Unfathomed Universe of Fungi: Penicillium

Among the diverse group of microorganisms, fungi make up a significant component of life on Earth. They are not only decomposers which facilitate the nutrient cycle in ecosystems, but also sources of medical agents and industrial domains. Amid the crowded world of fungi, one genus, Penicillium has been a major player in many advances such as the production of antibiotics and cheese-making. At least 300 species of Penicillium have been discovered, making it one of the largest groups of fungi. Nevertheless, the impacts of Penicillium fungi on human life are not only limited to positives, since some of its species can lead to food spoilage (Torres-Garcia et al., 2022). In addition, their ability to produce a wide range of secondary metabolites suggests that other species of this genus might be harmful to human health. In this article, the significance of the genus Penicillium will be emphasized within the frame of its morphological characteristics as well as benefits and drawbacks to nature and human life. From its life-saving origins in the medicinal field to its contributions to the nutrient cycling and food industry, it is aimed to provide a balanced understanding of the genus in the biological and medicinal context.

Classification and Morphology

Penicillium has several species that exhibit different morphological features. These species are considered to be ascomycota, and sexual reproduction is maintained within sac-like structures, "asci", filled with ascospores. However, there are some other fungi species among the genus that reproduce asexually, and the production of ascospores is not observed (Langlois et al., 2014). The "conicidia", asexual spores, in Penicillium, are localized within special structures called conidiophores that possess a brush-like appearance due to primary and secondary branching, which gives the genus its original name (Penicillium means brush in Latin). The fungi form hyphae, thread-like, filamentous structures whose cells are separated by septa, cross-walls which provide space between the cytoplasm and the organelles. These hyphae come together to form an inter-connected network called the mycelium (Visagie et al., 2014). During vegetative reproduction, the hyphae break up into short segments via a process called fragmentation, through which they form their own mycelium.

Figure 1: "Penicillium rubens", Fleming-Stamm, Kultur (ZfMK, 2022).

Figure 2: "Penicillium" morphology (Visagie et al., 2016).

The colonies of Penicillium species vary in colour, depending on the production of pigments. They are most commonly observed to vary between blue, green, and grey; however, species like P. camemberti and P. solitum may appear as beige or brown-orange colonies (Torres-Garcia D et al., 2022). The size of the colonies may also differ among species depending on the growth pattern of the fungi. Studies show that the reason for this colourful appearance is the presence of melanin within the spores (Yang et al., 2022).

Impacts to the Nature

Penicillium species are considered to be saprophytes, meaning that they obtain the nutrients they require via the decomposition of living organisms. The organic material such as plant residues found within the soil is broken into simpler compounds by the fungi (Pangging et al., 2021), resulting in the release of carbon back into the atmosphere, contributing to the carbon cycle. This phenomenon also has a significant impact on nutrient recycling since the process helps materials such as phosphorus and sulphur to travel back to the ecosystem. Another advantage is that the process of breaking down organic material in the soil increases the overall quality, contributing to the wellness of the ecosystem (Park et al., 2019). Some species of the genus are studied as tools for bioremediation, which is the phenomenon where biological organisms are used in order to remove environmental pollutants (Ojuederie & Babalola, 2017). By utilizing the biological mechanisms of Penicillium, the ecosystem is aimed to be restored so that contamination of the environment is minimized.

Figure 3: Process of Bioremediation (BYJU'S).

Mycorrhizal interactions of Penicillium with most plant species in terrestrial ecosystems also play essential roles in the growth and survival of their hosts. Mutualistic interactions of plant roots with Penicillium facilitate nutrient uptake such as phosphorus by plants, which affects carbon allocation as well as recycling. In fact, some species of Penicillium that were found in association with wheat are observed to induce the solubilization of phosphorus in the endosphere and rhizosphere (Toju & Sato, 2018). Although this might be the case, different species can also serve as pathogens which can cause serious diseases in various plant species, impacting organismal health. Some endophytic Penicillium species—that can live within a plant tissue without causing harm or disease—are known to aid the plant in terms of optimal nutrient uptake, protection against other pathogens, and enhancing the ability of the plant to conserve water, leading to drought tolerance (Kaur & Saxena, 2023).

Impacts to Human Life

The significance of Penicillium fungi on human health goes back to the 1920s, when Alexander Fleming came back to his office from vacation to realize that some of the organisms had grown on one of his bacteria plates. He realized that the bacteria cannot grow near the places where the fungus grows (Gaynes, 2017). After careful isolation and observation of the fungus, he decided to use this knowledge to generate penicillin, which is the first antibiotic known in history. Since then, other species of Penicillium have been utilized in research in order to develop other antibiotics that potentially save numerous lives when used against bacterial infections. Nevertheless, misuse of antibiotics leads to the presence of bacteria that are resistant to the antibiotics, causing more severe infections. Besides being popular in antibiotics generation, Penicillium species have been utilized for other drug discoveries. Research shows that these drugs possess antimicrobial, antiparasitic, anticancer, antioxidant, anti-obesity, anti-inflammatory, antidiabetic, neuroprotective, antifibrotic, or immunosuppressive properties (Toghueo & Boyom, 2020). In spite of having numerous benefits to pharmacological research, some species are able to produce allergenic substances that might trigger health problems in some people.

Figure 4: Alexander Fleming (Latson, 2015).

Other industrial applications of Penicillium include the production of organic molecules such as acids, enzymes, or secondary metabolites. In records, there are numerous Penicillium species which serve as biocatalysts, plant growth promoters, and phytoremediators. Also, a high number of neuroprotective agents have been reported to be extracted from Penicillium species. An integral example of this case is Huperzine A, which is a well-known acetylcholinesterase inhibitor that has been used for the management of Alzheimer's, a commonly observed disease among old people. An additional example can be the derivation of citric acid from Penicillium species which is commonly used in the food industry. In addition to bioactive compounds, there are examples of synthesis of nanoparticles by some species of the genus. This area remains to be appreciated across nanotechnology since the use of metals in the form of nanoparticles for the sake of human health might be possible. For example, Penicillium citrinum has the ability to synthesize a gold nanoparticle that has significant antioxidant properties (Toghueo & Boyom, 2020).

Another major role of Penicillium in human life includes the food industry, enabling cheese-making. Several species of the genus have been utilized in the market for the production of cheese varieties: the fungus is introduced to the cheese during the making process, giving it a unique flavor Well-known examples of cheese that include the utilization of Penicillium species are Camembert and Brie (Penicillium camemberti) and blue cheeses such as Roquefort, Gorgonzola and Stilton (Penicillium roqueforti). In addition to cheese production, some species are also used to stabilize the flavour of certain foods such as sausages and meat products as well as contributing to their process of fermentation. They can be used as additives in order to facilitate the fermentation process of foods such as bread or condiments such as soy sauce (Renato et al., 2011). Another contribution to the food industry includes some species being used as a biological control to prevent the spread of pathogens to certain nutrients such as fruits.

Figure 5: Roquefort cheese (Chef's Mandala).

Figure 6: Camembert cheese (Fisher of Newburry).

Although Penicillium make a significant contribution to the food industry, some species are known to contaminate various kinds of food such as fruits, grains etc. and cause their spoilage, resulting in reduced quality of food along with low yield. This contamination process is due to the production of mycotoxins which have serious consequences on animal and human health. Citrus fruits are among the fruits that have the highest rate of infection via fungicides which serves as a potential threat for decreasing the yield of production in a significant manner. Citrus blue mould (Penicillium italicum) and green mould (Penicillium digitatum) are the most common types of fungus that can cause citrus fruits to suffer from infections. Another post-harvest pathogen found in citrus species includes P. ulaiense which is known to be the cause of whisker mould (Carrillo, 1995). Since citrus fruits are among the fruits that have the highest rate of global production, their infection via Penicillium species serves as a potential threat for decreasing the yield of production significantly.

Figure 7: "Penicillium digitatum" on orange (iStock).

Figure 8: Whisker mould, "Penicillium ulaiense" (Invasive.org).

Conclusion

Penicillium fungi, having unique morphological characteristics and diverse impacts on human life and ecology, serve as important organisms that are utilized in research. There are several species of the genus which can be identified via microscopic and macroscopic observations combined with laboratory work. In the natural environment, the genus is important to endorse nutrient cycling as well as decomposing the organic material, contributing to the carbon cycle. Moreover, the activity of fungi such as Penicillium can help with environmental restoration. The medical industry benefits from Penicillium's contributions by the generation of antibiotic species and other potential drugs that might positively affect human health. Additionally, Penicillium species yield a range of bioactive compounds, including antimicrobial agents and neuroprotective substances. In the food industry, Penicillium is utilized in order to enhance the fermentation process of food and cheese production such as Camembert, Roquefort and Brie. In spite of having invaluable benefits, it is also possible for these fungi to present some challenges such as causing food spoilage, acting as plant pathogens and generating antibiotic-resistant bacterial strains. Understanding and managing these impacts would maximize the benefit that could be received from Penicillium species in the future.

Bibliographical References

Carrillo L. (1995). Penicillium ulaiense Hsieh, Su & Tzean, un patógeno post-cosecha de cítricos del noroeste argentino [Penicillium ulaiense Hsieh, Su & Tzean, a post-harvest pathogen of citrus fruits in northeastern Argentina]. Revista Argentina de microbiologia, 27(2), 107–113.

Gaynes R. (2017). The Discovery of Penicillin—New Insights After More Than 75 Years of Clinical Use. Emerging Infectious Diseases, 23(5), 849–853. https://doi.org/10.3201/eid2305.161556

Kaur, R., & Saxena, S. (2023). Penicillium citrinum, a Drought-Tolerant Endophytic Fungus Isolated from Wheat (Triticum aestivum L.) Leaves with Plant Growth-Promoting Abilities. Current microbiology, 80(5), 184. https://doi.org/10.1007/s00284-023-03283-3

Langlois, D. K., Sutton, D. A., Swenson, C. L., Bailey, C. J., Wiederhold, N. P., Nelson, N. C., Thompson, E. H., Wickes, B. L., French, S., Fu, J., Vilar-Saavedra, P., & Peterson, S. W. (2014). Clinical, morphological, and molecular characterization of Penicillium canis sp. nov., isolated from a dog with osteomyelitis. Journal of clinical microbiology, 52(7), 2447–2453. https://doi.org/10.1128/JCM.03602-13

Ojuederie, O. B., & Babalola, O. O. (2017). Microbial and Plant-Assisted Bioremediation of Heavy Metal Polluted Environments: A Review. International journal of environmental research and public health, 14(12), 1504. https://doi.org/10.3390/ijerph14121504

Pangging M, Nguyen TTT, Lee HB. Seven New Records of Penicillium Species Belonging to Section Lanata-Divaricata in Korea. Mycobiology. (2021) Aug 2;49(4):363-375. doi: 10.1080/12298093.2021.1952814. PMID: 34512080; PMCID: PMC8409940.

Park, M. S., Oh, S. Y., Fong, J. J., Houbraken, J., & Lim, Y. W. (2019). The diversity and ecological roles of Penicillium in intertidal zones. Scientific reports, 9(1), 13540. https://doi.org/10.1038/s41598-019-49966-5

Renato Chávez, Francisco Fierro, Ramón O. García-Rico, Federico Laich.(2011) .

Mold-Fermented Foods: Penicillium spp. as Ripening Agents in the Elaboration of Cheese and Meat Products, Mycofactories 1: 73. https://doi.org/10.2174/978160805223311101010073

Toghueo, R. M. K., & Boyom, F. F. (2020). Endophytic Penicillium species and their agricultural, biotechnological, and pharmaceutical applications. 3 Biotech, 10(3), 107. https://doi.org/10.1007/s13205-020-2081-1

Toju, H., & Sato, H. (2018). Root-Associated Fungi Shared Between Arbuscular Mycorrhizal and Ectomycorrhizal Conifers in a Temperate Forest. Frontiers in microbiology, 9, 433. https://doi.org/10.3389/fmicb.2018.00433

Torres-Garcia D, Gené J, García D (2022) New and interesting species of Penicillium (Eurotiomycetes, Aspergillaceae) in freshwater sediments from Spain. MycoKeys 86: 103-145. https://doi.org/10.3897/mycokeys.86.73861

Visagie, C. M., Houbraken, J., Frisvad, J. C., Hong, S. B., Klaassen, C. H., Perrone, G., Seifert, K. A., Varga, J., Yaguchi, T., & Samson, R. A. (2014). Identification and nomenclature of the genus Penicillium. Studies in mycology, 78, 343–371. https://doi.org/10.1016/j.simyco.2014.09.001

Yang, F., Cheng, L., Du, Y., Xia, L., & Long, C. A. (2022). Functional identification of the DHN melanin synthesis gene cluster and its role in UV-C tolerance in citrus postharvest pathogenic fungus Penicillium digitatum. Fungal biology, 126(9), 566–575. https://doi.org/10.1016/j.funbio.2022.07.002

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