PENICILLIUM: CHARACTERISTICS, TAXONOMY, MORPHOLOGY, HABITAT - BIOLOGY - 2025

Penicillium is a genus of bacteria found in the Ascomycota phyllum. It was first described by the German mycologist Heinrich Link in 1809 and its taxonomy has been complex.

At first it was located in the Deuteromycetes (imperfect fungi), since only its anamorphic (asexual) state was known. Later it was found that Penicillium corresponded to the teleomorphic (sexual) states of the Eupenicillium and Talaromyces genera of the Ascomycetes.

Conidiophore of Penicillium sp. By AJC1, via Wikimedia Commons.

The anamorphic state of Penicillium is characterized by septate hyaline (colorless) hyphae. When placed in laboratory culture media, colonies turn from blue, bluish-green, to pink.

The etymology of Penicillium comes from brush, since the conidiophores (structures that produce asexual spores) are branched.

Penicillium, together with other types of fungi, are known as molds, because they grow on different organic products, forming a layer of different colors. This genus is saprophytic and can grow in various environments.

Many species of the genus are of great importance to humans. Penicillin (the first antibiotic used in medicine) was obtained from P. chrysogenum. On the other hand, the so-called blue cheeses obtain their characteristics from the lipolytic and proteolytic activity of Penicillium species.

Some species are harmful to health. These produce various toxins such as ochratoxins that damage the kidney system. Other species damage various foods such as bread and citrus. Some are pathogens of animals and humans.

General characteristics

The group is characterized by presenting septate hyphae and the mycelia form filaments. When cultivated in the laboratory it produces colonies with a compact mycelium and well defined margins.

The colonies are fast growing and have a woolly or cottony appearance. They are initially white and then turn blue-green, greenish-yellow or pink.

Sex spores (ascospores) are produced in asci (fruiting bodies) that can be woody or softer in texture depending on the group.

The most outstanding characteristic of the genus is the development of branched, brush-shaped conidiophores. The different types of branching of the conidiophore allow to differentiate the species.

The conidiophore is well structured and has an axis from which branches (metallis) are formed. Phialides (bottle-shaped cells that produce conidia) are formed on the metules. This is the most important character to differentiate the species of the genus.

Types of conidiophores

Simple: the phialides are solitary.

Monoverticylates: a group of phialides occur at the apex of the conidiophore axis.

Divicariates: the methules branch out at different heights and on these are groups of phialides.

Biverticylates: three or more metallis are formed at the tip of the shaft, each with a group of phialides at the apex.

Terverticylates: they present a series of intermediate branches between the metules and phialides.

Production of mycotoxins

Penicillium species produce toxic substances that contaminate food, which are known as mycotoxins. The most common are ochratoxins and patulins.

Ochratoxins can contaminate cereals and cheeses, as well as accumulate in the fat of animals consumed by humans. These toxins affect the kidney system.

Patulins are found in cereals and nuts. It can weaken the immune system and attack the nervous system.

Nutrition

Penicillium species are saprophytes. They have a high capacity to degrade organic matter due to the fact that they produce a large amount of hydrolytic enzymes. These enzymes have the ability to speed up the decomposition process.

These fungi are known as molds and their spores are the main air pollutants in many enclosed buildings. In addition, they behave as pathogens of different crops both in the field and in post-harvest.

Many can produce toxins that cause harm to humans. Others favor the fermentation of some foods and are also capable of producing antibiotics.

Phylogeny and taxonomy

The taxonomy of Penicillium is quite complex, because in many groups of fungi it was frequent that the anamorphic and teleomorphic states were considered different taxa.

At first the genus was located in the artificial group Deuteromycetes (imperfect fungi) because their sexual forms were not known. Teleomorphs are located on the Ascomycota phylum within the order Eurotiales

The genus was separated into four sub-genera: Aspergilloides, Furcatum, Penicillium and Biverticillium, which are differentiated by the type of conidiophore. Later, Biverticillatum was associated with the teleomorphic genus Talaromyces and the other three subgenera with Eupenicillium.

Later, in a review of Penicillium senso stricto (not including Talaromyces), the teleomorph Eupenicillium was considered a synonym, since the name Penicillium is older.

Gender

It is characterized by presenting asci formed by a network of hyphae with smooth walls. Asci mature in a few weeks in culture medium. Eight ascospores are formed that are unicellular. The anamorph (subgenus Biverticillatum) has narrow phialides.

Molecular studies determined that this is a monophyletic group and is currently located in the Trichocomaceae family. Approximately 110 species are recognized, grouped into seven sections.

Among the species of interest, T. marneffei stands out, a pathogen that causes systemic mycosis in patients with HIV.

Gender

Currently the genus includes all species of Eupenicillium, as well as the genera Eladia, Torulomyces, Chromocleista and Hemicarpenteles.

The species present asci with isodiametric cells (equal sides), with very hard walls (sclerotized). Asci take months to mature, and in some cases, ascospores fail to mature.

The asexual state is characterized by ampulous or bottle-shaped wide phialides.

The genus is located in the Aspergillaceae family and has been sub-divided into two sub-genera (Aspergilloides and Penicillium) and 25 sections.

Habitat

Penicillium species can thrive in diverse environments and have a cosmopolitan distribution. These are capable of living in extreme conditions of temperature, salinity, pH or water stress.

The presence of Penicillium has been reported on more than 400 species of plants. They have been found in very cold areas such as the sub-arctic and the tundra.

In saline environments, the presence of Penicillium species has been reported as endophytes of seagrasses or growing in soils of high salinity.

Other species are capable of growing in highly acidified soils naturally or by human activities, such as mining effluents and industrial wastewater.

Reproduction

Penicillium and Talaromyces species reproduce asexually and sexually.

Asexual reproduction

It can occur by fragmentation of mycelia that later continue to divide. In other cases, a sclerotium (thick-walled mycelium) can form that can withstand adverse conditions and then begin to divide.

The most common form of asexual reproduction is the production of conidia (sex spores) from conidiophores. These are formed from the division of the nucleus of the phialides.

Sexual reproduction

Sex spores are produced in the asci. An antheridium (male structure) and ascogonium (female structure) are generated. The cytoplasms of both structures fuse (plasmogamy) and then the nuclei join (karyogamy).

Once the diploid cell is formed, meiosis occurs. Four haploid cells are formed that undergo mitosis, thus producing eight ascospores.

Ascospores are haploid with a single nucleus. They may have a smooth or ornamented surface and are blown away by wind or discharged by rain.

Pathogens and diseases

Various species of Penicillium are causal agents of food spoilage, particularly when conditions are humid and warm. Colonies develop on these and the enzymes that produce break them down quickly. Colonies are generally greenish in color.

The contamination of buildings by the spores of Penicillium and other genera is also frequent. Closed and humid environments favor the development of the fungus. There are people who are very sensitive to spores, which causes various respiratory diseases and allergies.

T. marneffei (formerly Penicillium marneffei) causes penicilliosis, a disease endemic to Southeast Asia. This affects only immunosuppressed patients, mainly those with HIV. Conidia infect the host by inhalation and then develop intracellularly, affecting the function of some organs.

Biotechnology

Some Penicillium species are widely used in the food and pharmaceutical industry.

In many meat products that are subjected to maturation processes, the presence of fungal colonies on the surface is common. These mushrooms enhance the characteristic aromas and flavors due to the production of various chemical compounds.

Penicillium species make up the surface microbial flora in some fermented sausages such as salami. These are antioxidants and prevent desiccation, in addition to their proteolytic capacity contributing to the curing of the products and avoiding the attack of pathogens.

In the case of the so-called blue cheeses, P. roqueforti is added during their maturation. Its activity produces enzymes that break down chemical compounds producing the characteristic aroma and flavor of these cheeses. The blue spots are the colonies of the fungus on the cheese.

Some species are capable of producing natural antibiotics. One of these is penicillin, which is obtained mainly from P. chrysogenum. Penicillin was the first antibiotic used in medicine.

References

1. Acosta R (2006) Selection of Penicillium producers of antifungal peptides for use in meat products. Memory to qualify for the title of Doctor. Faculty of Veterinary Medicine, University of Extremadura, Spain. 288 p.
2. Cabañes J, MR Bragulat and G Castellá (2010) Ochratoxin a producing species in the genus Penicillium. Toxins 2: 1111-1120.
3. Houbraken and RA Samson (2011) Phylogeny of Penicillium and the segregation of Trichocomaceae into three families. Studies in Mycology 70: 1-51.
4. Samson RA, N Yilmazi, J Houbraken, H Spierenburg, KA Seifert, SW Peterson, J Varga and JC Frisvad (2011) Phylogeny and nomenclature of the genus Talaromyces and taxa accommodated in Penicillium subgenus Biverticillium. Studies in Mycology 70: 159-183.
5. Visagie CM, J Houbraken, JC Frisvad, SB Hong, CGW Klaassen, G Perrone, KA Seifert, J. Varga, T Yaguchi and RA Samson (2014) Identification and nomenclature of the genus Penicillium. Studies in Mycology 78: 343-371.
6. Yadav A, P Verma, V Kumar, P Sangwan, S Mishra, N Panjiar, V Gupta and AK Saxena (2017) Biodiversity of the genus Penicillium in different habitats. In: KV Gupta and S Rodríguez-Canto (ed). New and future developments in microbial biotechnology and bioengineering. Penicillium system properties and applications. Elsevier, United Kingdom.