Penicillium chrysogenum is a mold that is widely distributed in nature, and is often found living on foods and in indoor environments. It was previously known as Penicillium notatum. It has rarely been reported as a cause of human disease. It is the source of several β-lactam antibiotics, most significantly penicillin. Other secondary metabolites of P. chrysogenum include various different penicillins, roquefortine C, meleagrin, chrysogine, xanthocillins, secalonic acids, sorrentanone, sorbicillin, and PR-toxin.
Like the many other species of the genus Penicillium, P. chrysogenum reproduces by forming dry chains of spores (or conidia) from brush-shaped conidiophores. The conidia are typically carried by air currents to new colonization sites. In P. chrysogenum the conidia are blue to blue-green, and the mold sometimes exudes a yellow pigment. However, P. chrysogenum cannot be identified based on color alone. Observations of morphology and microscopic features are needed to confirm its identity.
The airborne spores of P. chrysogenum are important human allergens. Vacuolar and alkaline serine proteases have been implicated as the major allergenic proteins. 
P. chrysogenum has been used industrially to produce penicillin and xanthocillin X, to treat pulp mill waste, to produce the enzymes polyamine oxidase, phospho-gluconate dehydrogenase, and glucose oxidase.
Science and History
Penicillin was discovered in 1928 when Alexander Fleming's lab assistant left a window open overnight and had mold spores cover his Staphylococcus bacterial specimens in a petri dish. At first he was very irritated at the contamination but as he was about to throw the specimens away when he noticed something interesting. He looked under the microscope at the bacteria surrounding the blue-green mold and noticed that many were dead or dying due to the mold preventing the bacteria from making new cell walls and reproducing. He identified the mold as Penicillium notatum, which releases the antibiotic penicillin G into the medium. After this he did some testing on humans and animals and discovered that not only did it kill bacteria, but that it was suitable for use in humans and animals. However, the discovery did not attract much attention until the 1940s when Howard Florey and Ernst Chain developed methods for mass production and application in humans, incited by the urgent war-time need for antibacterial agents. Army pilots sent back soil from around the world to be tested for the right kind of mold. Even the people of Peoria, Illinois were told to bring in any molds that they found around their homes. It has also been said that the scientists working on this project kept an eye out for similar looking molds while grocery shopping or when they were cleaning around the kitchen especially their refrigerators. The discovery of penicillin ushered in a new age of antibiotics derived from microorganisms.
Genetics and Evolution
The ability to produce penicillin appears to have evolved over thousands of years, and is shared with several other related fungi. It is believed to confer a selective advantage during competition with bacteria for food sources. However, some bacteria have developed the ability to survive penicillin exposure by producing penicillinases, enzymes that degrade penicillin. Penicillinase production is one mechanism by which bacteria can become penicillin resistant.
The principle genes responsible for producing penicillin, pcbAB, pcbC and penDE are closely linked, forming a cluster on chromosome I. Some high-producing Penicillium chrysogenum strains used for the industrial production of penicillin have been shown to have multiple tandem copies of the penicillin gene cluster.
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