Aspergillus: Characteristics, Morphology, Life Cycle, Habitat

Aspergillus is a genus of fungi that includes more than 100 species that are characterized by being filamentous. The fungi that belong to this genus are saprophytes and are found in habitats where there is a lot of humidity. They grow mainly on dead organic matter, which they help break down.

Likewise, some of the species that make up this genus are known human pathogens, causing pathologies mainly in the respiratory tract. These pathologies can range from simple sinusitis, to chronic aspergillosis and even a systemic infection.

Aspergillus niger seen under the electron microscope. Source: Mogana Das Murtey and Patchamuthu Ramasamy [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

Due to its pathogenic potential, this type of fungi is a genus that has been the subject of numerous studies, which is why there are many data on it.

Article index

  • one

    Taxonomy

  • two

    Characteristics

  • 3

    Morphology

  • 4

    Lifecycle

    • 4.1

      Asexual reproduction

    • 4.2

      Sexual reproduction

  • 5

    Habitat

  • 6

    Main species

    • 6.1

      Aspergillus fumigatus

    • 6.2

      Aspergillus flavus

    • 6.3

      Aspergillus niger

    • 6.4

      Aspergillus tubingensis

  • 7

    Diseases

    • 7.1

      Aspergillosis

    • 7.2

      Fungal sinusitis

    • 7.3

      Otomycosis

  • 8

    Treatments

  • 9

    References

Taxonomy

The taxonomic classification of the genus Aspergillus is as follows:

  • Domain: Eukarya.
  • Kingdom: Fungi.
  • Phylum: Ascomycota.
  • Class: Eurotiomycetes.
  • Order: Eurotiales.
  • Family: Trichocomaceae.
  • Genus: Aspergillus.

Characteristics

The genus Aspergillus is made up of more than 100 species. However, despite there being so many, they have certain aspects in common.

One of its distinctive characteristics is its morphology, made up of conidiophores that end in an apical vesicle and that in turn present a basal foot cell that inserts into the hypha at the opposite end. Of course, depending on the species, the characteristics of the gallbladder can vary somewhat.

Likewise, the fungi of this genus are saprophytes, which means that they feed on dead or decomposing organic matter. Due to this, these fungi are an important part of the food chains of the ecosystems in which they are found, since they are a powerful decomposer of organic matter, transforming it into compost for the soil.

With regard to reproduction, the vast majority of species reproduce asexually, through conidia (spores), although in some a sexual part is also observed in their life cycle.

Morphology

The fungi of the genus Aspergillus are filamentous, made up mainly of chain cells that in turn form a structure known as hypha.

The hyphae that make up the mycelium of this fungus are characterized by being septate and having an approximate diameter of between 2.6 and 8.0 microns. In the same way, these hyphae are branched, generating the so-called conidial heads when they come into contact with the air. These can produce up to 500,000 conidia.

The structure of the conodial heads is as follows: they have a conidiophore that at its terminal end has a widening, like a kind of vesicle. Likewise, they are covered by structures called phialids that have an elongated shape.

The function of phialids is to produce large columns of conidia that are mostly round in shape and have a diameter of between 2 and 5 microns. These conidia are considered the infectious propagules that constitute the starting point for the development of the mycelium of the fungus.

Viewed under the microscope, the hyphae are uniform and have a tree-like branching pattern. Importantly, the branches are dichotomous. Likewise, hyphae have parallel contours.

The colonies that are obtained by culturing in the laboratory are of various colors. At first they are white, but later that color can vary to yellow, brown, green or even black. This will depend on the species of Aspergillus that is being cultivated. When it comes to the texture of the colonies, they look like cotton or velvet.

Lifecycle

As in many organisms of the fungi kingdom, the fungi belonging to the genus Aspergillus contemplate both sexual and asexual reproduction in their life cycle.

Asexual reproduction

The type of reproduction most frequently observed in these fungi is asexual. It is produced through asexual spores known as conidia. These grow at the ends of the phialids.

The conidia are released and transported by the action of the wind. When it falls to the substrate, if the environmental conditions of humidity and temperature are ideal, they begin to germinate.

Initially, the first structure to form is a germ tube that eventually transforms into a new mycelium.

Sexual reproduction

On the other hand, sexual reproduction is extremely rare in these fungi, being observed in very few species such as Aspergillus fumigatus. Most of the fungi of this genus are homothallic. This means that they have both male and female sexual organs in the same mycelium and even formed from the same hypha. Both organs are elongated, multinucleated, and tend to wrap around each other.

The female sexual organ is divided into three parts: the terminal segment known as the trichogin which functions as the receptive part. The next segment is known as the ascogonium, and below this is the stem.

Similarly, the male sexual organ, the polynodium, can grow in the same hyphae or in an adjacent one. It has a unicellular antheridium at its end.

Gamete fusion or plasmogamy occurs when the tip of the antheridium bends over the trichogyne and fuses with it. From here, the ascogenic hyphae are formed, which begin to branch to form another structure known as the ascocarp, which in the fungi of the genus Aspergillus is hollow and closed and is called cleistothecium.

Within the cleistothecium, the asci are formed, which in turn contain the so-called ascospores. There, the ascospores are free, feeding on the nutritive fluid that is there. Finally, when they fully mature, they are released. When they fall into the substrate they germinate, giving rise to a new mycelium.

Habitat

The fungi of the genus Aspergillus have a wide distribution throughout the planet. The ideal habitat for these fungi is hay and compost. It is common to find it growing on cereals that are stored in unsuitable conditions of humidity and temperature.

Like many fungi, it grows on decaying organic matter.

Main species

The genus Aspergillus exceeds 100 species. However, not all have been studied and recognized equally. The most representative species of the genus will be described below.

Aspergillus fumigatus

This is one of the fungi of the genus Aspergillus that has been most studied, since it constitutes an important pathogen for humans. It is the cause of numerous respiratory tract infections, mainly due to its inhalation.

It is a filamentous fungus that is considered ubiquitous, that is, it can be found in any ecosystem. It has saprophytic customs, which means that it develops on dead organic matter, which it degrades. It has the typical appearance of the mushrooms of this genus, with short and round conidiophores.

Aspergillus fumigatus. Source: CDC / Dr. Libero Ajello (PHIL # 4297), [Public domain]

In cultures, their colonies are initially white and later adopt a color ranging from bluish-green to grayish-green. The texture of these is similar to that of velvet.

This fungus presents in its life cycle two types of reproduction: asexual, through the conidia and sexual, mediated by ascospores. These are very resistant to high temperatures, even reaching up to 70 ° C.

Infection in humans by this organism occurs, in most cases, when spores found in the environment enter the respiratory tract. It can also happen through infection of a previous wound or mucous membranes. Sometimes it can cause an infection known as invasive aspergillosis, which is very dangerous and can even be fatal.

Aspergillus flavus

This is a fungus considered pathogenic because it produces toxins that are harmful to humans, known as aflatoxins. This fungus produces a total of four toxins: B1, B2, G1 and G2. These toxins are particularly toxic to the liver, where they can trigger cirrhosis to cancer in this organ.

The conidiophores of this species do not present any type of color. They also present a globose-looking broadening, which is surrounded by phialides. The conidia that occur in the phialid, have a color that ranges from yellow to green. They are found, in a general way, forming chains.

Colonies of this species can take on a wide variety of appearances, such as granular or scattered dust-like. As with many Aspergillus species , Aspergillus flavus colonies initially have a color (yellow) and as they mature they change it, becoming darker.

This fungus is related to certain pathologies such as aspergillosis, onychomycosis, fungal sinusitis and otomycosis, among others.

Aspergillus niger

It is one of the best known species of the genus Aspergillus . It owes its name to the fact that it produces a kind of black mold on the vegetables in which it grows.

The hyphae that make up the mycelium of this fungus form a thread and are divided by a septum, and are transparent. In the conidiophores there are globose vesicles that are covered by phialides. These undergo a process called basiseptal conidiogenesis, through which so-called globose mitospores are produced, measuring between 3 and 5 microns.

This species has great importance in the field of biotechnology, since it produces some chemical substances of interest such as gluconic acid, citric acid and some enzymes such as phytase and galactosidase.

Likewise, Aspergillus niger produces a toxin known as Ochratoxin A, which can contaminate food, passing to humans and other animals when they eat it. The effect of this toxin in the body is mainly limited to the immune system, reducing the formation of antibodies, as well as the size of the immune organs. Similarly, it produces an alteration at the level of cytokinins.

Aspergillus tubingensis

This is a species that has great ecological value, since it has been found to be able to digest plastic, even without leaving any residue. From an environmental point of view this is very important, since it can be used to clean up our ecosystems.

The conidia of this species have an approximate diameter of between 2 and 5 microns. It reproduces exclusively asexually and its ideal growth temperature is between 20 and 37 ° C.

Similarly, Aspergillus tubingensis is a species that produces certain substances such as ochratoxin A and mycotoxies.

Diseases

Some of the species that make up the genus Aspergillus are known human pathogens. They mainly cause respiratory tract infections.

Aspergillosis

It is an infection caused by various species of Aspergillus, especially Aspergillus fumigatus. Since its entry into the body occurs through inhalation, the tissues that are affected are those of the respiratory tract.

However, aspergillosis can present in several clinical forms: allergic bronchopulmonary aspergillosis, chronic pulmonary aspergillosis and invasive aspergillosis.

Allergic bronchopulmonary aspergillosis

Among the symptoms of this pathology are:

– Fever.

– Dark mucous expectoration.

– Hemoptysis (bleeding from the lungs).

– General discomfort.

– Airway obstruction.

Chronic pulmonary aspergillosis

This pathology is a compendium of different clinical pictures that affect various structures of the respiratory system. These are:

– Aspergilloma: it is a kind of foreign body made up of hyphae of the fungus, as well as mucus, pus, fibrin and cellular debris. This is housed in a lung cavity or even in one of the paranasal sinuses. Its symptoms include chest pain, bloody sputum, fever and chronic cough, among others.

– Gavitate chronic aspergillosis: occurs when the lung tissue is so affected that it develops several cavities, mainly at the level of the upper lung lobes. The symptoms are similar to those of aspergilloma, but they last over time, in addition to being much more intense.

Invasive aspergillosis

It is the most serious presentation of the disease and is seen only in people whose immune system is very weakened; for example, people with diseases of the immune system such as AIDS, people with some type of cancer who have undergone chemotherapy or those who have had a bone marrow transplant. It occurs when the infection is no longer limited to the lung tissue, but spreads to other organs such as the heart or kidneys.

Symptoms that may occur are:

– High fever that does not improve.

– Cough with bloody expectoration.

– Chest pain.

– Pain in the joints.

– Difficulty breathing.

– Headache.

– Inflammation in one of the eyes.

– Difficulty speaking.

– Skin lesions.

Fungal sinusitis

It occurs when the fungus colonizes any of the cavities found in the face, known as the paranasal sinuses. The symptoms are:

– Purulent or seromucosal rhinorrhea.

– Nasal obstruction or foreign body sensation.

– Frequent sneezing.

– Pain in the jaw and teeth.

Otomycosis

It occurs when the fungus invades the ear canal. Among its most representative symptoms we find the following:

– Ear pain.

– Nonspecific itching in the ear.

– Desquamation of the epithelium.

– Inflammation.

– Hearing loss.

– Presence of dark colored residues, such as green, brown or black in the ear canal.

Treatments

The drugs used to treat infections caused by fungi of the genus Aspergillus are those that directly attack the fungus. The most used are:

– Amphotericin B.

– Itraconazole.

– Posaconazole.

– Echinocandins.

– Vorconazole.

Likewise, in some cases surgical excision of the lesions is recommended. However, this last option has practically stopped being used in recent times, thanks to the excellent results obtained with drug therapy.

References

  1. Bennet, J. and Klich, M. (2003). Mycotoxins. Clinical Microbiology Reviews. 16. 497-516.
  2. Fortún, J., Mije, Y., Fresco, G., Moreno, S. (2012). Aspergillosis. Clinical forms and treatment. Infectious diseases and clinical microbiology. 30 (4). 173-222
  3. García, P., García, R., Domínguez, I. and Noval, j. (2001). Otomicosis: clinical and microbiological aspects. Journal of Biological Diagnosis. 50 (1)
  4. Guerrero, V., Herrera, A., Urbano, J., Terré, R., Sánchez, I., Sánchez, F., Martínez, M. and Caballero, j. (2008). Chronic invasive fungal sinusitis of the maxillary sinus due to Aspergillus. Portuguese Journal of Otorhinolaryngology and Cervical Facial Surgery. 46 (2)
  5. Méndez, L. (2011). Aspergillosis. Retrieved from: http: /facmed.unam.mx/deptos/microbiologia/micologia/aspergilosis.html
  6. Germain, G. and Summerbell, R. (1996). Identifying filamentous fungi. Star Publishing company. 1st edition.

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