Moraxella

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, although Moraxella catarrhalis is also of immense clinical significance.

Morphologically, they are very small polymorphic Gram-negative rods or cocci, most commonly paired together (diplobacilli or diplococci). A characteristic feature of this paired arrangement is that the opposing sides of the cells are flattened. Sometimes they may form short chains. Their cell wall contains true waxes.

Moraxellae lack true flagella and are therefore considered non-motile. However, on solid nutrient media, they are capable of exhibiting a specific jerky motility («twitching motility»), which is provided by special pili — fimbriae.

Cultural properties and biochemistry

By type of respiration, moraxellae are strict aerobes. They are quite fastidious and demanding regarding nutrient media composition: for optimal growth, they require specific supplements such as amino acids, biotin, as well as lactate or succinate, which serve as the main sources of carbon and energy. The optimal temperature for their cultivation is slightly below the standard human body temperature, at 33–35 °C.

On solid nutrient media, the bacteria can form two types of colonies:

1. Rough (R-forms): have uneven edges, formed by strains possessing fimbriae. Capable of causing corrosion of the nutrient medium.
2. Smooth (S-forms): very small colonies characteristic of strains lacking fimbriae.

The biochemical activity of moraxellae is low: they belong to the group of non-fermenting bacteria, meaning they are incapable of producing acid during carbohydrate breakdown. At the same time, they actively produce a number of enzymes: catalase (+), oxidase (+), as well as nitrate reductase (+), through which they reduce nitrates.

Pathogenicity and clinical significance

Under natural conditions, moraxellae have low resistance in the external environment. Their natural habitat is the normal microflora of the mucous membranes of the upper respiratory tract in humans and animals.

Pathogenicity factors include:

• Endotoxin, released upon cell wall destruction;
• Fimbriae, ensuring reliable adhesion to the epithelium;
• DNase activity.

Infections caused by moraxellae are of an opportunistic, predominantly endogenous nature and develop mainly in patients with a reduced immune status. The spectrum of lesions is quite broad:

• Organs of vision and hearing: conjunctivitis, otitis media, sinusitis.
• Respiratory tract: pharyngitis, bronchitis, pneumonia.
• Systemic lesions: endocarditis, meningitis, as well as urethritis.

Diagnostics, treatment, and prevention

The basis of microbiological diagnostics is the bacteriological method (inoculation of material onto nutrient media). An important diagnostic criterion for the rapid identification of M. catarrhalis is the marker test for the production of the enzyme butyrate esterase.

In general, microorganisms are sensitive to $\beta$-lactam antibiotics. However, clinicians must take into account that M. catarrhalis very frequently produces $\beta$-lactamases — enzymes that degrade penicillins. For this reason, beta-lactamase-inhibitor combination penicillins or drugs from other pharmacological groups are recommended for empirical therapy.

To prevent recurrent respiratory infections, therapeutic vaccines are used, representing bacterial lysates (e.g., «Broncho-Vaxom», «Bronchomunal»). Among other components, they contain M. catarrhalis antigens that stimulate local immunity.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of the formation of rough (R) and smooth (S) colonies depending on the presence of fimbriae
• The mechanism of action of butyrate esterase and the methodology for performing the marker test
• True waxes in the cell wall of moraxellae: structure and biological role
• The role of DNase activity in the pathogenesis of moraxellar infections
• Composition and mechanism of action of bacterial lysates («Bronchomunal», «Broncho-Vaxom») for immunoprophylaxis

Other topics in the section «Gram-negative aerobic rods (Pertussis, Brucella, Tularemia, Legionella, Coxiella/Q fever, Bartonella, Pseudomonas, Burkholderia, Acinetobacter, etc.)»

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