Aspergillus niger
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Aspergillus niger is a mold classified within the Nigri section of the Aspergillus genus.<ref name=":4">Template:Cite journal</ref> The Aspergillus genus consists of common molds found throughout the environment within soil and water, on vegetation, in fecal matter, on decomposing matter, and suspended in the air.<ref name=":5">Template:Citation</ref> Species within this genus often grow quickly and can sporulate within a few days of germination.<ref name=":5" /> A combination of characteristics unique to A. niger makes the microbe invaluable to the production of many acids, proteins and bioactive compounds. Characteristics including extensive metabolic diversity, high production yield, secretion capability, and the ability to conduct post-translational modifications are responsible for A. niger's robust production of secondary metabolites.<ref name=":8">Template:Cite journal</ref> A. niger's capability to withstand extremely acidic conditions makes it especially important for the industrial production of citric acid.<ref name=":4" /><ref>Template:Cite journal</ref>
A. niger causes a disease known as "black mold" on certain fruits and vegetables such as grapes, apricots, onions, and peanuts, and is a common contaminant of food. It is ubiquitous in soil and is commonly found in indoor environments, where its black colonies can be confused with those of Stachybotrys (species of which have also been called "black mold").<ref name=Samson_2001>Template:Cite book</ref> A. niger is classified as generally recognized as safe (GRAS) by the US Food and Drug Administration for use in food production,<ref>Template:Citation</ref> although the microbe is capable of producing toxins that affect human health.<ref name=fris/>
Taxonomy
Aspergillus niger is included in Aspergillus subgenus Circumdati, section Nigri. The section Nigri includes 15 related black-spored species that may be confused with A. niger, including A. tubingensis, A. foetidus, A. carbonarius, and A. awamori.<ref name="Klich_2002">Template:Cite book</ref><ref name="Samson_2004">Template:Cite journal</ref> In 2004, a number of morphologically similar species were described by Samson et al.<ref name="Samson_2004" />
In 2007, the strain of ATCC 16404 Aspergillus niger was reclassified as Aspergillus brasiliensis (refer to publication by Varga et al.<ref name="Varga et al.">Template:Cite journal</ref>). This required an update to the U.S. Pharmacopoeia and the European Pharmacopoeia, which commonly use this strain throughout the pharmaceutical industry.Template:Citation needed
Cultivation
A. niger is a strict aerobe; therefore, it requires oxygen to grow.<ref name=":10">Template:Cite journal</ref> The fungus can grow in a range of environmental conditions; it can grow at temperatures ranging from Template:Convert.<ref name=":9">Template:Cite journal</ref> As a mesophile,<ref name=":14">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> its optimal temperature range is Template:Convert.<ref name=":10" /> It can tolerate pH ranging from 1.5 to 9.8.<ref name=":9" /> A. niger is xerophilic, meaning it can grow and reproduce in environments with very little water. It can also grow in humid conditions even tolerating environments with 90-100% relative humidity.<ref name=":14" /> The fungus is most commonly grown on potato dextrose agar (PDA), but it can grow on many different types of growth media including Czapek-Dox agar, lignocellulose agar, and several others. Template:Citation needed
Genome
Aspergillus niger has a genome consisting of roughly 34 megabases (Mb) organized into eight chromosomes.<ref name=":02">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The DNA contains 10,785 genes that are transcribed and translated into 10,593 proteins.<ref name=":02" /> Template:Infobox genome
Two strains of A. niger have been sequenced. Strain CBS 513.88 produces enzymes used in industrial applications while strain ATCC 1015 is the wildtype strain of ATCC 11414 used to produce industrial citric acid (CA).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Andersen_2011">Template:Cite journal</ref><ref name="Pel_2007">Template:Cite journal</ref> The A. niger ATCC 1015 genome was sequenced by the Joint Genome Institute in a collaboration with other institutions.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Completed sequences have been used to uncover orthologous genes and pathways involved in fungal metabolism, specifically the catabolism of monosaccharides.<ref name=":12">Template:Cite journal</ref> The ability of A. niger to change its metabolism depending on the carbon sources and other nutrients present in its environment has enabled the microorganism to survive and be found in almost all ecosystems. Further research is being done to study these mechanisms for all fungi using the complete sequenced genome of A. niger.<ref name=":12" />
Industrial uses
There are two ways in which Aspergillus niger can be grown for industrial purposes: solid state fermentation (SSF) and submerged fermentation (SmF).<ref>Template:Cite journal</ref> SSF uses a solid substrate with nutrients and minimal moisture to grow microorganisms. Nutrients such as nitrogen and carbon come from agricultural byproducts such as wheat bran, sugar pulp, rice husks, and corn flour.<ref name=":03">Template:Cite journal</ref> SSF gives better yield of microbe products and is more cost effective than SmF due to using agricultural byproducts.<ref name=":13">Template:Cite journal</ref> SSF is predominantly used over SmF.<ref name=":13" /> In SmF, microbes are grown in a liquid medium inside large aseptic fermentation vessels.<ref name=":03" /><ref name=":13" /> These vessels are expensive pieces of equipment that provide more water for growth and allow for tight control of environmental factors, such as temperature and pH, that affects microbial growth.<ref name=":13" />
Aspergillus niger is cultured to facilitate the industrial production of many substances.<ref>Template:Cite journal</ref> Various strains of A. niger are used in the industrial preparation of citric acid (E330) and gluconic acid (E574); therefore, they have been deemed acceptable for daily intake by the World Health Organization.<ref>Template:Cite journal</ref> A. niger fermentation is "generally recognized as safe" (GRAS) by the United States Food and Drug Administration under the Federal Food, Drug, and Cosmetic Act.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> A. niger is also being considered as a potential new source of natural food grade pigments.<ref>Template:Cite journal</ref>
The production of citric acid (CA) is achieved by growing strains of A. niger in a nutrient rich medium that includes high concentrations of sugar and mineral salts and an acidic pH of 2.5-3.5.<ref>Template:Cite journal</ref> Many microorganisms produce CA, but Aspergillus niger produces more than 1 million metric tons of CA annually via a fungal fermentation process.<ref name=":0">Template:Cite journal</ref> CA is in high demand for applications such as the control of microorganism growth, food and beverage flavor enhancement, acidity manipulation, pharmaceuticals, etc.<ref>Template:Cite book</ref>
A. niger produces many useful enzymes for the catabolism of biopolymers in order to obtain nutrients from its environment.<ref name=":3">Template:Cite journal</ref> The production of specific enzymes can be increased for industrial purposes.<ref>Template:Cite journal</ref><ref name=":3" /> For example, A. niger glucoamylase (Template:PDBe-KB) is used in the production of high-fructose corn syrup and pectinases (GH28) are used in cider and wine clarification. Alpha-galactosidase (GH27), an enzyme that breaks down certain complex sugars, is a component of Beano and several other products that decrease flatulence.<ref>Template:Cite journal</ref> Another use for A. niger within the biotechnology industry is in the production of magnetic isotope-containing variants of biological macromolecules for NMR analysis.<ref>Template:Cite journal</ref> Aspergillus niger is also cultured for the extraction of the enzyme, glucose oxidase (Template:PDBe-KB), used in the design of glucose biosensors, due to its high affinity for β-D-glucose.<ref>Template:Cite journal</ref><ref name="ghoshdastider">Template:Cite journal</ref>
In the food industry, A. niger is also cultured to isolate the enzyme fructosyltransferase to produce fructooligosaccharides (FOS).<ref name=":1">Template:Cite journal</ref> FOS are used to manufacture low-calorie and functional foods due to FOS characteristic ability to slow growth of pathogenic microorganisms in the intestines.<ref name=":1" /><ref name=":2">Template:Cite journal</ref> These foods have prebiotic fiber among other health promoting properties. A. niger is not the only organism to produce the enzyme fructosyltransferase, but it has been found to produce the enzyme at rates conducive to industrial production.<ref name=":1" /><ref name=":2" /> A specific use of A. niger within the food industry is its capability to produce enzymes like carbohydrase and cellulase, which are commonly used in the seafood industry for removing the bellies of clams during processing and removing the tough external skin of shrimp from their edible internal tissue.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Aspergillus niger can grow in gold-mining solutions containing cyano-metal complexes with gold, silver, copper, iron, and zinc. The fungus also plays a role in the solubilization of heavy-metal sulfides.<ref>Template:Cite book</ref> A. niger has also been shown to remediate acid mine drainage through biosorption of copper and manganese.<ref>Template:Cite journal</ref>
Toxicity
A. niger produces a wide variety of secondary metabolites,<ref name="fris">Template:Cite journal</ref> some of which are mycotoxins called ochratoxins,<ref name="Abarca_2004">Template:Cite journal</ref> such as ochratoxin A.<ref name="Samson_2001" /><ref name="On the safety of Aspergillus niger">Template:Cite journal</ref> Contamination by filamentous fungi, such as A. niger, occurs frequently in grapes and grape based products resulting in contamination by ochratoxin A (OTA). OTA, a clinically relevant mycotoxin, can accumulate in human tissue and cause a variety of serious health conditions.<ref>Template:Cite journal</ref> Potential consequences of OTA poisoning include kidney damage, kidney failure and cancer but the United States Food and Drug Administration (FDA) has not set maximum permissible levels of OTA in food unlike the EU that set maximum permissible levels in a variety of food products.<ref>Template:Cite journal</ref>
Pathogenicity
Plant pathogen
Aspergillus niger can cause black mold infections in certain legumes, fruits, and vegetables such as peanuts, grapes, and onions, leading to the fungus being a common food contaminant. This filamentous ascomycete has a tolerance to changes in pH, humidity, and heat, thriving in a temperature range from Template:Cvt.<ref>Template:Cite journal</ref> These characteristics make infections of A. niger a common cause of post-harvest decay in fruits and vegetables, which can lead to significant economic loss in the food industry.<ref name=":11">Template:Cite journal</ref> A. niger infection in plants can cause a reduction in seed germination, seedling emergence, root elongation, and shoot elongation, causing the plant to perish before maturation.<ref name=":11" /> Specifically, Aspergillus niger causes sooty mold on onions and ornamental plants.Template:Citation needed
Human pathogen
A. niger is pathogenic. Aspergillosis is a fungal infection caused by spores of indoor and outdoor Aspergillus mold species.<ref name=":6">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Due to the ubiquitous nature of A. niger, its spores are commonly inhaled by humans from their surrounding environment.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Aspergillosis infection customarily occurs in people with compromised immune systems or pre-existing lung conditions like asthma and cystic fibrosis.<ref name=":6" /> Types of aspergillosis include allergic bronchopulmonary aspergillosis (ABPA), allergic aspergillus sinusitis, azole-resistant aspergillus fumigatus, cutaneous (skin) aspergillosis, and chronic pulmonary aspergillosis.<ref name=":6" /> Out of the approximated 180 species of aspergillus molds, roughly 40 species have been found to cause health concern in immunocompromised humans.<ref name=":6" /> Aspergillosis is particularly frequent among horticultural workers who often inhale peat dust, which can be rich in Aspergillus niger spores. The fungus has also been found in ancient Egyptian mummies and can be inhaled when they are disturbed.<ref>Handwerk, Brian (May 6, 2005) Egypt's "King Tut Curse" Caused by Tomb Toxins?. National Geographic.</ref> Otomycosis, which is a superficial fungal infection of the ear canal, is another disorder that can be caused by overgrowth of Aspergillus molds like A. niger.<ref name=":7">Template:Cite journal</ref> Otomycosis caused by A. niger is frequently associated with mechanical damage of the ear canal's external skin barrier and often presents itself in patients living in tropical climates.<ref name=":7" /><ref name="On the safety of Aspergillus niger"/> A. niger is rarely reported to cause pneumonia compared to other Aspergillus species, such as Aspergillus flavus, Aspergillus fumigatus, and Aspergillus terreus.<ref>Template:Cite journal</ref>
Gallery
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A. niger growing in a Petri dish
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SEM micrograph of Aspergillus niger (strain: melanoliber) conodiophore possessing a large number of small conidospores (in false color). These spores release peptidomelanin into the surrounding medium during germination.
