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History and taxonomy
After being described by Charles Thom in 1930, Penicillium crustosum was characterized by Thom and Kenneth Raper in 1949. When it was originally described, it was thought to be an uncommon fungus. However, it is now known to be ubiquitous in food and animal feed. There are many synonyms for P. crustosum, as this species has been characterized as various variations of other Penicillium species. For example, it has been previously characterized as a variation of Penicillium expansum.

Growth and morphology
P. crustosum has been described as the most complex penicillium species, both due to its morphological features and the variety of secondary metabolites it is capable of producing. It has large conidia that are smooth-walled and spherical. This fungus exhibits terverticillate conidiophores that are born from subsurface hyphae. Each conidiophore has at least two branches (rami) that divide into metulae, which support the phialides. This morphology is typical for this subgenus. Both the rami and metulae are cylindrical, while the phialides are cylindrical and tapered.

The optimal growth temperature of P. crustosum is 25 &deg;C. The growth of P. crustosum has been described with respect to multiple media. Fast growing, dull green colonies are observed when this fungus is grown on both Czapek yeast autolysate (CYA) and malt extract agar (MEA). It is also important to note that when grown on MEA, large numbers of conidia are observed. Its highest rates of sporulation are noted when grown on yeast extract (YES) agar. P. crustosum has also been described as cyclohexamide tolerant fungus.

Physiology
P. crustosum has been shown to have the ability to survive in conditions of limited access to water, and has therefore been characterized as a psychrophile. P. crustosum has the ability to produce penitrems A-F, a group of mycotoxins. The synthesis of these penitrems can be induced through excess glutamate, oxidative stress, or cheese model medium. Penitrem A synthesis specifically can be stimulated by high environmental glucose. High salt concentrations can stop the synthesis of all penitrems by P. crustosum. Besides the penitrem family of mycotoxins, P. crustosum also has the ability to produce cyclopiazonic acid, roquefortine, and terrestric acid. This fungus can also produce alkali, but only when plated on creatine-sucrose agar.

Pathogenicity
Neurotoxicity outbreaks within populations of domestic animals were originally thought to be caused by a variety of other species within the Penicillium genus, including P. aurantiogriseum, P. commune, and P. viridicatum. It has since been established that the cause of these outbreaks was in fact P. crustosum. This was clarified by the discovery of Penitrem A as the causative toxin. Penitrem A is a tremorgenic neurotoxin primarily produced by P. crustosum. Outbreaks of this fungus result in tremors, brain damage and death in dogs, cows, horses, and sheep. Although Penitrem A has not been evaluated with respect to its toxicity in humans, it is assumed to be toxic since it affects large mammals. Part of the reason why its toxicity in humans is not well documented is the fact that Penitrem A is undetectable in post-mortem examinations. It does not induce any visible pathological effects. Some studies have shown that dizziness and vomiting may be the primary symptoms of ingestion. These effects were observed after the ingestion of drinks containing mould with P. crustosum. Since P. crustosum poses multiple health risks, it is important to correctly identify the fungi responsible for any food spoilage or mould growth.

A minor allergen from P. crustosum, known as Pen cr 26, has been recently discovered. It is genetically and morphologically similar to Pen b 26, a major allergen produced by Penicillium brevicompactum. The key difference between these allergens are the epitopes present on the surface of each molecule. The different epitopes lead to different immune responses, which explains the categorization of Pen cr 26 as a minor allergen, while Pen b 26 is known to be a major allergen. It has been proposed that Pen cr 26 may be a hypoallergenic variant of Pen b 26.

Habitat and ecology
P. crustosum is able to thrive in a wide range of habitats, and has been found in many countries across the globe. It thrives in a variety of environments including arctic seawater, sea ice, glacial ice, and salt marsh soil. This fungus, along with many fungi in the Penicillium genus, has also been shown to exist as an air contaminant. P. crustosum has also been described as a spoilage fungus found in cereal, animal feed, corn, nuts, cheese, processed meat, pomaceous and stone fruits. For example, it has recently been shown to cause fruit rot in Argentinian cherries and raspberries. Although its appearance in animal feed and human foods is equally common, its associated negative toxic effects have mainly been described in animal populations. It is important to note that the strains found in each environment produce different secondary metabolites. For example, Arctic strains are more likely to produce Andrastin A. Andrastin A is a farnesyltransferase inhibitor mainly associated with other Penicillium species, including P. albocoremium and P. roqueforti.