Moraxella osloensis is a Gram-negative coccobacillus belonging to the family Moraxellaceae. Classified as a non-fermenter, it does not utilize carbohydrates through fermentation for energy production. While often considered low-virulence, its presence in patient samples can indicate a potential opportunistic infection, especially in vulnerable individuals. Accurate identification of this organism is necessary in clinical microbiology.
Morphology, Habitat, and Growth Profile
Characterization of Moraxella osloensis begins with microscopic examination. Gram staining reveals short, plump Gram-negative rods, often described as coccobacilli, appearing in pairs or short chains. Some strains may exhibit pleomorphism, with variations in size and shape, especially in older cultures.
M. osloensis is a strict aerobe, requiring oxygen for growth, and typically grows well between 33°C and 37°C. It is non-fastidious, growing on common laboratory media like Sheep Blood Agar and Tryptic Soy Agar. Colonies are usually visible within 24 to 48 hours, appearing small to medium, circular, translucent, and unpigmented.
The organism is widely distributed in the environment and is a common saprophyte on human skin and mucosal surfaces, particularly in the respiratory tract. While it often grows on blood agar without causing hemolysis, colonies can sometimes cause slight pitting of the agar surface. The presence of M. osloensis as normal flora makes the context of its isolation from a clinical specimen important for determining its significance.
Standard Biochemical Identification Tests
To differentiate M. osloensis from other Gram-negative non-fermenters, characteristic biochemical tests are performed. Isolates consistently yield a positive oxidase test, indicating the presence of cytochrome c oxidase. The catalase test, which detects the enzyme catalase, is also typically positive for this species.
A defining feature of M. osloensis is its non-saccharolytic metabolism; it does not utilize or produce acid from carbohydrates like glucose, distinguishing it from Enterobacterales. This is confirmed using the Oxidation-Fermentation (O-F) test, where no acid is produced in either the oxidative or fermentative tubes. The species is also generally negative for nitrate reduction, which helps distinguish it from certain other Gram-negative rods.
Tributyrin hydrolysis is another differentiating test, often used to distinguish Moraxella catarrhalis from other members of the genus. M. osloensis is typically negative for tributyrin hydrolysis, which helps narrow down the identification within the Moraxella group. While commercial identification kits are commonly used in clinical laboratories, they can sometimes struggle to reliably identify M. osloensis due to the similarity in biochemical profiles among various Moraxella species.
Molecular and Automated Confirmation Methods
When conventional biochemical tests or commercial kits fail to provide definitive identification, modern, high-throughput technologies are employed. Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) is often the method of choice due to its speed and accuracy. This technique creates a unique protein fingerprint of the isolate, primarily focusing on ribosomal proteins, which is compared against a comprehensive database of known microbial spectra.
MALDI-TOF MS can identify M. osloensis rapidly, often within minutes of culture growth. However, its accuracy relies heavily on the completeness of the spectral database; misidentification can occur if the database lacks sufficient reference spectra for atypical strains. Despite limitations in complex cases, its reliability for identifying most clinical isolates has revolutionized the workflow for Gram-negative non-fermenters.
For ambiguous or novel isolates, 16S rRNA gene sequencing remains the gold standard for definitive identification. This method involves amplifying and sequencing the gene that codes for the small subunit of the ribosomal RNA, which serves as a highly conserved molecular clock for bacteria. A sequence match of 98% or greater with a known M. osloensis sequence in a public database, such as GenBank, provides the highest confidence for species assignment.
Context of Isolation
Understanding the source and condition of the patient is paramount when Moraxella osloensis is isolated, as it frequently acts as a harmless colonizer. The bacterium is considered an opportunistic pathogen, possessing low virulence.
It primarily causes infection in individuals with compromised immune systems or pre-existing conditions. Immunosuppression due to cancer, chemotherapy, or chronic disease significantly increases the risk of invasive infection.
Infections due to M. osloensis are varied but often involve sites related to indwelling medical devices, such as central venous catheter infections. It has also been implicated in serious, though rare, conditions including bacteremia, endocarditis, meningitis, and osteomyelitis.
Determining whether the isolate is a true pathogen or simply a contaminant requires careful correlation with the patient’s clinical presentation and the source of the specimen.