitis produce similar symptoms, the main one being a cough that produces mucus and often lasts for weeks. However, the symptoms of pneumonia are usually more severe.

Sinus infection

M. catarrhalis can also cause sinus infections in children as well as adults with weakened immune systems. Symptoms of a sinus infection are similar to those of a cold, but tend to get worse over the course of a week rather than better. They can also cause greenish-yellow discharge in your nose, pressure or pain in your face, and a fever.


COPD refers to a group of lung diseases that worsen over time. These include chronic bronchitis, emphysema, and refractory asthma, which is asthma that doesn’t get better with regular treatment.

The main symptoms of COPD are coughing, wheezing, coughing up mucus, chest tightness, shortness of breath, and difficulty breathing.

While COPD slowly gets worse over time, infections can speed up the process and cause serious complications, including death.

M. catarrhalis is the bacterial cause of worsening COPD. It can increase mucus production, make mucus thicker, and make it even harder to breath.

Pink eye

Conjunctivitis, commonly known as pink eye, is an infection of the outer layer of your eye. M. catarrhalis can cause pink eye in both children and newborns.


In very rare cases, M. catarrhalis can cause meningitis, especially in newborns. Meningitis refers to inflammation of the meninges, which are layers of tissue that surround the brain. While most cases of meningitis are preventable with a vaccine, there’s no vaccine for M. catarrhalis yet.

Can you treat it?

Infections caused by M. catarrhalis usually respond well to antibiotics. However, almost all strains of M. catarrhalis produce an enzyme called beta-lactamase, which makes them resistant to some common antibiotics, such as penicillin and ampicillin.

Common antibiotics used to treat M. catarrhalis infections include:

  • amoxicillin-clavulanate (Augmentin)
  • trimethoprim-sulfamethoxazole (Bactrim)
  • extended-spectrum cephalosporins, such as cefixime (Suprax)
  • macrolides, such as azithromycin (Zithromax)

Adults can also take tetracycline and fluoroquinolone antibiotics.

Regardless of which antibiotic you use, it’s very important to take them exactly as prescribed. Even if your symptoms start to improve and you don’t feel sick, make sure you complete the full course of antibiotics. Otherwise, your infection may return and be resistant to the original antibiotic used.

Can you prevent it?

Scientists are currently working to that protects against M. catarrhalis infections. This would be a major breakthrough in helping to prevent ear infections and pink eye in children. It will also be valuable for adults with COPD who are vulnerable to M. catarrhalis infections.

Until then, the best way to avoid M. catarrhalis infections is to keep your immune system healthy through following a balanced diet and getting regular exercise. If you have a compromised immune system or lung condition, make sure you regularly wash your hands and carry hand sanitizer. If you need to go to a hospital or doctor’s office, consider wearing an N95 respirator mask while you’re there.

The bottom line

Most people have M. catarrhalis in their respiratory tract at some point their lives, usually during childhood. While it was initially thought to be relatively harmless, more recent research has found that it can do more damage than previously thought, especially for people with weakened immune systems or lung conditions.

While M. catarrhalis infections are resistant to some common antibiotics, there are plenty of other antibiotics that do work. Just make sure to follow your doctor’s instructions for taking them.