Biochemical Tests for Moraxella Catarrhalis Identification

Moraxella catarrhalis is identified in the clinical laboratory through a short series of biochemical tests, typically starting with Gram stain and oxidase reaction and then confirmed with an esterase-based assay such as tributyrin hydrolysis or the butyrate esterase spot test. These few steps are enough to separate it from the organism it is most easily confused with, Neisseria species, and the whole process can be completed on the same day a culture turns positive. The simplicity of the biochemical approach is part of what makes M. catarrhalis one of the more straightforward respiratory pathogens to pin down in a routine microbiology lab, though the details of each test and the pitfalls worth watching for are more interesting than that summary suggests.

Colony Appearance and the Hockey Puck Sign

Before any biochemical reagent touches the isolate, the first clue comes from the culture plate itself. On blood agar, M. catarrhalis grows as small, grey or white, opaque colonies that are non-hemolytic, smooth, and dry, typically reaching one to three millimeters in diameter after a day of incubation. On chocolate agar, the colonies tend to look pinkish-brown.¹ The single most characteristic physical feature is the so-called “hockey puck sign”: if you nudge a colony with a bacteriological loop, it slides intact across the agar surface instead of smearing or breaking apart.² This behavior is unusual enough that experienced technologists use it as a quick presumptive indicator before any formal testing begins. The colony essentially stays cohesive, as though it were a tiny disc being flicked across ice.

Colony morphology alone is never definitive, though. Several other organisms can grow in similar-looking colonies, and the hockey puck sign is a helpful hint rather than a diagnostic test. What it does is narrow the list of suspects quickly enough that only a couple of additional tests are needed to clinch the identification.

The First-Line Biochemical Screen

Once a suspicious colony is spotted, laboratories rely on a small battery of rapid tests to make a preliminary identification. The combination that has proven most reliable in routine practice is Gram stain plus oxidase plus one esterase assay. A study that evaluated 153 strains across multiple test methods concluded that this three-test combination was both dependable and convenient for everyday lab use.³

On Gram stain, M. catarrhalis appears as Gram-negative diplococci, meaning pairs of kidney-bean-shaped cells that lose the crystal violet stain. This morphology immediately places it in the same visual category as Neisseria species, which is exactly why additional biochemical steps are required. The oxidase test is the next checkpoint: M. catarrhalis is oxidase-positive, which confirms that it belongs to the group of aerobic Gram-negative cocci. But Neisseria species are also oxidase-positive, so the oxidase result narrows the field without finishing the job.

The third leg of this screening trio is an esterase test, and here is where M. catarrhalis finally separates itself. The organism produces butyrate esterase (a C4-esterase) at levels that Neisseria species do not. Detecting that enzyme is the keystone of biochemical identification, and labs have several substrate options to choose from.

Esterase Tests That Separate Moraxella from Neisseria

The reason esterase testing works so well is that all M. catarrhalis strains test positive for butyrate esterase activity, while all Neisseria species test negative. That clean separation makes it one of the most reliable single differentiating tests in clinical microbiology for this group of organisms.⁴ Three substrates are commonly used, each detecting the same underlying enzymatic activity but through different chemistry:

Tributyrin: A lipid substrate that is hydrolyzed by the organism’s esterase. A positive result shows as clearing around colonies on tributyrin agar or as a color change on commercially available test strips. It is simple but can take longer to read than the fluorogenic alternatives.
4-Methylumbelliferyl butyrate (MUB): A fluorogenic substrate that releases a fluorescent compound when cleaved by butyrate esterase. The result can be read under a UV lamp within minutes, making it one of the fastest options. A study of 47 M. catarrhalis strains, 89 Neisseria strains, and 82 other organisms found that the MUB spot test correctly differentiated M. catarrhalis from all Neisseria species tested.⁵
Indoxyl acetate: A chromogenic substrate that produces a visible blue-green color when hydrolyzed. It is available as a disk or strip test, making it especially convenient for labs that prefer a visual readout without needing a UV lamp.

Both the MUB tube test and the indoxyl acetate strip test provide same-day identification from primary culture plates, meaning results can be reported on the day the colony is first seen.⁴ The choice between substrates is largely a matter of laboratory preference, available equipment, and cost. All three perform well enough that any one of them, combined with Gram stain and oxidase, constitutes a reliable identification.

Newer probe chemistry continues to build on the same esterase principle. Researchers have developed colorimetric and fluorogenic probes using a boron dipyrromethene (BODIPY) dye scaffold that also detects the C4-esterase activity characteristic of M. catarrhalis, aiming for improved signal brightness and dual-readout capability.⁶ These remain research-stage tools for now, but they illustrate how central butyrate esterase detection is to the identification workflow: even cutting-edge probes still target the same enzymatic signature.

DNase and Other Supplementary Tests

Some laboratories add a DNase test to their confirmation panel. M. catarrhalis is DNase-positive, meaning it produces an enzyme that degrades DNA. This is tested on DNase agar plates that contain methyl green dye; if the organism produces DNase, the dye is released and a clear zone appears around the growth. One large population-based study used a combination of oxidase, tributyrin, and DNase positivity, alongside Gram stain and colony morphology, to verify M. catarrhalis isolates.¹

DNase testing is not as universally employed as esterase testing because the esterase assays alone are usually sufficient. Where it adds value is as a belt-and-suspenders measure, particularly when dealing with a strain whose esterase result is ambiguous or when the lab wants extra confidence before reporting a clinically significant isolate.

M. catarrhalis is also catalase-positive and does not produce acid from common sugars (glucose, maltose, sucrose, lactose). The absence of sugar fermentation helps distinguish it from Neisseria meningitidis (which ferments glucose and maltose) and Neisseria gonorrhoeae (which ferments glucose only). Sugar utilization testing used to be a more central part of the identification algorithm, but the esterase tests have largely replaced it for day-to-day identification because they are faster and more definitive.

Nitrate Reduction and What It Tells You

M. catarrhalis reduces nitrate to nitrite, a characteristic that can be detected with a standard nitrate reduction test. Its genome encodes a truncated denitrification pathway capable of converting nitrate through to nitrous oxide.⁷ In practice, nitrate reduction is a useful confirmatory detail rather than a frontline test. Most Neisseria species do not reduce nitrate, so a positive nitrate reduction result in an oxidase-positive Gram-negative diplococcus adds one more tick in the M. catarrhalis column. That said, laboratories rarely need this test if the esterase result is cleanly positive, and it takes longer to perform than the esterase spot tests.

Commercial Kit and Card Systems

For labs that prefer standardized, packaged identification systems rather than assembling individual tests, several commercial products are available that incorporate M. catarrhalis identification into a broader panel for fastidious Gram-negative organisms.

The BactiCard Neisseria system uses four chromogenic substrates arrayed on a card, including indoxyl butyrate esterase for M. catarrhalis, along with separate substrates targeting enzymes characteristic of N. gonorrhoeae, N. meningitidis, and N. lactamica. In an evaluation of 125 M. catarrhalis isolates, the card correctly identified about 98% of them.⁸ The simplicity of a card format makes it attractive for labs without heavy automation.

The API NH strip, designed for Neisseria, Haemophilus, and Moraxella identification, takes about two hours to run. In an evaluation of 305 strains belonging to species covered by its database, roughly three-quarters were identified without any additional testing, and only a single strain was misidentified.⁹ The remaining strains needed supplementary tests to reach a final identification, which is common for any kit-based system handling the full diversity of fastidious Gram-negatives.

The VITEK 2 NH card takes the process further into automation. One multicenter evaluation comparing its results to 16S ribosomal RNA gene sequencing found correct identification rates above 96% for clinical isolates.¹⁰ A separate evaluation of 188 strains across ten genera found that the card identified 91% of strains without any extra tests, and with simple supplementary testing the correct-identification rate climbed to about 97%, with results available within six hours.¹¹ For high-throughput laboratories already running VITEK 2 instruments, the NH card slots neatly into the existing workflow.

Each of these systems still relies on the same core biochemistry as the manual tests; what changes is the packaging, the automation, and the breadth of organisms covered in a single panel. A lab using the BactiCard is still detecting butyrate esterase, just in a pre-formatted card rather than with a tube of MUB.

Beta-Lactamase Testing

Although not used for species identification per se, beta-lactamase testing is almost always performed alongside it and is worth understanding as part of the complete workup. The vast majority of M. catarrhalis isolates produce BRO-type beta-lactamases, enzymes that break down penicillin and ampicillin and render those antibiotics ineffective. In a European surveillance study of 419 isolates, 92% were beta-lactamase positive.¹² A separate study of 64 isolates from carrier children found 89% beta-lactamase positive by nitrocefin disk testing, with the majority typed as BRO-1.¹³

The nitrocefin disk test is the standard rapid method here. A colony is smeared onto a disk impregnated with nitrocefin, a chromogenic cephalosporin. If beta-lactamase is present, the nitrocefin ring is cleaved and the disk turns from yellow to red within minutes. The test does not tell you which type of BRO enzyme the organism carries (BRO-1 versus BRO-2), but it tells clinicians immediately whether first-line penicillins will work. Given that well over 90% of isolates in most surveys are beta-lactamase positive, the practical implication is that amoxicillin-clavulanate, second- or third-generation cephalosporins, or macrolides are typically needed instead of plain amoxicillin or ampicillin.

Where MALDI-TOF and Molecular Methods Fit In

In laboratories equipped with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), identification of M. catarrhalis has become almost trivially fast. The instrument ionizes proteins from a small smear of a colony and matches the resulting spectral fingerprint against a reference database. In one study of over 1,100 clinical isolates spanning bacteria and yeasts, MALDI-TOF MS correctly identified roughly 96% to the species level and 99.5% at least to the genus level.¹⁴ For M. catarrhalis specifically, the spectral pattern is distinctive enough that misidentification is rare.

Real-time PCR assays targeting M. catarrhalis-specific DNA sequences offer another route, and these can detect the organism directly in clinical specimens like nasopharyngeal secretions without waiting for a culture to grow. One such assay applied to secretions from 184 patients with respiratory infections showed 100% sensitivity and up to 98% specificity compared with culture.¹⁵ The speed advantage is significant: PCR can deliver results in hours from a raw specimen, while culture-based identification requires overnight incubation before biochemical testing even begins.

Despite these advances, biochemical tests have not disappeared. MALDI-TOF instruments cost hundreds of thousands of dollars and require ongoing database licensing, putting them out of reach for smaller or resource-limited laboratories. PCR assays require reagents, thermal cyclers, and technical expertise that may not be available everywhere. The simple, inexpensive combination of Gram stain, oxidase, and an esterase strip remains the workhorse of M. catarrhalis identification across much of the world.

A Name That Keeps Changing

If you encounter older literature referring to “Branhamella catarrhalis,” that is the same organism. M. catarrhalis was previously classified in the genus Branhamella and was for years considered a harmless commensal of the upper respiratory tract.¹⁶ Over the past few decades it has been recognized as a genuine pathogen, particularly in children with otitis media, adults with chronic obstructive pulmonary disease exacerbations, and elderly or immunocompromised patients with lower respiratory infections. The shift in how the organism is regarded has influenced how aggressively laboratories pursue its identification. Where it was once considered a contaminant or irrelevant colonizer, a positive identification now typically triggers an antimicrobial susceptibility report and clinical attention.

The taxonomic reclassification also explains why some older test evaluation papers use the name “Moraxella (Branhamella) catarrhalis” with both genus names stacked together, a convention that was common during the transitional period. For practical purposes in the lab, the identification workflow is the same regardless of which name appears on the bench card.

Common Pitfalls and Edge Cases

The biochemical identification of M. catarrhalis is straightforward most of the time, but a few scenarios can trip up even experienced laboratories. The most frequent source of confusion is Neisseria species. Because both organisms appear as oxidase-positive Gram-negative diplococci, relying on Gram stain and oxidase alone will not get you to a species-level identification. Labs that skip the esterase test occasionally misidentify M. catarrhalis as a Neisseria or vice versa, which can have real clinical consequences since the two genera differ in their clinical significance and antibiotic susceptibility.

Another potential stumbling block involves mixed cultures. In respiratory specimens, M. catarrhalis often coexists with other flora, and picking a colony that looks right on the plate but actually contains a mixture of organisms can produce confusing or inconsistent biochemical results. Making sure the colony being tested is pure is a basic quality control step that is easy to overlook when workloads are heavy.

Rare atypical strains do exist. Occasional isolates have been reported with weaker-than-usual esterase activity, which can produce a borderline or delayed positive result on the butyrate esterase test. When this happens, adding a second confirmatory test like DNase or checking sugar utilization patterns is prudent. The key is not to panic over a single faint result but to layer in another test rather than guess.

Finally, the hockey puck sign, while memorable and useful, is not 100% specific. Some other Moraxella species and even occasional non-Moraxella organisms can form cohesive colonies. Treat it as a pointer toward further testing, not as an endpoint in itself. Combined with the biochemical panel, it is a strong piece of the puzzle, but no single observation should stand alone when identifying a clinically significant respiratory pathogen.