Moraxella catarrhalis: Identification, How to Tell It from Neisseria, and Disease

A Gram stain from a sputum sample shows Gram-negative diplococci, pairs of kidney-shaped cocci that look exactly like Neisseria. But this is from an elderly patient with a chest infection, not a genital or meningitis sample. Is it a harmless commensal, a respiratory pathogen, or a dangerous Neisseria? The organisms look identical down the microscope, so the answer comes from a few quick tests, not the Gram stain.

That organism is often Moraxella catarrhalis, a common cause of ear, sinus, and lower respiratory infections that closely resembles Neisseria. This page is about how M. catarrhalis causes disease and, above all, how the laboratory tells it apart from the Neisseria it mimics.

Introduction

Moraxella catarrhalis is a Gram-negative diplococcus, a bacterium that grows in pairs and closely resembles the cocci of the genus Neisseria. It is a common cause of respiratory infections and is important in the laboratory largely because it must be told apart from the Neisseria species it mimics.

Figure: Important properties of Moraxella catarrhalis

The name carries its history. The genus Moraxella is named after Victor Morax, the Swiss ophthalmologist who first described it. The species name catarrhalis comes from "catarrh" (Greek for "to flow down"), describing the profuse nasal and eye discharge of the colds and inflammation the organism is associated with. It was formerly called Branhamella catarrhalis, a name still seen in older texts.

One point of identification worth noting at the outset: within the genus Moraxella, most species are coccobacilli, but M. catarrhalis is a diplococcus. So the coccal shape both makes it resemble Neisseria and separates it from its own genus relatives.

Where it lives and how it spreads

M. catarrhalis is found only in humans and lives as a commensal of the upper respiratory tract. It is carried more often by children and older adults than by healthy young adults. It spreads by respiratory droplets and causes opportunistic infection, meaning it usually causes disease only when conditions favor it (young age, old age, or damaged airways).

Diseases

M. catarrhalis is now recognized as a genuine respiratory pathogen, not just a harmless commensal. Its diseases fall into a clear pattern by age and airway health:

• In children: it is one of the three main causes of otitis media (middle ear infection) and sinusitis, alongside Streptococcus pneumoniae and Haemophilus influenzae.
• In older adults with chronic lung disease (COPD): it causes lower respiratory infections, including bronchitis and bronchopneumonia, often as an exacerbation of their existing lung disease.
• Rarely: it can cause invasive disease such as bacteremia, endocarditis, or meningitis.

A useful way to hold the diseases together: M. catarrhalis is a respiratory-tract organism that attacks at the two ends of life and in damaged airways, the child's middle ear and sinuses, and the older smoker's lower airways.

Virulence factors and how Moraxella causes disease

M. catarrhalis is an opportunist rather than an aggressive pathogen; its virulence factors help it stick, resist clearance, and survive antibiotics.

Adhesins. A set of surface proteins lets M. catarrhalis attach to the lining of the respiratory tract, the necessary first step in colonizing the nose, sinuses, middle ear, and airways. Attachment is what lets a commensal become an infection when the airway is vulnerable.

Lipooligosaccharide (LOS). Like other Gram-negative organisms, its outer membrane carries an endotoxin-like molecule (LOS, a shorter relative of the LPS in enteric bacteria) that triggers inflammation and contributes to the local tissue damage of otitis media and bronchitis.

Beta-lactamase (the key clinical factor). This is the most important practical point. The great majority of M. catarrhalis strains produce a beta-lactamase, an enzyme that destroys penicillin and ampicillin. This is why plain penicillins fail against it and why treatment relies on beta-lactamase-stable drugs. The near-universal beta-lactamase is the single most useful thing to remember about treating this organism.

Complement resistance and biofilm. Some strains resist killing by complement in serum, and M. catarrhalis can form biofilms (for example in the middle ear), which helps it persist and contributes to recurrent or hard-to-clear infection.

Putting it together

M. catarrhalis colonizes the upper airway using its adhesins, and when the airway is vulnerable (a child's short eustachian tube, an older smoker's damaged bronchi), it moves into the middle ear, sinuses, or lower airways. Its LOS drives the inflammation of the resulting otitis media or bronchitis, biofilm helps it persist, and its beta-lactamase lets it survive plain penicillins. The organism's whole clinical behavior, common, opportunistic, and penicillin-resistant, follows from these few factors.

Identifying Moraxella catarrhalis in the laboratory

The whole challenge with M. catarrhalis is that it looks like Neisseria. Distinguishing them is the main laboratory task.

Gram stain. M. catarrhalis appears as Gram-negative diplococci, oval, about 0.6 to 1 μm, with the adjacent sides flattened (kidney-bean or coffee-bean pairs). In sputum it is often seen with pus cells and sometimes inside them. This appearance is essentially identical to Neisseria, so the Gram stain alone cannot separate them.

Colony appearance: the one visual clue. On blood and chocolate agar, M. catarrhalis forms distinctive grey-white, dry, brittle colonies that can be pushed intact across the agar with a loop ("hockey-puck" behavior), a colony you can slide like a puck. This dry, movable colony is a useful early clue that distinguishes it from the more butyrous (buttery) colonies of Neisseria.

Telling Moraxella catarrhalis from Neisseria

Because they look identical on Gram stain, a few tests do the real work. The key separators, in order of usefulness:

• Sugar utilization is the classic distinction. M. catarrhalis is asaccharolytic: it does not produce acid from glucose, maltose, lactose, or sucrose. Neisseria species DO ferment sugars (for example, N. gonorrhoeae uses glucose only; N. meningitidis uses glucose and maltose). So an organism that looks like Neisseria but ferments no sugars points to Moraxella.
• DNase: M. catarrhalis is DNase-positive; Neisseria is DNase-negative. This is one of the most reliable separators.
• Butyrate esterase (tributyrin): M. catarrhalis is positive; Neisseria is negative. This is the basis of rapid spot tests for M. catarrhalis.
• Nitrate reduction: M. catarrhalis reduces nitrate; N. gonorrhoeae does not.
• Growth: M. catarrhalis grows on plain blood agar at 35°C, whereas the pathogenic Neisseria are fastidious and prefer enriched media.

Figure: Neisseria and Moraxella in Carboferm neisseria test

The full comparison:

Treatment

The defining fact is resistance: almost all M. catarrhalis strains produce a beta-lactamase, so plain penicillin and ampicillin do not work.

Treatment therefore uses beta-lactamase-stable options: a beta-lactam combined with a beta-lactamase inhibitor (such as amoxicillin-clavulanate), or agents such as certain cephalosporins, macrolides, or co-trimoxazole, guided by local patterns. Specific doses and durations are clinical decisions and are not covered here.

How to remember

Looks like Neisseria, ferments nothing. M. catarrhalis is a Gram-negative diplococcus that mimics Neisseria on the Gram stain, but it produces acid from no sugars (asaccharolytic), while Neisseria ferments at least glucose. Sugar fermentation is the classic separator.

The hockey-puck colony. Its colony is dry and brittle and can be slid across the agar with a loop like a puck. That "pushable colony" is a hands-on clue you won't forget, and it's unlike the buttery Neisseria colony.

DNase and butyrate positive. Two quick tests that are positive for Moraxella and negative for Neisseria. If you remember one, remember DNase-positive.

Two ends of life, plus damaged lungs. M. catarrhalis causes otitis media and sinusitis in children, and bronchitis and pneumonia in older adults with COPD. Young ears and old airways.

Beta-lactamase almost always. Nearly every strain makes beta-lactamase, so penicillin fails. Reach for a beta-lactamase-stable drug.

Key exam facts

Where students get confused

Moraxella vs Neisseria on Gram stain. They are indistinguishable as Gram-negative diplococci. You cannot separate them by microscopy alone. The separation is biochemical: sugar utilization (Moraxella ferments none, Neisseria ferments glucose), DNase, butyrate, and nitrate.

"Asaccharolytic" is the classic clue. A Neisseria-looking organism that produces acid from no sugars is Moraxella. Students expect a coccus that looks like Neisseria to behave like it; Moraxella deliberately does not ferment.

Moraxella catarrhalis is a diplococcus, but its genus relatives are coccobacilli. The coccal shape is what makes M. catarrhalis resemble Neisseria and what separates it from other Moraxella species, which are rod-like.

It is a real pathogen, not just flora. M. catarrhalis was long dismissed as a harmless commensal. It is now recognized as a genuine cause of otitis media, sinusitis, and COPD exacerbations.

Nearly all strains resist penicillin. Because almost all produce beta-lactamase, plain penicillin and ampicillin fail. This is a common treatment error; a beta-lactamase-stable drug is needed.

References

1. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Procop, G. W., et al. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.
3. Murphy, T. F., & Parameswaran, G. I. (2009). Moraxella catarrhalis, a human respiratory tract pathogen. Clinical Infectious Diseases, 49(1), 124–131. https://doi.org/10.1086/599375
4. Centers for Disease Control and Prevention. Identification of Neisseria gonorrhoeae and related species. CDC.