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Penicillium glabrum is one of the most common, and widely distributed species of Penicillium moulds. It is a causative agent of Suberosis, a type of Hypersensitivity pneumonitis. P. glabrum produces a wide variety of metabolites, one of which, sclerotiorin, may have therapeutic potential in preventing diabetes complications.

History
Penicillium glabrum was first described by Carl Wehmer in his 1893 book, Beiträge zur Kenntnis einheimischer Pilze, where he named the fungus Citromyces glaber. Using members of the Penicillium genus, Wehmer was the first to attempt producing citric acid commercially by fermentation. Wehmer began his research on Penicillium species and citric acid production in 1889 at the Leipzig Botanical Institute. In his research Wehmer discovered fungi that closely resembled Penicillium species, but were able to produce citric acid. He identified two new species, C. glaber (P. glabrum) and C. pferrerianus (likely P. spinulosum), which were almost indistinguishable. Wehmer was confident that these two species had been observed in the past, but were not yet described as distinct species due to their resemblance to Penicillium glaucum. Wehmer assigned the two species to a new genus, Citromyces, due to differences between these new fungi and existing members of the genus Penicillium: the ability to produce citric acid, and having simple, unbranched (monoverticillate) conidiophores.

After Wehmer, P. glabrum moulds were described by many other authors under different names. There is some uncertainty around which exact moulds Wehmer used in his research, but C. glaber is considered to be synonymous with P. glabrum and C. pferrerianus is likely P. spinulosum. Although Wehmer used his species to produce citric acid, P. glabrum and P. spinulosum produce little citric acid, and modern industrial production of citric acid by fermentation usually uses strains of Aspergillus niger.

Taxonomy
Penicillium glabrum belongs to the genus Penicillium and the subgenus Aspergilloides. P. glabrum was originally placed in genus Citromyces, Citromyces was the name assigned to monoverticillate Penicillium species by Carl Wehmer.

P. glabrum is a member of the Penicillium frequens series. members of the Penicillium frequens series are some of most widely distributed and most common species in the genus Penicillium. This series includes, P. purpurescens, P. odoratum, P. abeanum, P. luteo-aurantium, P. multicolor, P. cremeo-grisum, P. spinulosum, and P. tarraconenense. Members of this series grow quickly, and spread broadly. The stripe of the conidiphore is smooth walled or roughened, and is seldom branched. Conidia are globose to subglobose. P. glabrum is very closely related to and also closely resembles Penicillium spinulosum, the main difference between the two species being that P. spinulosum produces roughened spinose conidia, whereas P. glabrum conidia are smooth or roughened.

A commonly used name for Penicillium glabrum is Penicillium frequentans R. Westling (1911). Other names for P. glabrum include: Citromyces albicans Sopp (1912), Penicillium aurantio-brunneum Dierckx (1901) , Penicillium candido-fulvum Dierckx (1923) , Penicillium fluitans Tiegs(1919) , Penicillium frequentans Westling (1911) , Penicillium oledzkii Zalessky (1927) and Penicillium pfefferianum'' (Wehmer) Pollaci (1916).

Growth and morphology


Penicillium glabrum colonies are green-grey in colour, and have a velvety appearance. The underside (reverse) is yellow to yellow-orange in colour. P. glabrum colonies contain abundant crowded conidiophores, some trailing hyphae, and are heavily sporing.

P. glabrum grows rapidly on agar, reaching a diameter of 4-5 cm in 7 days. Grown on CYA (Czapek medium), colonies reach a diameter of 5-6 cm, and grown on MEA (malt extract agar), a diameter of 5.5-6.5 cm in 14 days at room temperature. Colonies have faint "mouldy" odour, or a strong "mouldy" odour when grown on CYA or MEA media.

P. glabrum has monoverticillate conidiophores, which have an unbranched stripe with a whorl of 10-12 flask-shaped phialides at terminus. The conidiophores may occasionally branch. The stripes of the conidiophore are 200 μm long by 3.0-3.5 μm wide. Chains of conidia at the terminal end of conidiophores may be up to 150 μm in length.

Physiology
Penicillium glabrum is a psychotroph, growing at 0 C, up to maximum of 30 C. The optimal temperature for growth is 23 C, ideal conditions for conidial growth are 23-30 C, at water potential of ― 200 bars. P. glabrum is osmophilic, able to grow at high osmotic pressure. P. glabrum cannot grow at 37 C.

P. glabrum produces the toxic metabolite citromycetin. Other metabolites include succinic, tartanic, citric and oxalic acids, Hydroxybutyric acid, sulochrin, ether asterric acid, emodin, hadacin, ergosterol palmitate, nephrotoxic cytromycetin, palitantin. The P. glabrum strain CFTRI A-24 produces sclerotiorin. Sclerotiorin is a potent inhibitor of aldose reductase and lipoxygenase. Aldose reductase is an enzyme involved in diabetes complications. Lipoxygenase inhibition by sclerotiorin may be useful in comercial packaging of food. Sclerotiorin also has antibacterial activity against Gram-positive Bacillus species.

Habitat and ecology
Penicillium glabrum is a commonly occurring species with a world-wide distribution. It is one of the most wide-spread species of Penicillia. Its habitat is indoors, on food, in soil and on decaying vegetable matter. It has been isolated from maize, peanuts, rice, jam, cured meats, canned carbonated beverages, as well as stored grapes and strawberries, bottled mineral water, and dried fruits, nuts, frozen cakes, and fruit juices. P. glabrum can also cause spoilage of cheese. It is also frequently found in stored wheat grain and fresh and stored corn.

P. glabrum is found in almost all soils, it is especially common is acidic forest soils such as podzol and grey forest soils. However, it is uncommon in arable and cultivated soils. It can also be found in pine litter, deciduous trees, barley and cabbage crops. The role of P. glabrum and other members of the Penicillium frequentans series on decomposition processes in nature has not been well studied, but their common distribution suggests they may play an important role as decomposers.

Health significance
Penicillium glabrum is the main causative agent of Suberosis, a type of hypersensitivity pneumonitis (HP) that affects cork workers. Suberosis a substantial cause of hypersensitivity pneumonitis in northern Portugal, with an estimated 1000 to 2000 workers in the Portugese cork industry suffering from the disease.

Suberosis is caused by inhalation of dust from mouldy cork. Precipitins for Penicillium glabrum are found in the sera of patients with suberosis. P. glabrum is commonly found in the air of cork factories. The air inside cork factories has a much higher concentration of P. glabrum spores compared to the outside air. Precipitins to P. glabrum, indicating a sensitivity, were found in 38% of cork workers exposed to mouldy cork particles. Less than 1% of the unexposed control population had this sensitivity. Precipitins to P. glabrum were found in 98% of cork workers affected by suberosis and only 7% of those not affected by suberosis. Long-term exposure to the cork factory environment, does not greatly increase the risk of developing a sensitivity to P. glabrum as 38% of workers with less than 5 years of exposure were sensitized, compared to 45% of workers exposed for 5-10 years.

P. glabrum is also a causal agent of the rare Chacinero's lung, another form of hypersensitivity pneumonitis. Dry sausages are dripped in mould and then fermented for 3-4 months. Afterwards, mould is brushed off by a worker. Long time exposure and inhalation to this mould dust leads to some developing Chacinero's lung. Some patients had specific antibodies (IgG) against P. Glabrum. Aspergillus fumigatus may also contribute to Chacinero's lung, as antibodies against A. fumigatus are found in patients as well.

The P. glabrum metabolite sclerotiorin, a potent aldose reductase inhibitor in vitro, may have therapeutic potential in preventing diabetes mellitus complications such as cataracts, neuropathy, nephropathy and retinopathy.