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Dynamics of Fungal Growth

By Dr. Harriet Burge, EMLab P&K Director of Aerobiology

Fungi that grow in indoor environments are primarily microscopic, and the asexual spores (conidia) are the dispersal units.  Spores of most common “molds” are designed for airborne dispersal and are released from mature fungal colonies in the outdoor environment.  These colonies are present on dead plant and animal material, on living plants, in soil, and on many other substrates.  Most spores are released by mechanical disturbance.  Outdoors this usually means wind.  Different fungi require different wind speeds to cause dispersal.  Once in the air, the smallest spores may be carried long distances, although they do eventually fall due to gravity.  The larger and more complex the spore is, the faster it will be removed from the aerosol.  (Wyatt et al., 2013)

Conidia

Spores penetrate indoor spaces through windows, doors, cracks, and on the surface of people and belongings that enter the space. Spores less than 4µm can pass through cracks at seams in the building.  Spores larger than 4 µm only enter through unobstructed openings (open windows, doors).  (Airaksinen et al. ) The airborne spores eventually impact or settle on a surface and wait for food and water.  Adherence to the surface involves activation of specific proteins, and hydrophobins [Aimanianda et al]. 

 

Aspergillus

Dormant conidia of Aspergillus and (probably) Penicillium are stress resistant and can survive desiccation, high temperature (50-60oc and UV radiation due to Melanin in the cell wall.   (Novikova et al., 2009) 

Penicillium

The dormancy state and conidial germination of Penicillium paneumis (and probably other species is regulated by the volatile auto inhibitor 1octen3ol. This compound has been used as an indicator of indoor fungal growth and is responsible for the characteristic of odor of many fungi. (Chitarra A. et al 2004)

Germination of dormant spores involves two steps:  Intake of water (I.e., swelling), and production of a germ tube.  Water intake requires breaking of the outer hydrophobic layer so that an underlying hydrophilic layer can be revealed.  In one study of Aspergillus conidia, this process lasted for 3 hours.  Asexual mold spores can produce a germ tube in water and do not require either carbon or nitrogen sources in the environment.   Sexual spores of these same fungi require accessible carbon and nitrogen for germ tube formation. Dague et al., 2008

Mycelium growth requires appropriate environmental conditions.  Especially important are water, temperature, and nutrients. Growth rate is dependent on the status of these factors, and on the genetics of the specific fungus.  Reduction of water supply (e.g., by lowering humidity will slow or even stop mycelium growth.  Each fungus has specific requirements for each relevant environmental factor. (Trinci 1969).  Water activity [Aw]  is a measure of water available for fungal growth.  Aw is defined as the ratio between the partial pressure of water in the atmosphere in equilibrium with the substrate to that of the atmosphere in equilibrium in pure water.  Aw for most fungi must be above .900. A few fungi can grow well at water activities below 0.8. For example, common Penicillium species grow rapidly at room temperature and water activity around 0.95.  Stachybotrys can grow at lower temperatures and  requires water activity at or near 1.0. Some Aspergillus species grow best at water activities around 0.8.

 

Stachybotrys

Most fungi have optimum temperatures in the range between 18 and 25oC.  Important exceptions include those fungi with optima near 37oC, including Aspergillus fumigatus. This temperature optimum is why these fungi can survive in the human body.   

Concentration of a single spore type and of multiple spore types influence both germ tube formation and mycelial growth.  This is caused by inhibitors released from the swollen spores and is one of the reasons why fungal growth reappears so quickly following cleaning of visible surface growth.  Cleaning does remove visible growth, but often leaves scattered spores.  These spores can readily germinate and grow in the absence of the inhibiting factors.  

To gain nutrients, fungi exude enzymes into the environment that can break down complex material such as polysaccharides and glycoproteins.  The mycelium can then absorb the glucose and amino acids from the environment.  In the process of breaking down the digestible portions of the substrate they weaken the material causing permanent damage. During this break down process, secondary metabolites are released into the environment.  These may be volatile (volatile organic compounds [VOC]) or nonvolatile liquids or solids.  Nonvolatile materials include mycotoxins. 

The question often arises “does fungal growth return more rapidly or extensively on material from which growth has been removed”.  As mentioned above, formerly inhibited spores may germinate creating new growth.  Note also, however, that concentrations of necessary nutrients have decreased and may slow such growth.

Overall, for fungal growth to occur on indoor substrates, enough water, appropriate nutrients and temperature must be present, and inhibitors must be absent.    Fungi for which the environment is ideal will germinate and grow most quickly.  Fungi for which conditions are suboptimal will grow much more slowly.  As conditions change to favor the slower growing fungi they will overgrow the original colonies.  Such is the case for the common succession of Penicillium and Stachybotrys as water begins to accumulate in an indoor substrate. 

The use of this information should be of help in formulating investigation strategies and in the interpretation of both visual and sampling data.  An excellent source for more information on the fungi is Bryce Kendrick’s book The Fifth Kingdom, which is available online at mycolog.com.

References:

Wyatt, Timon T.; Wosten, Han A. B.; Dijksterhuis, Jan Fungal Spores for Dispersion in Space and Time Advances in applied microbiology Vol 85   Book Series: Advances in Applied Microbiology   Volume: 85   Pages: 43-91   Published: 2013

Airaksinen, M; Kurnitski, J; Pasanen, P; et al.Fungal spore transport through a building structure Indoor Air  Volume: 14   Issue: 2   Pages: 92-104   Published: APR 2004

Belozerskaya, TA  Fungal hydrophobins: structure and function. Mikologya I Fitopathology Volume: 35   Issue: 1   Pages: 3-11   Published: 2001

Novikova, N.; Gusev, O.; Polikarpov, N.; et al.Survival of dormant organisms after long-term exposure to the space environment Conference: 17th International-Academy-of-Astronautics Humans in Space Symposium Location: Moscow, RUSSIA Date: JUN 07-11, 2009
Sponsor(s): Int Acad Astronaut Acta Astronautica   Volume: 68   Issue: 9-10   Special Issue: SI   Pages: 1574-1580   Published: MAY-JUN 2011

Chitarra, GS; Abee, T; Rombouts, FM; et al.Germination of Penicillium paneum conidia is regulated by 1-octen-3-ol, a volatile self-inhibitor Applied and Environmental Microbiology Volume: 70   Issue: 5   Pages: 2823-2829   Published: MAY 2004

Dague, Etienne; Alsteens, David; Latge, Jean-Paul; et al.High-resolution cell surface dynamics of germinating Aspergillus fumigatus conidia Biophysical Journal  Volume: 94   Issue: 2   Pages: 656-660   Published: JAN 15 2008

Trinci, APJ. A Kinetic study of growth of Aspergillus nidulans and other fungi. Journal of General Microbiology   Volume: 57   Pages: 11-&   Part: 1    Published: 1969

 


Published September 2018.