Aspergillus flavus characteristics, morphology, diseases

3748
Robert Johnston

Aspergillus flavus it is an environmental fungus that can affect as an opportunistic pathogen, producer of mycotoxins and as a contaminant of crops and food products. It can also be found contaminating leathers, fabrics, paints, broken dialysis bags, soft contact lenses, open medications, among others..

It is widely distributed in nature and together with other genera and species are important in the decomposition of organic matter. These play a fundamental role in the carbon and nitrogen cycle..

By Medmyco [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

This genus has great metabolic versatility, as well as a great capacity to spread and propagate its conidia, since its conidial head can produce more than 500,000 conidia..

The conidia are spread in the air, being able to reach many substrates. They are even found in deserts and high in the atmosphere. That is why anyone could cause allergic reactions due to hypersensitivity when there is continuous exposure..

It can also cause serious pathologies in immunosuppressed patients, behaving like an opportunistic pathogen.

On the other hand yeah A. flavus It develops on cereal grains such as corn, rice and peanuts, it will produce toxic substances on these. Among them: carcinogenic hepatotoxins and aflatoxins, which affect both humans and animals.

Article index

  • 1 Features
  • 2 Production of aflatoxins and other toxic substances
  • 3 Production of substances with antibacterial properties
  • 4 Taxonomy
  • 5 Morphology
    • 5.1 Macroscopic characteristics
    • 5.2 Microscopic characteristics
  • 6 Diseases and symptoms
    • 6.1 Fungal sinusitis
    • 6.2 Corneal infection
    • 6.3 Nasal-orbital aspergillosis
    • 6.4 Cutaneous aspergillosis
    • 6.5 Invasive pulmonary aspergillosis
    • 6.6 Human consumption of food contaminated with aflatoxins (aflatoxicosis)
  • 7 Prevention
    • 7.1 At an industrial level
    • 7.2 At the clinical level
  • 8 References

Characteristics

The genus Aspergillus is generally characterized by being anamorphic microorganisms (Deuteromycetes); that is, they only reproduce asexually. However, in some species, including A. flavus its teleomorphic forms (Ascomycetes) are known, that is, they have sexual reproduction.

Another important feature of Aspergillus flavus is that they can produce secondary metabolites. This means that they do not have a direct function in the physiological metabolism of the fungus, but rather act as a defense factor for a hostile environment..

These are made during fungal development, called aflatoxins, among other compounds. Although it is not a unique property of A. flavus, since they are also produced by A. parasiticus, and A. nomius.

The danger occurs when the fungus establishes itself and produces toxic substances on grains and legumes, which will later be consumed by humans and animals..

The fungus can also affect the leaves of plants previously damaged by insects in hot and humid climates, being very frequent in the tropics..

In turkeys and chickens there are epidemics of respiratory tract aspergillosis due to the consumption of grains contaminated with aflatoxins, causing 10% of deaths in chicks, while in cattle and sheep it causes abortions.

Production of aflatoxins and other toxic substances

Aflatoxins are said to be the most potent natural hepatocarcinogenic substances that exist. In this sense, Aspergillus flavus produces 4 aflatoxins (B1 and B2 G1 and G2), depending on the type of strain.

Aspergillus flavus It is classified into two groups according to the size of their sclerotia, where group I (L strains) have sclerotia greater than 400 µm and those of group II (S strains) have sclerotia smaller than 400 µm.

The most common aflatoxins (B1 and B2) are produced by the L and S strains, but the G1 and G2 aflatoxins are only produced by the S strains. However, the L strain is more virulent than the S strain, although it produces fewer aflatoxins..

Aflatoxin B1 is the most toxic, it has great hepatotoxic and carcinogenic potential, therefore it can cause from acute hepatitis to hepatocellular carcinoma.

In addition, Aspergillus flavus produces cyclopiazonic acid causing degeneration and necrosis of the liver, myocardial injury and neurotoxic effects.

In addition to this, it produces other toxic compounds such as sterigmatocystin, kojic acid, ß-nitropropionic acid, aspertoxin, aflatrem, gliotoxin, and aspergillic acid..

Production of substances with antibacterial properties

It's known that A. flavus produces 3 substances with antibacterial activity. These substances are aspergillic acid, flavicin and flavacidin..

Aspergillic acid has bacteriostatic or bactericidal activity against certain Gram positive and Gram negative bacteria depending on the concentration in which it is used..

The main bacteria affected are: Streptococcus ß- hemolytic, Staphylococcus aureus, Enterobacter aerogenes, Enterococcus faecalis Y Escherichia coli.  

For its part, flavicin has a bacteriostatic effect against Streptococcus H.H-hemolytic, Bacillus anthracis, Corynebacterium diphtheriae, Staphylococcus aureus, Staphylococcus epidermidis, Brucella abortus, Bacillus subtilis, Shigella dysenteriae Y Vibrio cholerae.

Meanwhile, flavacidin is a substance that has biological and chemical characteristics very similar to penicillin..

Taxonomy

Fungi kingdom

Phylum: Ascomycota

Class: Eurotiomycetes

Order: Eurotiales

Family: Aspergillaceae

Genus: Aspergillus

Species: flavus.

Morphology

Macroscopic characteristics

The colonies of A. flavus vary in appearance, they can be seen from granular, woolly or powdery.

The color of the colonies can also vary, at first they are yellowish, then they turn to yellowish-green tones and as time passes they turn to darker tones such as brownish-yellowish.

The reverse side of the colony may be colorless or yellowish brown..

Microscopic characteristics

Microscopically, colorless conidiophores that measure 400 to 800 µm long, thick-walled and with a rough appearance can be observed in the lower area where the globose vesicle is located..

The globose or subglobose vesicle measures between 25-45 µm in diameter. From there the phialides depart, surrounding the entire gallbladder. The phialides can be monoseriate, that is, with a single row of conidia or biseriate with double row of conidia..

The conidia are pyriform or globose yellowish green, smooth, but when mature they become slightly rough. The conidia in this species form relatively long chains.

As a sexual reproduction structure, they have subglobose or long sclerae of white or black color where ascospores develop..

Diseases and symptoms

Among the most frequent pathologies caused by  A. flavus there are fungal sinusitis, skin infection, and non-invasive pneumonia. It can also cause corneal and nasoorbital infections and disseminated disease..

Aspergilus flavus it is responsible for 10% of invasive diseases and is the third cause of oticomycosis in humans. It also causes aflatoxicosis.

Here is a brief explanation of the main diseases:

Fungal sinusitis

It is characterized by long-standing nasal congestion, rhinorrhea, post-nasal drainage, headache, and the appearance of nasal polyps, without invasion of the surrounding tissue..

Abundant eosinophils are present in the mucus and characteristic hyphae can be seen. Total IgE and IgG are elevated. In severe cases it can turn into invasive sinusitis.

Corneal infection

It manifests as conjunctivitis that worsens until the corneal perforation and loss of the affected eyeball. It is associated with a trauma with a stabbing element or by endogenous dissemination.

Nasal-orbital aspergillosis

It consists of an aspergilloma located in the paranasal sinuses that extends to the orbit of the eye. The most significant signs are unilateral protopsis and inflammation of the surrounding tissues..

Cutaneous aspergillosis

It is a local lesion that presents necrosis of the underlying tissue, which produces an angio-invasion and thrombosis.

Invasive pulmonary aspergillosis

It is defined as a necrotizing pneumonia with invasion of blood vessels secondary to colonization of the lung parenchyma.

The symptoms it reflects are fever, pulmonary nodules or infiltrates, hemoptysis, hemorrhagic infarction. The fungus can spread through the pleura into the pleural space, intercostal muscles, and myocardium..

It can also enter the bloodstream and spread to the brain, eyes, skin, heart, and kidney..

Human consumption of aflatoxin-contaminated food (aflatoxicosis)

The effects that it can produce in humans can be of 3 types: carcinogenic, mutagenic and teratogenic.

The metabolites that are generated from the biotransformation of consumed aflatoxins can affect any organ, however the target organ is the liver..

The manifestations that stand out are fatty liver, moderate and extensive necrosis, hemorrhage, gallbladder enlargement, damage to the immune, nervous and reproductive system.

Prevention

At an industrial level

To prevent infestation of grains and legumes, the storage humidity should be regulated below 11.5% and the temperatures below 5 ° C. In this way, the growth and proliferation of the fungus is prevented..

Fumigations should also be done to reduce the amount of mites and insects that are the main vectors carried by the conidia on their legs. The elimination of broken and immature kernels will help reduce the colonization of the fungus. 

On the other hand, a biological control has been proposed to reduce the development of toxigenic fungi on susceptible substrates. It consists of using strains of A. flavus non-toxigenic to displace toxigenic strains competitively.

At the clinical level

Placement of air filters and constant aeration of spaces, avoiding humidity and darkness.

References

  1. Amaike S. Keller N.  Aspergillus flavus. Annu Rev Phytopathol. 2011; 49: 107-133
  2. Ryan KJ, Ray C. SherrisMicrobiology Medical, 2010. 6th Ed. McGraw-Hill, New York, U.S.A
  3. Casas-Rincón G. General Mycology. 1994. 2nd Ed. Central University of Venezuela, Library Editions. Venezuela Caracas.
  4. Koneman, E, Allen, S, Janda, W, Schreckenberger, P, Winn, W. (2004). Microbiological Diagnosis. (5th ed.). Argentina, Editorial Panamericana S.A.
  5. Arenas R. Illustrated Medical Mycology. 2014. 5th Ed. Mc Graw Hill, 5th Mexico.
  6. Bonifaz A. Basic Medical Mycology. 2015. 5th Ed. Mc Graw Hill, Mexico DF.
  7. Rocha A. Antibacterial activity do Aspergillus flavus. Memories of the Oswaldo Cruz Institute Rio de Janeiro, Brazil. 1944; 41 (1): 45-57
  8. Cuervo-Maldonado S, Gómez-Rincón J, Rivas P, Guevara F. Update on Aspergillosis with emphasis on invasive Aspergillosis. Infectio. 2010; 14 (2): 131-144
  9. Majumdar R, Lebar M, Mack B, et al. The Aspergillus flavus Spermidine Synthase (spds) Gene, Is Required for Normal Development, Aflatoxin production, and Pathogenesis during infection of Maize Kernels. Frontiers in Plant Science. 2018; 9: 317
  10. Pildain M, Cabral D, Vaamonde G. Populations of Aspergillus flavus in peanuts grown in different agro-ecological zones of Argentina, morphological and toxigenic characterization. ESTUARY. 2005; 34 (3): 3-19
  11. Wikipedia contributors. Aspergillus flavus. Wikipedia, The free Encyclopedia. September 10, 2018, 11:34 UTC. Available at: Wikipedia.org.

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