Identification
Trichoderma reesei forms fast-spreading colonies, white at first and turning yellow-green to green as conidia develop. Under the microscope the conidiophores branch repeatedly and end in flask-shaped phialides that produce small, smooth, green conidia in slimy heads, as is typical for Trichoderma. Like most species in the genus, it can only be identified reliably by DNA sequencing.
Its sexual stage was described as Hypocrea jecorina; under the one fungus, one name rule the species is now called T. reesei. Industrial strains descend from the isolate QM6a and its mutants such as RUT-C30, selected over decades for higher enzyme secretion.
Habitat and ecology
T. reesei is a soil and wood-associated fungus of tropical regions; wild isolates of the species have been found in the tropics on several continents. Like other Trichoderma, it is a saprotroph that lives on decaying plant material and degrades cellulose and hemicellulose. The original strain was isolated in the 1940s on the Solomon Islands from cotton canvas and clothing of US Army equipment that was rotting in the humid climate, which led to the discovery of its powerful cellulases.
Uses and importance
T. reesei is one of the most important industrial enzyme producers. Its cellulases and hemicellulases are used to give denim a worn look (biostoning) and to soften fabrics, in detergents, in animal feed to improve digestibility, in pulp and paper processing, in fruit juice and brewing, and in breaking down plant biomass into sugars for second-generation bioethanol.
Because it can secrete grams of protein per litre, it is also used as a host for producing other enzymes and recombinant proteins. Its genome was published in 2008, and it is a model organism for studying how fungi regulate enzyme production and protein secretion. Genome editing with CRISPR is now routinely applied to improve strains.
Cultivation
In industry, T. reesei is grown in large, aerated, stirred fermenters in submerged fed-batch culture. Production of cellulases is induced by cellulose or by inducing sugars such as lactose or sophorose, while high glucose levels repress it. Temperature is usually kept around 28 °C, with tight control of pH and dissolved oxygen. The enzymes are secreted into the broth, separated from the biomass and formulated. This is industrial and laboratory work under quality control, not home cultivation.
Safety and edibility
Trichoderma reesei is not a food and is not cultivated to be eaten. Industrial production strains have a long record of safe use for producing enzymes, which are then purified and assessed for their intended use, for example in food or feed. The species is not considered a human pathogen, but related species such as Trichoderma longibrachiatum can cause rare infections in severely immunocompromised people. Concentrated enzyme dusts can cause respiratory allergy in exposed workers, so industrial plants control dust exposure. Laboratory work requires standard microbiological safety practice, and genetically modified strains are handled under the applicable biosafety rules.
If you suspect mushroom poisoning, call your local emergency number or poison centre straight away, even before any symptoms appear, and keep the remains of the mushrooms.
Look-alikes
Many Trichoderma species look almost identical: green, fast-growing moulds such as Trichoderma harzianum, used in biocontrol, and Trichoderma aggressivum, a serious green mould pest of button mushroom farms. Trichoderma longibrachiatum is a very close relative. Separation requires DNA barcodes, usually based on the ITS region together with genes such as tef1.
Around the world
Worldwide, enzymes from T. reesei are produced by major biotechnology companies and used in textiles, food, feed and biofuels on every continent. Research groups in Europe, the United States and Asia use the fungus as a model for protein secretion and strain engineering.