Moraxella catarrhalis

Moraxella catarrhalis is a common germ that lives in the nose and throat — so common in young children that for most of the twentieth century doctors assumed it was harmless, a mere passenger rather than a troublemaker. That view has changed. We now know Moraxella catarrhalis is a genuine respiratory pathogen: it is one of the top three causes of childhood ear infections and sinus infections, and a leading cause of flare-ups in adults with chronic lung disease. This page explains what the bacterium is, the illnesses it actually causes, who is most likely to run into trouble, how infections are diagnosed, and why a quirk of this particular germ — nearly all strains destroy ordinary penicillin — matters so much for treatment. The tone throughout is practical and honest: most infections are mild and manageable, but the details are worth understanding.

Overview
The Bacterium
The Illnesses It Causes
Who's Most at Risk
Diagnosis
Treatment
Prevention
The Honest Bottom Line
Research Papers
Connections
Featured Videos

Overview

For decades, Moraxella catarrhalis had an image problem. Because it turns up so often in the noses and throats of perfectly healthy babies, generations of textbooks dismissed it as a harmless mouth-and-throat commensal — a bystander with no real capacity to make people sick. Its very name reflects that history: it was shuffled between genera and once called Neisseria catarrhalis and later Branhamella catarrhalis, as if no one was quite sure it deserved serious attention.

That reputation has been thoroughly revised. Careful studies over the last few decades — sampling the fluid from behind children's eardrums, and the sputum coughed up during lung flare-ups — established that Moraxella catarrhalis really does cause disease, not just occupy space. It is now recognized as one of the three most important bacterial causes of middle-ear and sinus infections in children, and among the top handful of bacteria that drive worsening symptoms in adults with chronic obstructive pulmonary disease (COPD). In other words, the same germ can be a quiet resident in a healthy toddler and a genuine cause of illness in a vulnerable ear, sinus, or lung.

None of this makes Moraxella catarrhalis a fearsome organism. Compared with germs that cause life-threatening bloodstream or brain infections, it is usually a nuisance rather than a menace. But it is a common, treatable nuisance — and understanding it helps explain why an ear or sinus infection sometimes shrugs off the first antibiotic prescribed.

The Bacterium

Moraxella catarrhalis is a gram-negative diplococcus. Under the microscope, the cells are round and pair up, and each pair has flattened facing sides — microbiologists often describe the appearance as two kidney beans set next to each other. "Gram-negative" refers to how it takes up a standard laboratory stain, a property tied to the structure of its outer wall; it means the organism belongs to a different broad family than gram-positive germs such as the streptococci and staphylococci.

The natural home of Moraxella catarrhalis is the human upper respiratory tract — the lining of the nose, the back of the throat, and the nasopharynx (the space where the nasal passages meet the throat). It is exclusively a human organism; there is no animal or environmental reservoir that matters for everyday infections. It spreads person to person through respiratory droplets and close contact, which is why it circulates so freely among children in households, daycare, and school.

What stands out most is how common colonization is in young children. Carriage of Moraxella catarrhalis in the nose and throat is the norm rather than the exception in the first years of life: many surveys find that a majority of infants and toddlers are colonized at some point, with carriage rates highest in the first two years and then falling as children grow. Colonization tends to rise and fall over weeks, with new strains arriving and old ones clearing. In healthy adults, by contrast, carriage is much less common. Being colonized is not the same as being sick — most colonized children are perfectly well — but colonization is the necessary first step, and the reservoir, from which infections arise.

The Illnesses It Causes

This is the heart of the matter. Moraxella catarrhalis causes disease almost entirely in the respiratory tract, and its illnesses split neatly into two groups: ear and sinus infections in children, and lower-airway flare-ups in adults with chronic lung disease.

Acute otitis media (middle-ear infection)

Along with Streptococcus pneumoniae and non-typeable Haemophilus influenzae, Moraxella catarrhalis is one of the top three bacterial causes of acute otitis media — the classic painful middle-ear infection of early childhood, with fever, ear-tugging, and fussiness. When doctors sample the fluid from behind an inflamed eardrum, these three organisms account for the large majority of bacteria found, and Moraxella catarrhalis is a consistent member of that group. It has a reputation for causing somewhat milder ear infections than pneumococcus, and — importantly for treatment decisions — ear infections due to Moraxella catarrhalis have among the highest rates of clearing up on their own without antibiotics.

Sinusitis (sinus infection)

The same three bacteria that cause ear infections are also the leading causes of acute bacterial sinusitis — infection of the air-filled sinus cavities around the nose and eyes — in both children and adults. The result is the familiar picture of facial pressure, blocked nose, thick discharge, and symptoms that drag on or worsen after a cold. Most sinus congestion during a cold is viral and does not need antibiotics, but when a true bacterial sinus infection develops, Moraxella catarrhalis is one of the usual suspects.

Exacerbations of COPD

In older adults, the story shifts to the lower airways. Moraxella catarrhalis is a leading bacterial cause of acute exacerbations of chronic obstructive pulmonary disease — the episodes in which someone with long-standing lung damage (usually from smoking) suddenly has more cough, more sputum, discolored phlegm, and more breathlessness than usual. Research that repeatedly cultured the sputum of COPD patients over time made the case convincingly: acquiring a new strain of Moraxella catarrhalis that the person's immune system has not seen before is associated with the onset of an exacerbation, and the immune system mounts a fresh response as the flare resolves. By most estimates, Moraxella catarrhalis accounts for roughly one in ten bacterial COPD exacerbations, placing it just behind non-typeable Haemophilus influenzae and Streptococcus pneumoniae. Because COPD flares are so common, that adds up to a large number of episodes.

Bronchitis and, less commonly, pneumonia

Moraxella catarrhalis can also cause bronchitis (inflammation of the larger breathing tubes) and, less commonly, pneumonia (infection of the lung tissue itself). Pneumonia from this organism is mainly seen in older adults and people with weakened immune systems or underlying lung disease, rather than in healthy young people. In these vulnerable patients it can be a meaningful cause of lower respiratory infection.

Rare invasive disease

Very occasionally, Moraxella catarrhalis escapes the respiratory tract and causes invasive disease — infection of the bloodstream (bacteremia) or, rarely, the lining of the brain (meningitis). This is uncommon and tends to occur in newborns or in people who are seriously immunocompromised. For the ordinary reader, invasive infection is a rarity worth knowing exists but not worth worrying about.

Who's Most at Risk

The pattern of who gets sick follows the two-part story above:

Young children — the highest-risk group for ear and sinus infections. Because carriage in the nose and throat peaks in the first years of life, and because young children's short, horizontal eustachian tubes drain the middle ear poorly, this is the age when Moraxella catarrhalis ear infections cluster. Attending daycare, having older siblings, and exposure to tobacco smoke all increase the risk.
Adults with COPD or other chronic lung disease — the group at risk for bronchitis, exacerbations, and pneumonia. Damaged, inflamed airways are more easily colonized and more easily tipped into a flare when a new strain arrives.
Older adults and people with weakened immunity — more likely to develop pneumonia and, rarely, invasive infection. Age, immune-suppressing medication, and other serious illnesses all raise the stakes.
Season — infections are noticeably more common in the colder months. Carriage rates and disease both rise in autumn and winter, tracking the general seasonality of respiratory illness.

A healthy adult with normal lungs and a normal immune system is, in practice, unlikely to become ill from Moraxella catarrhalis even while carrying it.

Diagnosis

For the everyday infections this germ causes, diagnosis is usually clinical — based on the history and physical examination rather than on identifying the specific bacterium. A clinician diagnoses a middle-ear infection by looking at the eardrum with an otoscope, and diagnoses sinusitis from the pattern and duration of symptoms. In routine cases, no one cultures the fluid to find out whether Moraxella catarrhalis, pneumococcus, or Haemophilus is responsible, because the initial treatment choice would be the same regardless.

Specific microbiological testing is reserved for situations where it will change management:

Culture. The organism can be grown from an appropriate sample — middle-ear fluid obtained by a specialist, or sputum in a lung infection. In the laboratory it is identified by its appearance, its growth characteristics, and simple biochemical tests. Culture also allows antibiotic-susceptibility testing when that information is needed.
Respiratory PCR panels. Multiplex PCR tests, which look for the genetic fingerprints of many respiratory pathogens at once, can detect Moraxella catarrhalis quickly and are increasingly used in hospitals when a precise answer matters.
Sputum testing in COPD exacerbations. When someone with COPD has a severe flare, is not improving, or needs hospital care, a sputum sample may be sent to identify the bacteria involved and guide antibiotic choice — one of the more common reasons Moraxella catarrhalis is specifically identified.

Treatment

Here is the single most practical fact about treating Moraxella catarrhalis: nearly all strains now produce an enzyme called beta-lactamase. Beta-lactamase chops apart the chemical ring at the heart of ordinary penicillins — including amoxicillin, the workhorse antibiotic for childhood ear and sinus infections — and inactivates them before they can kill the bacterium. This capability is carried by an enzyme known in this organism as BRO, and it is now present in more than 90 percent of isolates; in most surveys it is close to universal. It was not always so: decades ago these bacteria were routinely susceptible to plain penicillin, but the beta-lactamase spread rapidly and is now the default.

The consequence is straightforward: plain amoxicillin often fails against Moraxella catarrhalis. When treatment is needed, clinicians reach for options that are stable against beta-lactamase or belong to a different class altogether:

Amoxicillin-clavulanate (for example, the combination sold as Augmentin) — pairs amoxicillin with clavulanate, a companion drug that blocks the beta-lactamase enzyme so the amoxicillin can work again. This is a common go-to.
Certain cephalosporins — oral agents such as cefdinir, cefuroxime, or cefpodoxime, whose structure resists the enzyme.
Macrolides — azithromycin or clarithromycin, a different class useful for people with penicillin allergy.
Doxycycline — a tetracycline antibiotic, generally reserved for older children and adults (it is avoided in young children).

Reassuringly, aside from its beta-lactamase, Moraxella catarrhalis has stayed largely susceptible to these agents, so treatment failures for lack of an effective drug are unusual once the right antibiotic is chosen.

Just as important is knowing when not to rush to antibiotics. Many ear infections clear up on their own, and those caused by Moraxella catarrhalis have some of the highest spontaneous-resolution rates. For this reason, professional guidelines endorse a “watchful waiting” approach for many children with mild, non-severe ear infections: treating the pain, giving the illness a couple of days to resolve, and starting antibiotics only if the child is not improving. This spares needless antibiotics and the side effects and resistance that come with them. Whether watchful waiting is appropriate depends on the child's age and how sick they are, which is a conversation to have with the treating clinician.

Prevention

There is no licensed vaccine against Moraxella catarrhalis, though researchers have been working on one for years — ideally a shot that would also cover the other main causes of ear infections. Until that arrives, prevention rests on sensible, low-tech measures:

Hand hygiene and respiratory manners. The germ spreads by droplets and close contact, so regular handwashing, covering coughs, and the ordinary habits that reduce colds also reduce its transmission — particularly in households and daycare settings with young children.
Avoid tobacco smoke. This is one of the most important and best-supported steps. Secondhand smoke raises a child's risk of ear infections, and smoking is the dominant cause of the COPD that sets adults up for exacerbations. Not smoking — and keeping children away from others' smoke — lowers the risk on both ends of the age spectrum.
Manage COPD well. For adults with chronic lung disease, good day-to-day control is the best defense against flares: taking inhaled maintenance medications as prescribed, stopping smoking, staying up to date on recommended vaccinations (such as influenza and pneumococcal vaccines, which prevent other triggers of exacerbations), and engaging in pulmonary rehabilitation where available. Prompt attention to early flare symptoms can keep a mild exacerbation from becoming a hospital stay.
General measures for children. Breastfeeding, avoiding bottle-feeding while lying flat, and limiting large-group daycare exposure in the first year where feasible are all associated with fewer ear infections overall.

The Honest Bottom Line

Moraxella catarrhalis is a good example of a germ that was underestimated and then correctly promoted from harmless bystander to real pathogen — without becoming something to fear. For most people it is either a quiet resident of a healthy nose and throat or the cause of a treatable ear or sinus infection. Its greatest impact falls on two groups: young children, in whom it is a top cause of ear and sinus infections, and older adults with chronic lung disease, in whom it drives flare-ups and occasional pneumonia.

The practical takeaways are modest but genuinely useful. Because nearly all strains now destroy plain penicillin, an ear or sinus infection that does not respond to basic amoxicillin may simply need a beta-lactamase-stable alternative — not a sign of anything dire. Many mild ear infections do not need antibiotics at all. And the two most powerful prevention tools — good hand hygiene and steering clear of tobacco smoke — are the same ones that protect against a long list of other respiratory problems. This is a common germ with an uncommonly manageable set of solutions.

This page is for general education and is not a substitute for professional medical advice. If you or your child has a suspected ear, sinus, or chest infection — especially with high fever, severe pain, breathing difficulty, or symptoms that worsen or fail to improve — see a qualified clinician.

Research Papers

Murphy TF, Parameswaran GI. Moraxella catarrhalis, a human respiratory tract pathogen. Clinical Infectious Diseases. 2009;49(1):124–131. doi:10.1086/599375 — The definitive modern review establishing M. catarrhalis as a genuine cause of otitis media and COPD exacerbations rather than a harmless commensal.
Verduin CM, Hol C, Fleer A, van Dijk H, van Belkum A. Moraxella catarrhalis: from emerging to established pathogen. Clinical Microbiology Reviews. 2002;15(1):125–144. doi:10.1128/CMR.15.1.125-144.2002 — Comprehensive account of how the organism's reputation shifted and of its biology, disease spectrum, and resistance.
Karalus R, Campagnari A. Moraxella catarrhalis: a review of an important human mucosal pathogen. Microbes and Infection. 2000;2(5):547–559. doi:10.1016/S1286-4579(00)00314-2 — Reviews colonization, virulence factors, and the case for treating the organism as a true mucosal pathogen.
de Vries SPW, Bootsma HJ, Hays JP, Hermans PWM. Molecular aspects of Moraxella catarrhalis pathogenesis. Microbiology and Molecular Biology Reviews. 2009;73(3):389–406. doi:10.1128/MMBR.00007-09 — Detailed look at the adhesins and surface molecules the bacterium uses to colonize and cause disease.
Ngo CC, Massa HM, Thornton RB, Cripps AW. Predominant bacteria detected from the middle ear fluid of children experiencing otitis media: a systematic review. PLOS ONE. 2016;11(3):e0150949. doi:10.1371/journal.pone.0150949 — Systematic review confirming M. catarrhalis among the top bacteria recovered from infected middle-ear fluid.
Vergison A. Microbiology of otitis media: a moving target. Vaccine. 2008;26(Suppl 7):G5–G10. doi:10.1016/j.vaccine.2008.11.006 — Reviews the shifting bacterial causes of ear infection, including the rising relative role of M. catarrhalis.
Lieberthal AS, Carroll AE, Chonmaitree T, et al. The diagnosis and management of acute otitis media. Pediatrics. 2013;131(3):e964–e999. doi:10.1542/peds.2012-3488 — The American Academy of Pediatrics guideline that frames “watchful waiting” for many non-severe ear infections.
Sethi S, Murphy TF. Infection in the pathogenesis and course of chronic obstructive pulmonary disease. New England Journal of Medicine. 2008;359(22):2355–2365. doi:10.1056/NEJMra0800353 — Authoritative review of how respiratory bacteria, including M. catarrhalis, drive COPD exacerbations.
Sethi S, Evans N, Grant BJB, Murphy TF. New strains of bacteria and exacerbations of chronic obstructive pulmonary disease. New England Journal of Medicine. 2002;347(7):465–471. doi:10.1056/NEJMoa012561 — Landmark study showing that acquiring a new bacterial strain, including M. catarrhalis, is associated with the onset of a COPD flare.
Murphy TF, Brauer AL, Grant BJB, Sethi S. Moraxella catarrhalis in chronic obstructive pulmonary disease: burden of disease and immune response. American Journal of Respiratory and Critical Care Medicine. 2005;172(2):195–199. doi:10.1164/rccm.200412-1747OC — Quantifies how often M. catarrhalis causes exacerbations and documents the immune response it provokes.
Khan MA, Northwood JB, Levy F, et al. bro β-lactamase and antibiotic resistances in a global cross-sectional study of Moraxella catarrhalis from children and adults. Journal of Antimicrobial Chemotherapy. 2010;65(1):91–97. doi:10.1093/jac/dkp401 — Global survey documenting the near-universal BRO beta-lactamase that inactivates plain penicillins.
Perez AC, Murphy TF. A Moraxella catarrhalis vaccine to protect against otitis media and exacerbations of COPD: an update on current progress and challenges. Human Vaccines & Immunotherapeutics. 2017;13(10):2322–2331. doi:10.1080/21645515.2017.1356951 — Reviews why no vaccine yet exists and the candidates in development to prevent ear infections and COPD flares.

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