Aspergillus niger is recognized as the most common species within the Aspergillus genus.
Its name originates from the Latin word aspergillum, meaning “holy water sprinkler,” due to its sprinkler-like appearance when observed under a microscope.
This fungus is commonly associated with the formation of black mold on fruits and vegetables such as grapes, apricots, onions, and peanuts.
It is known to cause food contamination and spoilage, making it a concern in food storage and processing.
Aspergillus niger exhibits mild opportunistic pathogenic characteristics, capable of causing respiratory infections, including pneumonia, in immunocompromised individuals—though such infections are less frequent compared to those caused by other Aspergillus species like A. flavus, A. fumigatus, and A. clavatus.
It is ubiquitously found in soil and can also occur indoors, where it typically appears black, contributing to its identification as black mold.
Beneficially, Aspergillus niger has been utilized for centuries in the production of citric acid, which serves as a common food preservative in canned fruits, as well as an ingredient in shampoos and a preservative in blood products.
Certain strains of Aspergillus niger are capable of producing mycotoxins, including ochratoxin A and the isoflavone orobol inhibitor.
History of Aspergillus niger
Aspergillus niger is the most common and most extensively studied species within the Aspergillus genus, particularly in terms of its morphology, physiology, benefits, and effects.
It is widely recognized as being less pathogenic to both humans and animals compared to other Aspergillus species.
In 1917, food chemist James Currie discovered that Aspergillus niger could produce high concentrations of citric acid when cultivated in a sugar-containing medium. He extracted this acid and extensively studied its benefits, especially as a food preservative.
Further research revealed its usefulness in the production of industrially important enzymes, including glucoamylase and α-galactosidase, among others.
Studies on its morphology and industrial applications led scientists to conclude that Aspergillus niger has multiple strains, each with distinct properties.
In 2004, researchers studying the production of ochratoxins by Aspergillus niger identified several closely related species within the subgenus Circumdati, section Nigri.
This group includes 15 black-spored species similar to A. niger, such as A. tubingensis, A. foetidus, A. carbonarius, and A. awamori.
Habitat of Aspergillus niger
Aspergillus niger is highly thermotolerant, enabling it to survive and thrive in both extremely low and extremely high temperature conditions.
Its asexual mode of reproduction allows it to grow in a wide range of environments whenever conditions are favorable, contributing to its opportunistic nature.
It typically inhabits decaying vegetation such as compost piles and dead leaves, as well as soil.
It can also be found in various locations including stored grains, dried fruits, dry nuts, and even on polyester surfaces.
A recent study conducted on the International Space Station revealed that Aspergillus niger is highly adaptable to space radiation, an environment exposed to intense UV radiation, X-rays, and solar flares.
This ability to survive in such extreme environments further highlights its remarkable adaptability to harsh and diverse conditions.
Morphology of Aspergillus niger
Aspergillus niger is a filamentous fungus with filamented hyphae, giving it a small plant-like appearance.
Its colonies initially appear white but turn black after a few days due to the production of conidial spores, with pale yellow edges and radial fissures.
Under the microscope, it shows smooth, colored conidiophores and conidia, with conidiophores arising from septate, hyaline hyphae as protrusions.
The conidial heads are radial in arrangement and split into columns (biseriate), with the conidiophore vesicle producing sterile cells called metulae that support the phialides.
Conidiophores measure 400–3000 µm in length, are smooth and hyaline, darken at the apex, and terminate in a globose vesicle measuring 30–75 µm in diameter.
The vesicle is completely covered by metulae and phialides.
The phialides produce conidia that have a rough texture, are dark brown in color, and measure 4–5 µm in diameter.
Cultural Characteristics of Aspergillus niger
Aspergillus niger generally has a cottony appearance, starting white to yellow and later turning black, composed of felt-like conidiophores, with the reverse side appearing white to yellow.
Under microscopy, the conidial heads are radiate with biseriate conidiogenous cells, and the conidia are brown in color.
On potato dextrose agar at 25°C, colonies are initially white but quickly turn black with conidial production; the reverse is pale yellow, and growth may produce radial fissures in the agar.
Malt Extract Agar – On malt extract agar, after 7 days of incubation at both 25°C and 37°C, colonies appear slightly brown with smooth-walled conidia.
Czapek Yeast Agar –On Czapek yeast agar, after 5 days of incubation at both 25°C and 37°C, colonies are black and woolly with smooth-walled conidia.
Life Cycle of Aspergillus niger
Aspergillus niger reproduces asexually through the formation of conidial spores.
The life cycle begins when conidia are dispersed onto a surface with favorable conditions, typically at temperatures between 25–40°C.
The conidia germinate to form a vegetative cell.
The vegetative cells develop into hyphal mycelium, which branches dichotomously and produces aerial hyphae.
The aerial hyphae grow to form conidiophores, which swell at the apex to develop the vesicle portion of the conidiophore.
From the vesicles, primary sterigmata known as phialides are produced.
The primary sterigmata give rise to secondary sterigmata, which begin producing conidial spores.
The spores are arranged in columns, forming several rows on top of the phialides.
Pathogenesis of Aspergillus niger
Plant manifestations
Aspergillus niger causes black mold in onions and ornamental plants by infecting onion seedlings, with the infection potentially becoming systemic and manifesting when conditions are favorable.
It is responsible for a common postharvest disease of onions, where black conidia can be observed between the scales of the bulb.
It can also cause disease in peanuts and grapes.
Human and animal disease
Compared to other common Aspergillus species, it is less likely to cause disease in humans and animals.
In rare cases, it can cause opportunistic invasive pulmonary aspergillosis in immunocompromised individuals following inhalation, particularly when the epithelial lining and respiratory tract are damaged, leading to severe lung disease.
It can cause aspergillosis in horticultural workers who are frequently exposed to peat dust rich in Aspergillus spores.
Aspergillus niger is a common cause of otomycosis, a fungal ear infection that may lead to temporary hearing impairment, pain, and in severe cases, damage to the ear canal and tympanic membrane.
Laboratory Diagnosis of Aspergillus niger
Microscopic examination reveals dark brown, rough-edged conidial spores and brown conidiophores.
Cultural examination can be performed using Potato Dextrose Agar, Czapek Yeast Agar, and Malt Yeast Agar.
Genomic sequencing can be carried out for precise identification and differentiation from other fungal species.
Thin-layer chromatography can be used to identify and quantify the presence of ochratoxin mycotoxin.
Treatment of Aspergillus niger Infections
Antifungal drugs such as itraconazole and Amphotericin B are used for the treatment of opportunistic invasive aspergillosis caused by Aspergillus niger.
For otomycosis, treatment involves the use of itraconazole along with nonsteroidal anti-inflammatory drugs (NSAIDs) such as acetaminophen to relieve pain.
Prevention and control of Aspergillus niger
Removal of Aspergillus niger spores and growth can be achieved through chemical and antifungal treatments such as:
70% ethanol or isopropyl alcohol applied for about 10 minutes, which penetrates the spore’s cell wall and hyphae, effectively killing them.
Phenols, which kill Aspergillus niger spores within 20 minutes and can be incorporated into scrub soaps, mouthwashes, and surface disinfectants.
Bleach containing hypochlorite, which inhibits the growth of spores.
Prevention of otomycosis infection includes:
Avoiding water entry into the ears while swimming or surfing.
Drying the ears thoroughly after showering.
Avoiding the use of cotton swabs inside the ears.
Refraining from scratching the skin outside or inside the ears.
Using acetic acid ear drops after water exposure to the ears.
Industrial Uses of Aspergillus niger
Aspergillus niger is widely used for the production of citric acid, which serves as a major food preservative for canned fruits, dry nuts, and dried fruits.
It produces glycoside hydrolase, an enzyme that breaks down cellulose and hemicellulose from plant cell walls into substrates that can be converted into ethanol, aiding in biofuel production.
It can be utilized for the production of bioactive metabolites and other pharmaceutical products.
Due to the high transfructosylating activity of its enzymes, Aspergillus niger can be adapted to produce large amounts of fructooligosaccharides.
It exhibits strong bioabsorption abilities, and its colonies are used to enhance the absorption of certain dyes such as Congo red and Blue 9 dye, as well as to aid in the removal of their impurities.
References
News Medical. What is Aspergillus niger? Available at: https://www.news-medical.net/life-sciences/What-is-Aspergillus-niger.aspx
National Center for Biotechnology Information (NCBI). Aspergillus niger research article. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3052473/
BustMold. Aspergillus niger – Mold Library Resource. Available at: https://www.bustmold.com/resources/mold-library/aspergillus-niger/
LIFE Worldwide. Aspergillus niger – Fungal Diseases Information. Available at: http://www.life-worldwide.org/fungal-diseases/aspergillus-niger
Microscope Master. Aspergillus – Microscopy and Identification. Available at: https://www.microscopemaster.com/aspergillus.html
ResearchGate. Morphological characteristics of Aspergillus niger. Available at: https://www.researchgate.net/figure/Morphological-characteristics-of-Aspergillus-niger-volumetric-power-input-100-W-m-3_fig1_268427977
ResearchGate. Aspergillus niger spores are highly resistant to space radiation. Available at: https://www.researchgate.net/publication/340398703_Aspergillus_niger_Spores_Are_Highly_Resistant_to_Space_Radiation
Indian Journal of Pharmaceutical Sciences. Fungal biodiversity of a library and cellulolytic activity of some fungi. Available at: https://www.ijpsonline.com/articles/fungal-biodiversity-of-a-library-and-cellulolytic-activity-of-some-fungi.html?view=mobile
Pakistan Journal of Medical Sciences. Aspergillus niger study. Available at: https://www.pjms.com.pk/issues/octdec207/article/article9.html
Wikipedia. Aspergillus niger. Available at: https://en.wikipedia.org/wiki/Aspergillus_niger
