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Aspergillus niger is a mold classified within the Nigri section of the Aspergillus genus. The genus Aspergillus consists of common molds found omnipresent in the environment like on vegetation, within soil and water, in fecal matter, on decomposing matter, and suspended in the air. Species within the Aspergillus genus are often fast growing and can sporulate within a few days after germination.The species A. niger is a filamentous fungus capable of growing in a variety of environments due to its versatile metabolism and robustness to extreme acid environments. A. niger's robustness to acidic environments is one reason why the specific mold is of great economic importance due to its industrial use in citric acid production.

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"). A. niger is classified as "Generally Regarded as Safe" (GRAS) by the US Food and Drug Administration, though the microbe is capable of producing toxins that can negatively affect human health.

The genus Aspergillus is ubiquitous, meaning it can be commonly found in all environments, and it is the most dominant genus of spore producing fungi. Species of Aspergillus are tolerant to elevated temperatures, thriving in a temperature range of 15°C-53°C.

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. Potential consequences of OTA poisoning include kidney damage, kidney failure and cancer. However, 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.

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The black aspergilli, Aspergillus niger, is one of the most common fungi that causes food to spoil and other materials to biodeteriorate.

Aspergillus niger is rarely reported to cause pneumonia compared to other Aspergillus species, such as Aspergillus flavus, Aspergillus fumigatus, and Aspergillus terreus.

Being a saprophyte, Aspergillus niger can have a negative economical impact due to it causing rotting in various of fruits and vegetables, like mangoes, tomatoes, and shallots.

Aspergillus niger produces more than 1 million metric tons of citric acid annually via a fungal fermentation process.

Cultivation

A. niger is a strict aerobe; therefore, it requires oxygen to grow. A. niger can grow in a range of environmental conditions; it can grow at temperatures ranging from 6 to 47ºC . However, as a mesophile, its optimal temperature range is 35-37ºC It can also tolerate pH ranging from 1.5 to 9.8 . A. niger is xerophilic, meaning it can grow and reproduce in environments with very little water. However, the fungi can also can also grow in humid conditions. A. niger is most commonly grown on Potato Dextrose Agar (PDA), but it can grow on many different types of growth medium including Czapek-Dox Agar, Lignocellulose Agar, and several others.

Fermentation

There are two ways in which Aspergillus niger can be grown for industrial purposes: solid state fermentation (SSF) and submerged fermentation (SmF). 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. SSF gives better yield of microbe products and is more cost effective than SmF due to using agricultural byproducts. SSF is predominantly used over SmF. In SmF, microbes are grown in a liquid medium inside large aseptic fermentation vessels. 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.

Solid state fermentation (SSF) - better yield and lower cost than SmF

Submerged fermentation (SmF) - used in production of industrial enzymes. method chosen due to the ease of handling variables and greater control of environmental factors (ex. temperature and pH)

Industrial Uses

Aspergillus niger has many industrial uses including the isolation of enzyme fructosyltransferase for the production of fructooligosaccharides (FOS). FOS are used to manufacture low-calorie and functional foods due to FOS characteristic ability to slow growth of pathogenic microorganisms in the intestines. These foods have prebiotic and 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 for industrial processes.

Aspergillus niger is used in the production of citric acid (CA). This process is performed by growing strains of A. niger in nutrient medium with high concentrations of sugar and mineral salts as well as an acidic pH of 2.5-3.5. Again, there are many microorganisms that produce CA, but Aspergillus niger produces more than 1 million metric tons of citric acid annually via a fungal fermentation process. CA is in high demand for uses such as control of microorganism growth, food and beverage flavor enhancement, adjust acidity, pharmaceuticals, etc..

Enzymes obtained from the cellular processes of A. niger include carbohydrase and cellulase, which are commonly used enzymes 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.

Genome

Aspergillus niger has a genome comprised of roughly 34 megabases (Mb) organized into eight chromosomes. The DNA contains 10,785 genes which are transcribed and translated into 10,593 proteins.

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). The completed sequences have been used to uncover orthologous genes and pathways involved in fungal metabolism, specifically the catabolism of monosaccharides. 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.

Pathogenicity

A.niger as a Human Pathogen

A. niger has been found to cause physical manifestations in humans. Aspergillosis is a fungal infection caused by spores of indoor and outdoor Aspergillus mold species. Due to the ubiquitous nature of A. niger, it's spores are commonly inhaled by humans from their surrounding environment. However, aspergillosis infection customarily occurs in people with compromised immune systems or pre-existing lung conditions like asthma and cystic fibrosis. Types of aspergillosis include allergic bronchopulmonary aspergillosis (ABPA), allergic aspergillus sinusitis, azole-resistant aspergillus fumigatus, cutaneous (skin) aspergillosis, and chronic pulmonary aspergillosis. Out of the approximated 180 species of aspergillus molds, roughly 40 species have been found to cause health concern in immunocompromised humans. 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. 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.

A. niger as a Plant Pathogen

Aspergillus niger can cause black mold infections in certain fruits and vegetables like peanuts, grapes, and onions which leads 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 15°C–53°C. These characteristics make infections of this mold a common of post-harvest decay in fruits and vegetables, which can cause significant negative economic loss for the food industry. A. niger can also cause a reduction in seed germination, seedling emergence, and root elongation causing the plant to perish before maturing. Specifically, Aspergillus niger causes sooty mold on onions and ornamental plants. Infection of onion seedlings by A. niger can become systemic, manifesting only when conditions are conducive. A. niger causes a common postharvest disease of onions, in which the black conidia can be observed between scales of the bulb.^{[citation needed]}