Identification
Penicillium rubens forms fast-growing, velvety colonies that are blue-green on top, often with clear to yellow droplets of exudate, and with a yellow pigment that diffuses into the agar on the reverse. Under the microscope it shows the brush-shaped conidiophores typical of Penicillium, branched several times, with smooth stalks and long chains of small, smooth conidia.
It belongs to the Penicillium chrysogenum species complex. In 2011 a taxonomic study using DNA sequences showed that two species had been treated as P. chrysogenum, and that both Fleming’s original isolate and the strain behind most industrial penicillin production are P. rubens. The two species cannot be reliably told apart by appearance; identification relies on DNA sequencing and metabolite profiles.
Habitat and ecology
The species is cosmopolitan. It is common in soil, on decaying plant material and on foods, and it is one of the most frequent moulds in indoor environments: house dust, damp walls, wallpaper, wood and other building materials affected by water damage. Its airborne spores are found almost everywhere. Like many penicillia, it grows at moderate temperatures and tolerates relatively dry conditions.
Uses and importance
In 1928 Alexander Fleming noticed that a mould contaminating a bacterial culture plate inhibited the growth of staphylococci; he named the active substance penicillin. During the Second World War, researchers in the United States searched for higher-yielding strains, and a strain isolated from a mouldy cantaloupe in Peoria became the ancestor of most industrial lines after repeated mutation and selection. Modern production strains produce many times more penicillin than the original isolates.
Penicillin and the semi-synthetic beta-lactams derived from it transformed medicine. P. rubens is also a model organism for studying secondary metabolism and fungal genetics: its genome was published in 2008, and research continues on its gene clusters for antibiotics and other natural products.
Cultivation
Industrial penicillin is produced in large, aerated stainless-steel fermenters by submerged fed-batch culture. Sugar is fed slowly, and a side-chain precursor such as phenylacetic acid is added to direct production towards penicillin G. Temperature, pH and dissolved oxygen are tightly controlled. The antibiotic is then extracted and purified from the broth. This is industrial and laboratory work under strict quality control, not home cultivation.
Safety and edibility
Penicillium rubens is not a food, and mouldy food or mould cultures should never be eaten or used to make home remedies: mould growth does not provide a reliable or safe dose of penicillin, and the fungus also produces other metabolites. Antibiotics should only be taken as prescribed by a doctor. Spores of P. rubens and related species are common indoor allergens and can contribute to allergic and respiratory symptoms in damp buildings; persistent mould in a home should be addressed by fixing the moisture problem, with professional help where needed. Invasive infections are rare and occur mainly in severely immunocompromised people. Laboratory work requires standard microbiological safety practice.
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 blue-green moulds look alike. Penicillium chrysogenum in the strict sense is almost identical and is separated by DNA. Penicillium roqueforti, the blue cheese mould, is darker green, without yellow pigment, and tolerates low oxygen. Aspergillus species such as Aspergillus niger have conidiophores ending in a swollen vesicle rather than a brush.
Around the world
Worldwide, P. rubens is among the most frequently identified moulds in house dust and water-damaged buildings, and its industrial descendants supply global penicillin production. Fleming’s original culture, now re-identified as P. rubens, is preserved in culture collections.
