Aspergillus fumigatus characteristics, morphology, diseases

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Egbert Haynes

Aspergillus fumigatus is a cosmopolitan environmental fungus that produces opportunistic infections in man andIt is part of the usual microbiota found in the environment; decaying air, soil and vegetation.

It is also one of the main environmental fungi that contaminates important areas of clinics and hospitals, causing nosocomial infections in patients with a weakened immune system. Without a doubt, the most vulnerable patients are bone marrow transplants and patients with cystic fibrosis..

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Of the four Aspergillus species most frequently isolated from hospitalized patients, A. fumigatus is the one that causes the majority of allergic and invasive lung diseases.

The main outbreaks have been associated with construction sites near hospitals and pipelines contaminated with pigeon nests. Immunosuppressed patients become infected by inhaling the conidia contained in the contaminated dust.

Apparently the bronchi constitute an ecological niche, due to the presence of nutrients and temperature conditions for the fungus. This causes high rates of morbidity and mortality in these individuals..

Also on some occasions this microorganism can be seen contaminating food. The most commonly attacked are those rich in carbohydrates and fibers such as bread, sweets and grains..

Contaminated food can be recognized by showing a cottony film on the gray-green surface. This causes economic losses.

Article index

  • 1 Features
  • 2 Virulence factors
    • 2.1 Production of elastase
    • 2.2 Production of other substances implicated in pathogenicity
    • 2.3 Adhesion factors to host tissue
  • 3 Taxonomy
  • 4 Morphology
    • 4.1 Macroscopic characteristics
    • 4.2 Microscopic characteristics
  • 5 Diseases and symptoms
  • 6 Main pathologies and symptoms
    • 6.1 Allergic bronchopulmonary aspergillosis
    • 6.2 Invasive aspergillosis
  • 7 Microbiological diagnosis
  • 8 Treatment
  • 9 Prevention
  • 10 References

Characteristics

This fungus lives in nature fulfilling an important job in it, since it participates in the decomposition of vegetables and a great variety of organic materials.

On the other hand, Aspergillus fumigatus It is capable of growing at 37 ° C, but it can also grow at 50 ° C. Therefore, it is said to be a thermotolerant species. Its conidia can survive at 70 ° C.

It is believed that A. fumigatus reproduces almost exclusively asexually through the production of conidia.

Virulence factors

Elastase production

Elastase has been seen to be an important pathogenicity factor in other lung pathogens, because elastase acts on elastin present in total lung proteins.

However, the role of this enzyme in A. fumigatus It is not clear, since certain differences have been noted between some strains and others. What this suggests is that elastase activity may be due to the synthesis of different elastinolytic proteases and not to the same enzyme.

Production of other substances involved in pathogenicity

These include mitogilin, which has cytotoxic activity, a metalloprotease with residual collagenolytic activity, and a galactomannan excreted as an exoantigen of diagnostic importance..

Among other substances are a protein similar to that of heat shock that could intervene in the immunoregulatory mechanisms of aspergillosis and 2 cilioinhibitory factors that allow colonization of the mucosa.

The first is gliotoxin and the second has not yet been well characterized. However, it is known that gliotoxin together with other mycotoxins, such as helvolic acid and fumagillin, appear to have an immunosuppressive effect..

These substances inhibit the oxidative mechanism of destruction of phagocytes and help the spread of the microorganism..

Adhesion factors to host tissue

Aspergilllus fumigatus it has a special tropism to bind to fibrinogen deposited in damaged epithelia. It also interacts with laminin, which is one of the main components of the lung basement membrane..

Taxonomy

Fungi kingdom

Phylum: Ascomycota

Class: Eurotiomycetes

Order: Eurotiales

Family: Aspergillaceae

Genus: Aspergillus

Species: fumigatus.

Morphology

Macroscopic characteristics

It has a filamentous mycelium with hyaline hyphae. Your colonies can have a velvety to cottony appearance..

Its color varies from bottle green, greenish-gray or greenish-brown. A white layer is observed at the edge of the colony. The reverse can be colorless or yellowish red.

Microscopic characteristics

By observing a direct examination of the pure cultures, the characteristic structures of A. fumigatus. It is made up of smooth and short or semi-long conidiophores (300-500 µm). Generally with greenish spots especially in the terminal area.

It has vesicles 30 to 50 µm in diameter in the shape of a typically fertile bottle, showing a greenish pigmentation. This structure supports a tight row of parallel phialides.

The phialides are formed by long chains of spherical or slightly ovoid equinulate conidia of green color. These tend to curve in the direction of the central axis.

As a sexed reproduction structure, they have 500 µm yellow globose cleistothecia and ascospores with equatorial ridges. These characteristics are what allow the fumigatus species to be identified from the rest.

Diseases and symptoms

Aspergillosis can manifest itself in a number of ways, some more serious than others. It can present as an allergic picture, that is, allergic bronchopulmonary aspergillosis and allergic sinusitis.

This affects patients sensitized by repeated exposure to conidia or fungal antigens..

Other pathologies caused by this microorganism are chronic pulmonary aspergillosis, invasive aspergillosis, and extrapulmonary aspergillosis, which include skin, ear, and ophthalmic infections..

It can also present systemically, such as endocarditis and infection of the central nervous system..

The most common aspergillosis caused by A. fumigatus is allergic and invasive bronchopulmonary infection, while it ranks second in ear infections.

Main pathologies and symptoms

Allergic bronchopulmonary aspergillosis

It occurs in people allergic to spores, especially asthmatics when they come into contact with the allergen.

The predisposition to develop allergy to this fungus has been associated with the discovery of single nucleotide polymorphisms in the gene that encodes mannan-binding lectins and in the gene for surfactant protein D. This contributes to the susceptibility to allergic bronchopulmonary aspergillosis..

Allergic bronchopulmonary aspergillosis involves the bronchi and bronchioles. Peribronchiolar inflammation with eosinophil infiltrate also occurs. It is characterized by general malaise, cough, and respiratory distress.

The samples of brown sputum, with the presence of abundant eosinophils and Charcot-Leyden crystals, suggest that this clinical entity is present..

If the disease becomes chronic, it can lead to bronchiolar fibrosis and severe respiratory deterioration after several years. In general, the patient usually has marked eosinophilia in peripheral blood and high levels of Total IgE.

Invasive aspergillosis

Aspergilus fumigatus is responsible for 85-90% of invasive aspergillosis.

The invasive form is the aspergilloma or fungal ball. This entity develops from a pre-existing cavity due to a previous disease, for example tuberculosis. In this cavity the fungal ball is formed, which consists of a tangled mass of hyphae, often dead.

In general, the fungal ball does not invade other organs, but it is not ruled out that it causes the erosion of nearby structures, which can cause death..

These pathologies occur mainly in immunosuppressed or neutropenic patients, for example transplant patients and patients with leukemias or lymphomas..

Microbiological diagnosis

The microbiological diagnosis of aspergillosis is difficult to interpret. Although the microorganism is observed in clinical samples and is isolated in the different culture media, it is not indicative of disease.

This is because gender Aspergillus it can be an environmental pollutant or be present in the respiratory tract without causing pathology.

Ideally, to make a good diagnosis is to correlate everything, that is, results of the culture, detection of antigens of the fungal cell wall (galactomannan), clinical manifestations and characteristics of the patient's immune system (neutropenic, immunosuppressed, transplanted, among others).

In this way, an excellent diagnostic approach can be made, especially in severe cases of invasive aspergillosis..

Treatment

In allergic presentations, bronchodilators, antihistamines such as cromolyn disodium or glucocorticoids such as prednisone are given, 25 mg / day orally for one week, with progressive reduction.

In invasive aspergillosis, azole compounds (voriconazole, itraconazole), caspofungin, and amphotericin B are used in various combinations..

However, even with treatment, the mortality rate is very close to 100%. Surgical removal of localized lesions (lobectomy) is sometimes necessary..

Prevention

As effective protection measures to prevent nosocomial aspergillosis, special filters should be used, capable of retaining the conidia, in addition to a good renewal of the air in the hospital spaces..

The most important thing is to prevent exposure and contamination of susceptible or high-risk patients.

References

  1. Arenas R. Illustrated Medical Mycology. 2014. 5th Ed. Mc Graw Hill, 5th Mexico.
  2. Bonifaz A. Basic Medical Mycology. 2015. 5th Ed. Mc Graw Hill, Mexico DF.
  3. Blanco J, Guedeja J, Caballero J, García M. Aspergillosis: pathogenicity mechanisms involved and approach to laboratory diagnosis. Rev Iberoam Micol 1998; 15: (1): 10-15.
  4. Koneman, E, Allen, S, Janda, W, Schreckenberger, P, Winn, W. (2004). Microbiological Diagnosis. (5th ed.). Argentina, Editorial Panamericana S.A.
  5. Ryan KJ, Ray C. SherrisMicrobiology Medical, 2010. 6th Ed. McGraw-Hill, New York, U.S.A
  6. Casas-Rincón G. General Mycology. 1994. 2nd Ed. Central University of Venezuela, Library Editions. Venezuela Caracas.
  7. Wikipedia contributors. Aspergillus fumigatus. Wikipedia, The Free Encyclopedia. September 10, 2018, 11:46 UTC. Available at: wikipedia.org/. Accessed September 15, 2018.
  8. Bandres MV, Sharma S. Aspergillus fumigatus. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2018.
  9. Guazzelli L, Severo C, Hoff L, Pinto G, Camargo J, Severo L. Aspergillus fumigatus fungus ball in the pleural cavity. J. bras. pneumol.  2012; 38 (1): 125-132. Available from: scielo.br.

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