ASPERGILLUS NIGER: CHARACTERISTICS, MORPHOLOGY AND PATHOLOGIES - BIOLOGY - 2025

Aspergillus niger is an environmental mycelial fungus, formed by septate hyaline hyphae. It is a ubiquitous fungus with a worldwide distribution of saprophytic life. This means that its life cycle is in nature, without involving man. Therefore, its implantation in human tissues is incidental to its normal cycle.

That is why all species of this genus are considered opportunistic pathogens. In the case of A. niger, it is the third most isolated species of this genus in opportunistic infections in humans.

By Bruno Alexandre Quistorp Santos, Seteno Karabo Obed Ntwampe and James Hamuel Doughari (), via Wikimedia Commons

In invasive infections Aspergillus niger represents 3-7%, being more frequent in otychomycotic infections and skin affections. Although it can cause opportunistic pathologies, it has a beneficial side on an industrial level.

This microorganism is used for the biodegradation of industrial waste and from there substances and enzymes are elaborated that are useful in the manufacture of a great variety of edible and inedible products.

characteristics

Reproduction

Aspergillus niger reproduces asexually through the production of conidia. Its conidia can be found in the soil and in a large number of natural substrates. They are spread thanks to the wind, to settle on different surfaces.

Contagion

In general, this microorganism preferentially affects adults more than children and men more than women. All breeds can be affected and the diseases it produces are not contagious.

Benefits

On the other hand, A. niger presents another side of the coin, with beneficial uses for environmental sanitation by degrading industrial waste that is then used to produce beneficial products.

So much so, that fermentation with A. niger is recognized as GRAS (Generally recognized as safe) by the FDA (Food and Drug Administration by the United States).

Despite the extensive industrial application of this microorganism, the genetic map of this fungus is only partially understood.

Taxonomy

Fungi kingdom

Phylum: Ascomycota

Class: Eurotiomycetes

Order: Eurotiales

Family: Aspergillaceae

Genus: Aspergillus

Species: niger.

Morphology

Macroscopic characteristics

Colonies of A. niger grow rapidly and are easily recognizable by their characteristic dusty appearance. At first the mycelium is white, then it becomes dark and finally they acquire different colors, ranging from jet black to dark brown.

The reverse side of the colony looks like a gray-yellowish suede cloth, which distinguishes A. niger from other fungi with dark colonies called dematiaceous fungi.

Microscopic characteristics

Aspergillus niger has a smooth or slightly granular conidiophore that is 1.5 to 3 mm long, with a thick wall. They are usually hyaline or brown.

Under the microscope, abundant conidia with variable appearance can be observed: among them globose, subglobose, elliptical, smooth, equinulate, warty or with longitudinal striae, all black.

The vesicles are globose, hyaline, or stained dark brown, measuring 75 µm in diameter. They are generally not observable, due to the dense accumulation of black conidia.

The phialides appear in two radiated series.

It does not have sexual reproduction structures.

Pathologies and clinical manifestations

Otomycosis

It is one of the pathologies caused by the genus Aspergillus, where the niger species is the main causal agent. This pathology is characterized by affecting the ear canal secondary to the implantation of a previous bacterial infection.

Bacterial infection provides the necessary moisture for the fungus to progress to internal structures.

The symptoms it causes are itching, pain, otorrhea and deafness due to irritation of the tissue, plus the mycelial plug and debris. The symptomatology disappears with the lavage of the canal. In this way the plug is removed.

On the other hand, antibacterial treatment must be given to eliminate the bacteria present, which are the primary cause of the infection and those that provide the optimal conditions for the development of the fungus.

In earwax samples the structures of the fungus can be seen.

Bronchial aspergilloma

Aspergillus niger is the second leading cause of bronchial Aspergilloma in America. This disease is characterized by the formation of a ball or compact colony of the fungus that can measure 3-4 cm in diameter.

This generally sits at the apex of the lung and adheres to the bronchial wall without penetrating it. Its evolution can take years.

The clinical signs are intermittent hemoptysis, due to irritation of the bronchial wall with the rubbing of the ball, there is no fever or expectoration.

Primary and secondary skin diseases

When the lesions are primary they consist of multiple nodules, the skin becomes thick, edematous with a purplish color. Black scabs with a raised erythematous border may form.

The fungus is found in the superficial, middle and deep dermis. It can be accompanied by stinging and pain. Histologically there are numerous giant cells and central necrosis. It can be confused with lepromatous leprosy.

It is treated with nystatin topically. In disseminated cases where cutaneous aspergillosis occurs secondarily, the lesions usually start as small, discrete red papules that turn into pustules.

Small granulomas with central necrosis are seen on biopsy. The organism can be visualized as radiant colonies.

Culture

Sabouraud-dextrose agar, yeast extract malt agar and Czapek are used to grow A. niger. Antibiotics generally need to be added to restrict the growth of contaminating bacterial microorganisms.

The use of cycloheximide as an antibiotic in culture media should be avoided, as some strains are affected by this drug.

Once seeded, the samples are incubated at room temperature or 37 ° C. They grow in 3 to 4 days.

KOH and Parker ink are used to visualize the structures of the fungus on direct examination.

Uses / applications

Aspergillus niger has a complex metabolic network, made up of 1,190 reactions and 1,045 metabolites, distributed in three compartments: extracellular, cytoplasmic, and mitochondrial.

The industry has taken advantage of these characteristics of A. niger and therefore has had to control certain important factors that regulate A. niger morphology and the fermentation process.

These factors are: nutrient levels and environmental conditions, such as pH, agitation, temperature, metal ions, phosphate concentration, nitrogen source, carbon source, alcohols and additives.

Citric acid

Among the most important substances that A. niger produces and accumulates is citric acid, although there are other microorganisms that also do it, such as Citromyces, Penicilium, Monilia, Candida and Pichia.

Citric acid is useful in the preparation of beverages, sausages, medicines, cosmetics, plastic and detergents. The most effective strains for its production are those with low activity of the enzymes isocitrate dehydrogenase and aconitase hydratase. Meanwhile, they must have high citrate synthetase activity.

Whey has been found to be an excellent substrate for the production of citric acid by Aspergillus niger, as it easily assimilates lactose without the need for prior hydrolysis.

Another use that the industry gives to Aspergillus niger is to obtain enzymes, such as α-amylase, aminoglycosidase, catalase, cellulase, α-galactosidase, ß-galactosidase, ß-gluconase, glucoamylase or glucose aerodehydrogenase. As well as glucose oxidase, α-glucosidase, α-D-glucosidase, ß -glucosidase, lipase, invertase, hesperidinase, hemicellulase, pectinase, pytase, protease and tannase. All for industrial use.

References

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