What to Know About M. catarrhalis

Medically Reviewed by Melinda Ratini, MS, DO on May 26, 2023
4 min read

Moraxella catarrhalis — abbreviated as M. catarrhalis — is a bacterium commonly found in the respiratory tract of small children and even some primates. Up until the past few decades M. catarrhalis was thought to be a relatively harmless bacterium that rarely caused infections. 

While M. catarrhalis can be responsible for mild sinus and ear infections in children, it can be much more dangerous in people with a compromised immune system. M. catarrhalis usually stays in the respiratory tracts of adults who have illnesses like cystic fibrosis or an autoimmune disease.

COPD. COPD stands for chronic obstructive pulmonary disease. Its symptoms usually include mucus in the lungs, problems breathing, and frequent coughing. Usually, you can get COPD because of particle inhalation over time, so the disease is more common among smokers. About 10% of COPD patients with worsened symptoms may have M. catarrhalis in their lungs.

Ear infections. Alongside Haemophilus influenzae and Streptococcus pneumoniae, M. catarrhalis is one of the most common causes of ear infections (otitis media) in children. About 15% to 20% of acute ear infections are caused by this bacterium.

Although not fully understood yet, recurring ear infections of the middle ear may be due to a bacterial biofilm — a small ecosystem of microorganisms living on a surface that is more resistant to antibiotics. M. catarrhalis is especially good at producing biofilms, some of which have been seen in some children with middle ear infections.

Sinus infections. M. catarrhalis sometimes also causes sinus infections. These infections are typically mistaken for a cold or allergies until the symptoms get worse. Some symptoms include discolored drainage from the nose, high fever, fatigue, swelling in the face, and pain in the forehead or behind the eyes.  

Pneumonia. Pneumonia is one of the most common issues related to respiratory bacterial infections. Patients with underlying conditions or a history of smoking are more likely to get pneumonia. People with COPD may have a higher chance of getting pneumonia because they have immune problems and spend a lot of time in the hospital.

Bronchitis. Although less severe than pneumonia or COPD, bronchitis is the temporary inflammation of the lungs. Even though it's rarely caused by bacteria, M. catarrhalis can cause bronchitis in patients who are already in the hospital for lung problems. 

Meningitis. Meningitis is rarely caused by M. catarrhalis. Meningitis is the inflammation of the spinal cord and brain due to bacterial or viral infection. Its symptoms include severe headaches, a stiff neck, high fever, and even seizures. 

If the bacteria make their way into the bloodstream causing bacteremia, it can become one of the quickest ways for them to reach the brain and lead to meningitis. Local infections in the ear or sinuses can also lead to meningitis if left untreated. 

‌In the past, doctors often overlooked M. catarrhalis as being a dangerous respiratory pathogen because it appears similar to Neisseria, another normal bacterium in your lungs. As a result, M. catarrhalis infections have been very difficult to diagnose until recently. 

If your doctor thinks you have an M. catarrhalis infection, they may ask you to undergo a culture test. The doctor will take a sample from the area of your infection (ear, upper respiratory tract, etc.) and see if M. catarrhalis grows on an agar surface. M. catarrhalis will form round, cloudy colonies with a pink hue. They grow differently from other bacteria that may be present in the area of infection because they can slide along the agar rather than sticking to it. Also, an M. catarrhalis colony is larger than that of Neisseria.

Some biochemical or genetic tests may be used if the above test fails to be definitive.

M. catarrhalis can have high antibiotic resistance, so not all antibiotics will work to get rid of an infection. Especially, simple antibiotics like ampicillin and penicillin may not work on M. catarrhalis infections. Some of the antibiotics that may work on M. catarrhalis include:

Most of these antibiotics are given out for ear infections in young children. But, it’s important to reduce the number of antibiotics given out as some bacteria can become resistant in a short period. More resistance leads to a more dangerous strain of bacteria that is harder to treat. 

There has also been a push to develop a bacterial vaccine to help with M. catarrhalis ear infections.