Taxonomy and Morphological Features
The genus Moraxella belongs to the family Moraxellaceae. These microorganisms were first isolated by researchers V. Morax and K. Axenfeld in 1896. The type species of the genus is Moraxella lacunata, though Moraxella catarrhalis is of immense clinical significance.
Morphologically, they are very small, polymorphic gram-negative rods or cocci, most commonly paired as diplobacilli or diplococci. A characteristic feature of this pairing is that the opposing sides of the cells are flattened. Occasionally, they may form short chains. Their cell walls contain true waxes.
Moraxellas lack true flagella and are considered non-motile. However, on solid growth media, they can exhibit a specific 'twitching motility', which is mediated by specialized hair-like structures called fimbriae.
Cultural Properties and Biochemistry
In terms of respiration, Moraxella species are strict aerobes. They are fastidious and have high nutritional requirements: for optimal growth, they need specific supplements such as amino acids, biotin, and lactate or succinate, which serve as primary carbon and energy sources. The optimal cultivation temperature is slightly below standard human body temperature, ranging from 33–35 °C.
On solid growth media, the bacteria can form two types of colonies:
- Rough (R-forms): Have uneven edges; formed by strains possessing fimbriae. They can cause pitting or corrosion of the agar.
- Smooth (S-forms): Very small colonies typical of strains lacking fimbriae.
The biochemical activity of Moraxellas is low: they belong to the group of non-fermenting bacteria, meaning they do not produce acid from carbohydrate breakdown. However, they actively produce several enzymes: catalase (+), oxidase (+), and nitrate reductase (+), which allows them to reduce nitrates.
Pathogenicity and Clinical Significance
Under natural conditions, Moraxella species have low environmental persistence. Their natural reservoir is the normal flora of the upper respiratory tracts of humans and animals.
Pathogenicity factors include:
- Endotoxin, released upon cell wall lysis;
- Fimbriae, ensuring secure adhesion to the epithelium;
- DNase activity.
Infections caused by Moraxella are opportunistic and predominantly endogenous in nature, developing mainly in immunocompromised patients. The spectrum of manifestations is quite broad:
- Eyes and ears: Conjunctivitis, acute otitis media, sinusitis.
- Respiratory tract: Pharyngitis, bronchitis, pneumonia.
- Systemic involvement: Endocarditis, meningitis, and urethritis.
Diagnosis, Treatment, and Prevention
Microbiological diagnosis is primarily based on culture methods (inoculating clinical specimens onto growth media). An important diagnostic criterion for the rapid identification of M. catarrhalis is a marker test for the production of the enzyme butyrate esterase.
In general, these microorganisms are susceptible to $\beta$-lactam antibiotics. However, clinicians must keep in mind that M. catarrhalis very frequently produces $\beta$-lactamases—enzymes that hydrolyze penicillins. Therefore, beta-lactamase inhibitor combinations (protected penicillins) or drugs from other pharmacological classes are recommended for empiric therapy.
To prevent recurrent respiratory infections, therapeutic vaccines composed of bacterial lysates are used. These preparations include antigenic components of M. catarrhalis that stimulate local mucosal immunity.