|Blue Stilton cheese, showing the blue-green mold veins produced by Penicillin roqueforti.|
Penicillium roqueforti is a common saprotrophic fungus from the family Trichocomaceae. Widespread in nature, it can be isolated from soil, decaying organic matter, and plants. The major industrial use of this fungus is the production of blue cheeses, flavouring agents, antifungals, polysaccharides, proteases and other enzymes. The fungus has been a constituent of Roquefort, Stilton, Danish blue, Cabrales and other blue cheeses eaten by humans since about 50 AD; blue cheese is mentioned in literature as far back as AD 79, when Pliny the Elder remarked upon its rich flavour.
First described by Thom in 1906, P. roqueforti was initially a heterogeneous species of blue-green sporulating fungi. They were grouped into different species based on phenotypic differences, but later combined into one species by Raper and Thom (1949). The P. roqueforti group got a reclassification in 1996 thanks to molecular analysis of ribosomal DNA sequences. Formerly divided into two varieties ― cheese-making (P. roqueforti var. roqueforti) and patulin-making (P. roqueforti var. carneum), P. roqueforti was reclassified into three species named P. roqueforti, P. carneum and P. paneum.
As this fungus does not form visible fruiting bodies, descriptions are based on macromorphological characteristics of fungal colonies growing on various standard agar media, and on microscopic characteristics. When grown on Czapek yeast autolysate (CYA) agar or yeast-extract sucrose (YES) agar, P. roqueforti colonies are typically 40 mm in diameter, olive brown to dull green (dark green to black on the reverse side of the agar plate), with a velutinous texture. Grown on malt extract (MEA) agar, colonies are 50 mm in diameter, dull green in color (beige to greyish green on the reverse side), with arachnoid (with many spider-web-like fibers) colony margins. Another characteristic morphological feature of this species includes the production of asexual spores in phialides with a distinctive brush-shaped configuration.
The chief industrial use of this species is the production of blue cheeses, such as its namesake Roquefort, Bleu de Bresse, Bleu du Vercors-Sassenage, Brebiblu, Cabrales, Cambozola (Blue Brie), Cashel Blue, Danish blue, Fourme d'Ambert, Fourme de Montbrison, Lanark Blue, Shropshire Blue and Stilton, and some varieties of Bleu d'Auvergne and Gorgonzola. (Other blue cheeses, including Bleu de Gex and Rochebaron, use Penicillium glaucum.)
Strains of the microorganism are also used to produce compounds that can be employed as antibiotics, flavours, and fragrances (Sharpell, 1985), uses not regulated under the U.S. Toxic Substances Control Act (TSCA). Its texture is chitinous.
Secondary metabolites 
Considerable evidence indicates that most strains are capable of producing harmful secondary metabolites (alkaloids and other mycotoxins) under certain growth conditions. Aristolochene is a sesquiterpenoid compound produced by P. roqueforti, and is likely a precursor to the toxin known as PR toxin, made in large amounts by the fungus. PR-toxin has been implicated in incidents of mycotoxicoses resulting from eating contaminated grains. However, PR toxin is not stable in cheese and breaks down to the less toxic PR imine.
Secondary metabolites of P. roqueforti, named andrastins A-D, are found in blue cheese. The andrastins inhibit proteins involved in the efflux of anticancer drugs from multidrug-resistant cancer cells.
The organism can also be used for the production of proteases and speciality chemicals, such as methyl ketones including 2-heptanone. Other strains of Penicillium species are also useful in biodeterioration.
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