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Succession of Fungi | Fungus of the Month: Penicillium

Succession of Fungi

By Dr. Harriet Burge

Is there a predictable pattern of fungal succession on wetted indoor materials?

This sounds like a relatively straightforward question, and we have the impression that there is some consistency, at least for specific materials. When you actually stop and think about it, however, the number of variables that controls succession is enormous, to the point where any consistency is surprising. A few of the possible variables that control fungal growth are:

  1. Amount of available water
  2. Available nutrients
  3. Time with specific set of conditions
  4. Resident spore populations
  5. Inhibitors
  6. Temperature
  7. Genetics of the fungus

These variables are inextricably intertwined so that one cannot really discuss any one by itself. It is also impossible in one of these short papers to cover all of these factors, so perhaps this would be better as a series of articles. For today, we will stick with available water.

Without water, there will be no fungal growth. That is simple and straightforward. However, when water is present, the availability of the water and the solvent concentration in the water will (in part) control the kinds of spores than can germinate and grow. Note that requirements for germination and growth may differ. Some kinds of spores will only germinate when water with relatively little dissolved solute is freely available. Stachybotrys chartarum is one of these fungi. Other fungi need high solute concentrations for germination. For some fungi, these dissolved solutes must be utilizable carbon (e.g., sugars). The common species of Eurotium (e.g., Aspergillus glaucus group) are included in this category. Other fungi are intermediate in these requirements (e.g., many Penicilliium species).

The literature makes a point of the importance of water activity as it controls fungal growth. However, the water activity studies that are referenced were done by people interested in the growth of fungi on food. Thus, in most cases, water activity is controlled by the addition of utilizable carbon sources (e.g., sugars). Under these conditions there is liquid water, but it has a very high solute concentration so that the equilibrium relative humidity of the space above the substrate is relatively low. The fungi that can grow at these low water activities have high solute concentrations inside their cells, so that the water can move from the substrate into the cell. This is not the same as saying that the same fungus will grow on gypsum board at the same water activity. In the gypsum board, the water activity is probably controlled by the bonds that hold the water to the cellulose fibers rather than high solute concentration.

Now, what does this mean with respect to real life succession? Considering only water availability and solute concentration, we would expect very wet gypsum board to support the growth of fungi like Stachybotrys chartarum (as it does). As the water begins to evaporate, one might expect the invasion of intermediate organisms such as Penicillium or some Aspergillus species, providing the Stachybotrys has released enough soluble sugars to make the water appropriately available. As water becomes limiting for each organism, sporulation may be initiated, then growth will stop.

These are purely hypothetical scenarios. As far as I know, no one has actually done the succession studies on building materials that would allow testing of these hypotheses. I have referred to various articles in the literature to formulate them, and list them here for those who want additional information. The abstracts and many of the articles are available free online through www.pubmed.gov.


Role of water mobility on mold spore germination. Pham X, Vittadini E, Levin RE, Chinachoti P.J Agric Food Chem. 1999 Dec;47(12):4976-83

Ecological determinants for germination and growth of some Aspergillus and Penicillium spp. from maize grain. Marin S, Sanchis V, Saenz R, Ramos AJ, Vinas I, Magan N. Appl Environ Microbiol. 2004 May;70(5):2823-9

SEM study of water activity and temperature effects on the initial growth of Aspergillus ochraceus, Alternaria alternata and Fusarium verticillioides on maize grain. Scanning electron microscopy.
Torres MR, Ramos AJ, Soler J, Sanchis V, Marin S. Int J Food Microbiol. 2003 Mar 25;81(3):185-93

Influence of temperature, water activity and pH on growth of some xerophilic fungi. Gock MA, Hocking AD, Pitt JI, Poulos PG. Int J Food Microbiol. 2003 Feb 25;81(1):11-9


Fungus of the Month: Penicillium

By Dr. Fernando Fernandez

The genus Penicillium refers to a very important group of fungi that can be friends or foes, depending on what / where / or when they occur and the chemical compounds they produce. The genus is most famous for the ability of some of its members to produce Penicillin, the most notable antibiotic of fungal origin that revolutionized the treatment of infections and human health. This compound was discovered accidentally when Alexander Fleming noticed that his Staphylococcus cultures were contaminated and inhibited by a mold, later found to be a strain of Penicillium chrysogenum. We now know that many species produce spores that become airborne and are frequently encountered as laboratory contaminants. Most recently Penicillium has become an important player in the discovery and development of the cholesterol lowering agents known as statins, specifically Mevastatin, produced by Penicillium citrinum.

Penicillium is able to grow on and exploit a variety of substrates, particularly foodstuffs. It is among the most commonly encountered fungi in the indoor air environment with perhaps Penicillium chrysogenum as the most common species found in household dust. Some species such as P. expansum, P. digitatum and P. italicum cause important post-harvest decays of apples and citrus fruits. Other species such as P. roqueforti and P. camemberti are highly prized and used for ripening and flavoring the "blue cheeses". Penicillium can also be found on wallpaper, wallpaper glue, and moist chipboards in water-damaged buildings. It is rather fast growing on substrates when compared to other indoor fungi such as Stachybotrys or Chaetomium. Generally, Penicillium species grow well in low moisture conditions however, certain species are adapted to even dryer conditions (i.e. they are xerophilic). Nearly all species are active producers of mycotoxins, although the genus is not as notorious for this ability as the genus Aspergillus. Penicillium is morphologically similar and closely related to Aspergillus, another important fungus when dealing with indoor air quality. On spore traps, the distinction between spores of Penicillium and Aspergillus has always been problematic as they both produce morphologically similar spores. That is why these spores are often clumped together as "Penicillium/Aspergillus type spores", in laboratory reports. Other types of spores also appear similar to Penicillium and Aspergillus spores and are consequently included in the "Penicillium/Aspergillus type spores" category on laboratory reports including those belonging to genera such as Paecilomyces and certain species of Acremonium just to name a few.

There are approximately 223 species of Penicillium. The genus is widespread and its members are the most abundant fungi in soils in temperate regions. Penicillium colonies grow rapidly on artificial medium in the laboratory. They are usually powdery with green, grey, yellow or white color and, rarely, red. The microscopic spore-bearing structures are colorless or slightly pigmented, and bear one or several whorls of branches with terminal spore-producing cells. Spores are one-celled, colorless or slightly pigmented and produced in dry chains. Some Penicillium species can also be identified by shifts in the pH of the medium during growth, when the medium contains creatine as a nitrogen source and sucrose as a carbon source. Some isolates acidify the growth medium and turn it yellow and, as the pH increases over time, the medium turns purple. Penicillium also reproduces sexually. It has four associated sexual names (or genera): Eupenicillium, Hamigera, Talaromyces, and Trichocoma.

Penicillium speciation in culture has always been challenging and problematic and requires years of training. Each species must be grown on a specific media and grown for 7 days under specific laboratory conditions. To make the matter even more complicated, each species is identified through very specific taxonomic keys in which a combination of microscopic and macroscopic characters are used. Many of these characteristics are inconsistent even within the same "species". Despite these difficulties, the scientific community has developed a good understanding of one of the most commonly occurring species in the indoor environment.


Alexopolous, C. J., Mims, C. W., and Blackwell, M. 1996. Introductory Mycology. 4th edition, John Wiley, New York.

Bills, G. F., M. Christensen, M. Powell, and G. Thorn. 2004. Saprobic Soil Fungi. Pp. 271-302. In: Biodiversity of Fungi, Inventory and Monitoring Methods. Eds. G. M. Mueller, G. F. Bills and M. S. Foster. Elsevier Academic Press, Oxford.

Hoog, G. S. de, J. Guarro, J. Gené, and M. J. Figueras. 2000. Atlas of Clinical Fungi. 2nd edition, Centraalbureau voor Schimmelcultures/Universitat Rovira I Virgili, Utrecht/Reus.

Scott, J., W. A. Untereiner, B. Wong, N. A. Straus, and D. Malloch. 2004. Genotypic variation in Penicillium chrysogenum from indoor environments. Mycologia 96: 1095-1105.


This article was originally published on January 2005.