Aspergillus albus var. thermophilesNakaz, Y.Takeda & Suematsu(1932)
Aspergillus triticiB.S. Mehrotra & M. Basu(1976)
Aspergillus triticusB.S. Mehrotra & M. Basu(1976)
Aspergillus candidus (also called A. triticus, A. albus, and A. okazakii) is a white-spored species of fungus in the genus Aspergillus.[1] Despite its lack of pigmentation, it is closely related to the most darkly-pigmented aspergilli in the Aspergillus niger group.[1] It is a common soil fungus worldwide and is known as a contaminant of a wide array of materials from the indoor environment[2] to foods and products.[3][4] It is an uncommon agent of onychomycosis and aspergillosis.[5] The species epithet candidus (L.) refers to the white pigmentation of colonies of this fungus.[6] It is from the Candidi section.[7] The fungi in the Candidi section are known for their white spores.[7] It has been isolated from wheat flour, djambee, and wheat grain.[7]
Aspergillus candidus is a member of the genus Aspergillus in the family Trichocomaceae, phylum Ascomycota.[8][9] A number of subspecies and varieties have been recognised including: A. candidus subsp. tjibodensis, A. candidus var. amylolyticus, A. candidus var. candidus, A. candidus var. densior, and A. candidus var. tenuissima.[8][9] Synonyms of A. candidus include A. albus, A. okazakii, A. tritici, and A. triticus,[8] and the fungus is closely related to A. niger and A. ochraceus.[9]
Early reports of white-spored aspergilli in the eighteenth and nineteenth century variously named these based on their gross appearance and white pigmentation, coining Latin species epithets such as 'candidus', 'albus', and 'niveus'.[1] The species epithet 'candidus' was first used by Link in 1809, and later in 1824, he added two varieties, A. candidus var. tenuissima and A. candidus var. densior.[8]
In 1877, Wilhelm described a species with biseriate sterigmata, globose vesicles, and short conidiophores as A. albus, which shared similarities between the A. albus described by Haller in 1742 and A. candidus described by Link in 1809.[1] One reason for the confusion of identifying different species of fungus was that, two different fungus looked very similar to each other, but one was uniseriate (meaning there is one row of cells on the spore-bearing surface) and the other was biseriate (meaning there are two rows of cells on the spore-bearing surface). Wehmer appeared to have been the first one to suggest that a single culture of Aspergillus can contain both uniseriate and biseriate sterigmata in 1901.[1] Wehmer then suggested that the species A. albus, St. candida, and A. candidus were synonymous, but there wasn't enough information to prove any clear identities of these species. In 1926, Charles Thom, who was a scientist at the U.S. Department of Agriculture in Peoria, Illinois,[10] and Church suggested to group A. candidus, A. Okazakii, St. szurakiana, and St. ochroleuca together as being synonymous because of their white-spore appearance. However, in 1945, Thom and Kenneth Raper (also a scientist at the U.S. Department of Agriculture in Peoria, Illinois)[10] rejected this idea because there were many more white-spored strains found in other genus, and that the white-coloured property alone was not enough reason to group these species together in one category.[1] Today, there are five species that are accepted to be synonymous to A. candidus. (listed in the 'Taxonomy' section)[1]
Growth and morphology
Aspergillus candidus is exclusively an asexually reproducing species.[9] It is closely related to the darkly pigmented group of aspergilli that includes A. niger, and is in the clade of aspergilli that includes the sexually-reproducing genus, Sterigmatocystis.[2][11]A. candidus forms typical branching, septate hyphae,[12] which produce conidial heads at maturity.[13] Conidiogenesis is favoured in reduced oxygen atmospheres.[13] Optimal growth of A. candidus occurs at 25–28°C at 0.016 atm partial pressure of oxygen.[3] Growth of this species has been reported at temperatures as low as 11–13°C and as high as 41–42°C.[3]A. candidus is xerophilic, meaning it can grow in low water availability.[3] When grown on stored grains in 15% moisture content, it has the ability to increase the moisture level (up to 18%) and elevate the temperature to as high as 55°C[3] In the laboratory, ' was observed on media containing substances like p-hydroxy-benzoic aldehyde, lignin sulphonate, and tannin (which contains substances that A. candidus uses and degrades).[3]Aspergillus candidus exhibits a broad range of microscopic and colony morphology.[11] Typically, the colonies of A. candidus are white[4] and conidial heads are globose (spherical) or subglobose (subspherical) producing globose, smooth, thin-walled, conidia.[9][1][12][2][3] En masse, the conidia are typically white to cream-coloured, but some strains may show a pinkish to reddish purple colour when they absorb the pigment from the sclerotia.[1][2] By contrast, the closely related species A. niger has darkly pigmented conidia with a rough surface.[1] Some strains of A. candidus make sclerotia in culture, which are nodules consisting of densely aggregated hyphae. Sclerotial production is often induced by incubation under stressful conditions such as relatively low temperature or interference competition from co-culture with other microorganisms.[14] Conidial heads are typically 2.5–4.0µm in diameter,[2] with phialides either directly attached to the apical swelling of the conidiophore (uniseriate) or situated upon a palisade of basal cells (biseriate) – the latter being the most common presentation.[9] Macroscopically, young A. candidus colonies may have a cottony texture in the outer surface which eventually becomes granular as the fungus matures and produces conidia.[15]
A. candidus has been cultivated on both Czapek yeast extract agar (CYA) plates and Malt Extract Agar Oxoid® (MEAOX) plates. The growth morphology of the colonies can be seen in the pictures below.
Aspergillus candidus growing on CYA plate
Aspergillus candidus growing on MEAOX plate
Ecology and physiology
A. candidus is a common type of fungus found worldwide,[9][4] particularly in tropical and subtropical regions.[1][3] There have been reports of this fungus in Bangladesh, Pakistan, Kuwait, Sri Lanka, South and West Africa, Somalia, Sahara, Libya, Egypt, Syria, Israel, Argentina, Bahamas, New Guinea, the Solomon Islands, China, Central America, Chile, Nepal, and the US, indicating a very wide distribution in the world.[3]A. candidus has been reported as a contaminant of many agricultural food products (such as wheat, oats, barley, corn, and rice), decay matter,[4][15] soil, fur, compost,[12] dead bees, dung, animal nests, and wood submerged in seawater.[3] The species is also known from indoor environments and has been isolated from air, flooring, carpets, mattresses, and dust.[1]
Contamination of maize by A. candidus has been shown to increase fat acidity content from the breakdown maize oil,[1] leading to discoloration of the germ and loss of viability,[3][1]A. candidus can oxidize tryptamine to traptophol,[3] and use glucose and other sugars in the substrate and converts it into D-mannitol.[3]A. candidus may sometimes cause self-heating which increases the temperature of the contaminated crop, in order to increase its competitiveness with other fungi living in the organism or environment at low relative humidity.[3] There have been cases reported where contamination by A. candidus resulted in an increase in surrounding temperature by 20–26°C.[1]A. candidus has also been reported to use and degrade straw lignin, arabinoxylan, and cellulose.[3]
Pathogenicity
Aspergillus candidus has been associated with a variety of diseases such as hypersensitivity diseases, and infectious diseases including aspergillosis, otomycosis, and onychomycosis.[2][11] The dry conidia produced by A. candidus are easily dispersed in air,[9] leading to inhalation by humans and animals.[12] Once in the lung and lower airway, the fungus induces can induce an allergic response[12] (IgE hypersensitivity response).[4] Organic dust toxic syndrome (ODTS) results from frequent inhalation of large amounts of organic dust containing a mixture of microorganisms often including A. candidus.[11]
Immunosuppressed individuals (such as organ transplant recipients and HIV patients) are may be susceptible to infection by A. candidus.[4][9][15] Invasive aspergillosis may be caused by A. candidus in humans, particularly in immunocompromised individuals, with high associated mortality.[11] Diagnosis of invasive aspergillosis is difficult due to the non-specific clinical measures and symptoms.[5] Some symptoms include fever, cough, and dyspnea (difficulty breathing).[5] Radiologic examination may reveal an alveolar hemorrhage with the appearance of a nodule.[5] Serum tests may show positive results for galactomannan and beta-D-glucan which are both the cell wall materials for A. candidus.[5] Although these may provide strong evidence for invasive aspergillosis, false positive results are also common because many other fungal pathogens also show similar symptoms test results.[5] If the invasive aspergillosis is kept untreated, the disease can spread from the lungs to any of the organs in the body.[5] Dissemination to the central nervous system is especially fatal, resulting in seizure or stroke.[5] Intravenous oral therapy by voriconazole has been used successfully in the treatment of invasive aspergillosis.[5] Although voriconazole is most recommended, amphotericin B deoxycholate is a suitable alternative option for patients intolerant of voriconazole.[5]
Hospital acquired infections by this species have been reported.[9][11] The reason for this is due to the ability of A. candidus to reside in a wide range of environments, from showerheads to storage tanks and potted plants.[12] The fungus can be observed microscopically in infected tissued mounted in potassium or sodium hydroxide solution containing the fluorescent brightener, calcofluor white, or in fungal-specific tissue stains such as Gomori methenamine silver (GMS) stain.[9]Aspergillus candidus secretes cytotoxic metabolites like terphenyl compounds and terprenins,[11] as well as citrinin[12] and immunotoxic (1→3)-β-D-glucans.[9][12] In addition to humans, there have also been reports of A. candidus disease in pigs[11] and birds.[2]
Genome sequencing
The genome of A. candidus was sequenced in 2014 as a part of the Aspergillus whole-genome sequencing project – a project dedicated to performing whole-genome sequencing of all members of the genus Aspergillus.[16] The genome assembly size was 27.32 Mbp.[16]
Related Research Articles
Aspergillus niger is a mold classified within the Nigri section of the Aspergillus genus. 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. Species within this genus often grow quickly and can sporulate within a few days of germination. 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. A. niger's capability to withstand extremely acidic conditions makes it especially important to the industrial production of citric acid.
Aspergillus flavus is a saprotrophic and pathogenic fungus with a cosmopolitan distribution. It is best known for its colonization of cereal grains, legumes, and tree nuts. Postharvest rot typically develops during harvest, storage, and/or transit. Its specific name flavus derives from the Latin meaning yellow, a reference to the frequently observed colour of the spores. A. flavus infections can occur while hosts are still in the field (preharvest), but often show no symptoms (dormancy) until postharvest storage or transport. In addition to causing preharvest and postharvest infections, many strains produce significant quantities of toxic compounds known as mycotoxins, which, when consumed, are toxic to mammals. A. flavus is also an opportunistic human and animal pathogen, causing aspergillosis in immunocompromised individuals.
Aspergillus is a genus consisting of several hundred mould species found in various climates worldwide.
Aspergillus terreus, also known as Aspergillus terrestris, is a fungus (mold) found worldwide in soil. Although thought to be strictly asexual until recently, A. terreus is now known to be capable of sexual reproduction. This saprotrophic fungus is prevalent in warmer climates such as tropical and subtropical regions. Aside from being located in soil, A. terreus has also been found in habitats such as decomposing vegetation and dust. A. terreus is commonly used in industry to produce important organic acids, such as itaconic acid and cis-aconitic acid, as well as enzymes, like xylanase. It was also the initial source for the drug mevinolin (lovastatin), a drug for lowering serum cholesterol.
The fungi imperfecti or imperfect fungi are fungi which do not fit into the commonly established taxonomic classifications of fungi that are based on biological species concepts or morphological characteristics of sexual structures because their sexual form of reproduction has never been observed. They are known as imperfect fungi because only their asexual and vegetative phases are known. They have asexual form of reproduction, meaning that these fungi produce their spores asexually, in the process called sporogenesis.
Aspergillus ochraceus is a mold species in the genus Aspergillus known to produce the toxin ochratoxin A, one of the most abundant food-contaminating mycotoxins, and citrinin. It also produces the dihydroisocoumarin mellein. It is a filamentous fungus in nature and has characteristic biseriate conidiophores. Traditionally a soil fungus, has now began to adapt to varied ecological niches, like agricultural commodities, farmed animal and marine species. In humans and animals the consumption of this fungus produces chronic neurotoxic, immunosuppressive, genotoxic, carcinogenic and teratogenic effects. Its airborne spores are one of the potential causes of asthma in children and lung diseases in humans. The pig and chicken populations in the farms are the most affected by this fungus and its mycotoxins. Certain fungicides like mancozeb, copper oxychloride, and sulfur have inhibitory effects on the growth of this fungus and its mycotoxin producing capacities.
Aspergillus sydowii is a pathogenic fungus that causes several diseases in humans. It has been implicated in the death of sea fan corals in the Caribbean Sea.
Aspergillus penicillioides is a species of fungus in the genus Aspergillus, and is among the most xerophilic fungi.
Aspergillus versicolor is a slow-growing filamentous fungus commonly found in damp indoor environments and on food products. It has a characteristic musty odor associated with moldy homes and is a major producer of the hepatotoxic and carcinogenic mycotoxin sterigmatocystin. Like other Aspergillus species, A. versicolor is an eye, nose, and throat irritant.
Aspergillus ustus is a microfungus and member of the division Ascomycota. It is commonly found in indoor environments and soil. Isolated cases of human infection resulting from A. ustus have been described; however the majority of these are nail infections.
Aspergillus clavatus is a species of fungus in the genus Aspergillus with conidia dimensions 3–4.5 x 2.5–4.5 μm. It is found in soil and animal manure. The fungus was first described scientifically in 1834 by the French mycologist John Baptiste Henri Joseph Desmazières.
Aspergillus alabamensis is a soil fungus in the division Ascomycota first described in 2009 as a segregated taxon of A. terreus. Originally thought to be a variant of A. terreus, A. alabamensis is situated in a distinctive clade identified by genetic analysis. While A. alabamensis has been found to be morphologically similar to Aspergillus terreus by morphological studies, the two differ significantly in active metabolic pathways, with A. alabamensis producing the mycotoxins citrinin and citreoviridin but lacking mevinolin.
Aspergillus tubingensis is a darkly pigmented species of fungus in the genus Aspergillus section Nigri. It is often confused with Aspergillus niger due to their similar morphology and habitat. A. tubingensis is often involved in food spoilage of fruits and wheat, and industrial fermentation. This species is a rare agent of opportunistic infection.
Aspergillus lacticoffeatus is a species of fungus in the genus Aspergillus. It belongs to the group of black Aspergilli which are important industrial workhorses. A. lacticoffeatus belongs to the Nigri section. It was first described in 2004. It has been found on coffee beans in Venezuela and Indonesia, and is an effective producer of ochratoxin. It has an unfunctional PKS gene (pksA) for the production of black conidium pigment, which makes it the only species in the Nigri section which is brown and not black. It has been proposed that this species might not be a separate species but instead a mutated A. niger.
Aspergillus chevalieri is a species of fungus in the genus Aspergillus. It is from the Aspergillus section. The fungi in the Aspergillus section are known for their ability to grow at extremely low water activities. The species was first described in 1926. It has since been reported as an opportunistic pathogen causing skin infections.
Aspergillus campestris is a species of fungus in the genus Aspergillus. The species was first described in 1982. It is from the Candidi section. The fungi in the Candidi section are known for their white spores. It has been shown to produce a high number of secondary metabolites.
Aspergillus taichungensis is a species of fungus in the genus Aspergillus. The species was first described in 1995. It is from the Candidi section. The fungi in the Candidi section are known for their white spores. It has been isolated from soil in Taiwan. A. taichungensis has been shown to produce candidusin C, terphenyllin, and 3-hydroxyterphenyllin.
Aspergillus wentii is an asexual, filamentous, endosymbiotic fungus belonging to the mold genus, Aspergillus. It is a common soil fungus with a cosmopolitan distribution, although it is primarily found in subtropical regions. Found on a variety of organic materials, A. wentii is known to colonize corn, cereals, moist grains, peanuts and other ground nut crops. It is also used in the manufacture of biodiesel from lipids and is known for its ability to produce enzymes used in the food industry.
Aspergillus carneus is a fast-growing, filamentous fungus found on detritus and in fertile soil worldwide. It is characterized by its yellow, thick-walled hyphae and biseriate sterigmata. The fungus produces citrinin and 5 unique depsipeptides, Aspergillicins A-E.
Meristacrum is a fungal genus in the monotypic family Meristacraceae, of the order Entomophthorales. They are parasites of soil invertebrates, they typically infect nematodes, and tardigrades.
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