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Stachybotrys chartarum is recognized as "Toxic Black Mold" by the general public^[1][2][3] because it associates with water-damaged buildings^[4] and some related health issues to the residents,^[1][5][6] such as increasing susceptibility to respiratory symptoms^[1] and allergies.^[3]

It is a cellulolytic saprophyte growing on artificial building surfaces indoor that contain cellulose, such as wallpaper and cardboard, and can also be found in soil with different acidity, textiles, seeds and dead plant materials. ^[1] It has an abundant sporulation distributing all over the planet^[2][5] though it only reproduces asexually via sporogenesis.^[3] And it has two chemotypes characterized by different secondary metabolites secreted. Chemotype S secreting macrocyclic trichothecenes which are inhibitor to protein biosynthesis in humans, and chemotype A secreting atranones which associate to inflammations in humans.^[1][6][7] It does not only induce human infections but also associates with infections in animals^[3][8] and plants.^[9] Avoid exposures to water-damaging buildings^[10] and eliminate the condensation on the walls indoor^[2] are the best treatments to related diseases currently.

History and taxonomy

In 1818, Christian Gottfried Ehrenberg first discovered it on the cardboard and named it Stilbospora chartarum.^[5] Later in 1837, August Carl Joseph Corda independently discovered the fungus on wallpaper and named it Stachybotrys atra.^[5] However, in 1930s, a mysterious death of horses in some Ukrainian towns firstly disclosed that S. alternans has contaminated the horse fodder before consumption.^[3][8] The naming was united by S. Hughes in 1958 as he found that actually Ehrenberg and Corda described the same fungus.^[11] In 1990s, the exposure to heavy indoor mold with macrocylic trichothecenes was first reported inducing severe pulmonary hemorrhage and hemosiderosis in human infants in Cleveland, USA.^[8][10] S. chartarum is distinguishable by its unique morphology from some similar species such as Melanopsamma poniformis, S. albipes, S. microspora and S. zeae.^[12][5]

Morphology

Stachybotrys chartarum is usually greenish-black to blackish mold from appearance as it matures and it is recognized as "Toxic Black Mold" due to its mycotoxin production with high toxicity.^[1][2][3] A study has grown S. chartarum on six agar dish before, ALK, CYA, CMA, PSA, V8 and SYES respectively; and weirdly it produced yellow pigments non-consistently in CYA (Czapek yeast agar) and PSA (Potato sucrose agar).^[13]

About 90% sample collected from the nature cannot be incubated on culture.^[3][14] Its conidiophores are uniquely ornamented^[15], immature condiophores are colorless then become darker in color as they mature, usually 100 micrometers in length and can be up too 1000 micrometers long, 3-6 micrometers in width, about 6-10 phialide structures at the tips.^[3][5] Its conidia are single-celled and ellipsoidal, with slimy masses on surface, immature conidia are colorless and has smooth surface while the mature ones are darker in color and have coarsely rougher surfaces.^[3][5][15] In addition, young conidia is narrower compare to the mature ones. Have been mistakenly described as "two-celled conidia" by Corda before, corrected to "unicellular conidia" by Bisby in 1943.^[15]

There are two main chemotypes in S. chartarum characterized by the metabolites.^[1][6][7] The strain that could secrete both macrocyclic trichothecenes and atranone has not been observed yet, and the chemotype A and S can be defined as "mutually exclusive" to each other. Chemotype A is usually more common compared to chemotype S. Sample taken from Stachybotrys-contaminated building surface indoor proved that both chemotypes can co-exist.^[16] And there is another new species, Stachybotrys chlorohalonata, that is distinguishable from both morphology and molecular markers.^[17]

Both chemotypes have similar morphology. Chemotype A has a slightly faster growth rate on culture ^[6] and slightly smaller colonies in size compare to chemotype S when incubated on culture dish in lab.^[13] Genetics can also help differentiate two chemotypes. In both tri5 and chs1 gene fragment, a single nucleotide substitution could help differentiate almost all chemotype A and S isolates and there is no difference between two chemotypes in tub1 gene fragment.^[6] Either ingestion or inhalation of macrocyclic trichothecenes can inhibit the protein biosynthesis.^[3] The spores contain mycotoxins, either macrocylic trichothecenes or atranones, and toxicity of the conidia has not yet been reported.^[2]

Growth and reproduction

Growth rate can be fast or slow depending on the media that provided.^[1][2][3] The amount of growth and sporulation is associated with the concentration of glucose, as it has the maximum growth with sucrose which can be decomposed into glucose and fructose by simply breaking the glycosidic bond when incubating in the laboratory. Biotins are required for growth and sporulation.^[5] It can be found in soil with relatively rich copper. It is one of the contributors to stabilize and aggregate the soil but a significant reducing growth of tomato. Flax and rape occur after inoculation of S. chartarum in soil. However, germination of S. chartarum is inhibited by soil mycostasis.^[5]

Coexistence with Acremonium has been discovered.^[4] Another Stachybotrys species, Stachybotrys chlorohalonata was also reported its coexistence with both chemotypes in water-damage buildings.^[13][15] And sometimes it can be found in water-damaged buildings together with Aspergillus species.^[10] To compete with Penicillium and Aspergillus that sharing the same habitat, substrates for optimal growth should be high in cellulose, low in sugar and nitrogen for S. chartarum.^[14] And it is antagonistic to numerous fungi with unknown metabolites, such as Bacillus subtilis. Besides, it suppresses the growth of Chalara elegans in citrus seedlings. Its growth can also be inhibited by Memnoniella echinata and it would be trapped once interact the hyphae with Tuber melanosporum.^[5]

S. chartarum used to be included in the Deuteromycetes formerly because it was reported exclusively reproduce asexually, and currently it is defined as a anamorph but now as the language corrected. Although most species in Ascomycota can switching their reproductive state under certain environmental conditions, there was no literature has ever reported this switching in S. chartarum up till now.^[3][18]

As one of the common indoor molds, S. chartarum shares the same life cycle with other indoor molds that reproduce asexually. It may have an infinite growth if the bioavailable resources in the living environment are unlimited, which can be considered as a genetic propagation with no meiosis. The life cycle of S. chartarum is simply cycling between a single spore and a conidiophore with a whole bunch of conidia.^[19]

Physiology

S. chartarum is a mesophile that usually lives in a moderate environmental temperature range, and it has the maximum growth in the temperature range around 23-27 °C.^[3][5] It can grow in a variable pH range 3.6-7.7. It is moderate osmotolerant and highly tolerant to UV irradiation. Although it lives better in environment that rich in cellulose and low in nitrogen and sugar, it still needs to acquire nitrogen that are necessary for lives from several sources. S. chartarum cannot acquire these nitrogen from sodium nitrite and the best nitrogen resource is (NH₄)₂SO₄.^[5]

Primary metabolism

S. chartarum synthesizes and utilizes several enzymes to degrade cellulose, for example, it produces

And it also secretes antigenic proteins SchS21 and SchS34 that have been discovered its role inducing inflammations in goats, mice and humans. These two proteins cannot be detected in the spores or the mycelium. SchS21 is an extracellular Mg-dependent alkaline DNase and its activity can be enhanced by the coexistence of Mg²⁺ and Ca²⁺ divalent ions. SchS34 can be found on the surfaces of conidia and hyphae if tested by immunogold staining, thus it has an undetermined extracellular function in nature.[20]

Habitat

It has a worldwide distribution.^[2][5] and it can simply incubated with stable moisture like damp-damaged buildings, wall paper that are rich in cellulose and low in nitrogen, and air.^[1][14]

Ecology and health issues

Mycotoxins that S. chartarum produced could cause stachybotryotoxicosis but S. chartarum is not recognized as a pathogen which directly gives infections to humans and animals.^[1][5][6] Exposure to the indoor contamination caused by S. chartarum may weaken respiratory tract (including lungs) and also affect the health of non-respiratory system, immune system, cognitive system in the brain. Besides, the maojority of DBRI (Damp-building related illness) patients have been reported the inhalation of mycotoxins.^[1][21] Though though the incidence is quite rare, chemotype S with higher toxicity is a cytotoxic inhibitor to protein biosynthesis in humans if it reaches certain concentration indoor. Its essential role inducing infantal pulmonary hemorrhage, hemosiderosis and pulmondary damages have been proven by rat models.^[10] Chemotype A with lower toxicity associates with diverse environment-related illnesses including chronic fatigue, allergy, impaired lung function.^[1][6][18] However, young or immunosuppressed individuals are more susceptible to its spore germination.^[5]

There were some famous cases threating global health of animals and humans in the history. In 1930s, horses died mysterically within 24 hours in a Ukrainian town due to inflammations on skin and respiratory tracts together with hemorrhages and tissue necrosis. The cause of the horse death was found tightly linking to the contamination by macrocyclic trichothecenes because the fodder was wet, a mycotoxic metabolites from S. chartarum chemotype S.^[3] And soon, an infantal acute pulmonary hemorrhage case caught the attention of the public. High exposure to indoor moulds including S. chartarum led respiratory symptoms, fatigue and dermatitis. ^[3][18] However, the cause of this case is still controversial nowadays since S. chartarum was just one of the common indoor moulds detected in the residences of the patients.^[3]

S. chartarum may also threat the health of plants and crops. For example, Li et al. significantly found both chemotypes of S. chartarum from root lesions of both parent and daughter soybean plants evidenced by detecting the presence of mycotoxins and RT-PCR result. However, there is still no direct evidence to determine if S. chartarum is playing a role in soybean infection. Thus, more studies are required to prove its pathogenicity risking plant health.^[9] But S. chartarum can be found in spices. Chemotype S has been found in dried culinary herbs and it has a greater potential to induce pulmonary inflammations if it contaminates food in this way.^[22]

No resistance to antibiotics or chemicals has been detected in S. chartarum. But a recent study found that S. chartarum was susceptible to twenty antifungal EOs (essential oils). There were 100% elimination by Carum carvi, Origanum vulgare, Pimenta racemosa, Thymus satureoides and Thymus vulgaris respectively^[23], and S. chartarum had a decreasing growth as the concentration of AgNPs (silver nanoparticles) on culture increased.^[24]

Prevention and treatments

The Stachybotrys allergy can be tested out by IgE blood allergy tests and about 83% S. chartarum allergic patients had been detected having Sta c 3, which is an extracellular alkaline Magnesium-dependent exodesoxyribonuclease which is able to induced specific anti-S. chartarum IgE in humans.^[3] SchS21 and SchS34, antigenic proteins from S. chartarum discovered in humans and mice and goats, can respetively bind to IgE or IgG to trigger inflammatory reactions. Therefore, these antibodies can also be used to detect the presence of these two antigenic proteins indoor and the related allergens, such as house dust mite allergens and Asp f1 from Aspergillus fmigatus. ^[20]

S. chartarum cannot be detected by air sample since the stickiness of its slime heads made it hard to be airborne,^[16] which is hard to test their presence by the air sampling. To identify if the indoor environment is contaminated by indoor mold, residents can complete a questionnaire and call health care workers for scheduling a home visit. The upper limit indicating "no contamination" is usually 100-1000 colony-forming units (CFU) per m³. High indoor exposure to mold usually associates with less vetilation and house-keeping, old carpets that are poorly washed, pets, existence of visible molds and so on. ^[14]

It has been proved that there is a strong relationship between wallpaper and Stachybotrys species, in particular S. chartarum.^[4] Thus S. chartarum can easily lead to an epidemic in the Stachybotrys-contaminated moldy buildings.^[10] Such that, living free from the stachybotryotoxicosisis is the best treatment currently, and the residents should eliminate the condensation on the walls indoor, in particular the basement walls, to prevent the growth of S. chartarum.^[2]

Furthermore, the builders should make better decisions about building materials.^[4] A study has ensured if addition of certain substances to building materials affect the growth of S. chartarum. One of the interesting finding of this study was that the growth and sporulation of S. chartarum have been completely suppressed the addition of a biocide, 1% Parmetol DF 17, in the plasterboard liners; but biocide itself cannot inhibit the growth of S. chartarum.^[25]

References