Coelomycetes | Factors of Importance In Determining the Prevalence of Indoor Molds
By Griselda Hernandez
Coelomycetes are fungi that produce their conidiophores and conidia within fruiting structures called conidiomata. It is due to this fruiting structure for which they are named. The term coelomycete comes from the Greek word koilos meaning "hollow" and the suffix mycete is derived from the Greek word meaning "fungus".
Coelomycetes are predominantly found in tropical and subtropical regions where they mainly exist as plant pathogens to a variety of different plant species (Figure 1). However, they are also known to exist as saprobes or parasites of fungi, lichens and vertebrates. As with most fungi, they have been isolated from soil. Although they require hot and humid conditions for optimal growth, they have been found infrequently in temperate regions in a variety of cellulose-based substrates in indoor environments. Examples of such substrates include wallboard, wood, drywall paper and wallpaper to name a few.
Figure 1: Coelomycete infection in a leaf (left) and pycnidium of coelomycete (right) used with permission by www.apsnet.org
To better understand coelomycetes, it is important to start with a basic description of what a fungi is as well as a description of their basic characteristics. Fungi are eukaryotic heterotrophs with an absorptive mode of nutrition; they typically have a hyphal body (yeasts are exceptions) and reproduce by means of spores. A eukaryotic organism is made up of cells that have a nucleus and other membrane-bound organelles. This differs from prokaryotic cells which do not have a nucleus and membrane-bound organelles such as those of bacteria and archaea. The term heterotroph means “other-eaters”. In other words, fungi require organic molecules (carbon containing molecules) as a food source. Fungi are therefore not photosynthetic. A hypha (pl. hyphae) is the fungal body which is basically a tube. Hyphae are analogous to stems in higher plants. Hyphae are made up of chains of cells whose walls are made up of chitin, a polymer of N-acetylglucosamine (a sugar). Also, hyphae are essential to a process called cytoplasmic streaming. Cytoplasmic streaming is the "circulatory system" of fungi and allows water and nutrients to be translocated to all areas of the mass of fungal hyphae, called a mycelium. Finally, spores and conidia are the "seeds" of fungi. Although the terms "spores" and "conidia" are used interchangeably, there is a significant difference. Spores are sexual entities that have been produced as a result of meiosis (a.k.a. reduction division) while conidia are asexual and are typically produced apically or laterally as a result of a type of cell division, mitosis. When spores or conidia colonize a suitable substrate under optimal environmental conditions, they germinate giving rise to more hyphae which collectively make up the mycelium of fungi which we are able to see with the naked eye.
In coelomycetes, the fungal hyphae germinate from conidia and form an aggregated mass until they form a fertile layer of densely packed conidiophores. Conidiophores are simple or branched hyphae that develop specialized cells (conidiogenous) from which asexual conidia are produced. It is this mass of hyphae that create the fruiting structure known as a conidiomata. The conidiophores may either partially or completely cover the base of the fruiting structure. Since coelomyetes are mostly plant pathogens they usually develop just beneath the plants’ tissue which can be colonized at various depths. They can develop immediately beneath the plant’s cuticle (subcuticular), within the epidermal layer (intraepidermal) or just beneath the epidermal layer (subepidermal) depending on the type of coelomycetes. Most solely consist of fungal tissue although some may contain both plant and fungal tissue. There are two main types of morphologically distinct conidiomata: pycnidia and acervuli (Figure 2) . As illustrated by the pictures below, conidiomata appear to have a "blister-like" appearance. They release their spores by rupturing fungal and sometimes plant tissue.
Figure 2: Acervulus (left) and pycnidium (right) lined with conidiophores. Permission granted by Sutton.
Pycnidia are enclosed structures that are spherical to obpyriform (inversed pear-shape) and contain an apical opening through which conidia are released. As the production of conidia progresses, pressure builds until the opening, the ostiole, ruptures thereby releasing the conidia. Pycnidia resemble the fruiting structures of ascomycetes called ascomata (Figure 3) . However, they may be easily distinguished from one another at the microscopic level since ascomata produce numerous sacs called asci (figure 3) which contain spores (sexual) usually in counts of eight. Examples of coelomycetes having a pycnidial conidiomata are the genera Phomopsis, Botryodiplodia, and Phoma. The latter of which is found occasionally in indoor environments.
Figure 3: Ascomata with spores (brown) within asci from MycoAlbum CD by G.L. Barron
The second main type of conidomata are acervuli. Like pycnidia, acervuli also produce a mat of closely packed conidiophores but instead grow in a raised mass. Unlike pycnidia, acervuli are typically open and cup-shaped and the opening by which conidia are released is much larger than that of a pycnidium. Coelomycetes of the genus Colletotrichum and Pestaloptiopsis are examples of fungi containing acervular fruiting structures. Pestaloptiopsis spores (Figure 4) which originate from an acervular conidiomata are occasionally seen in outdoor air samples. Since the fruiting structures of coelomycetes are not easily aerosolized, identification of a coelomycetous fungus in air samples may not be possible. The presence of coelomycetes on direct analyses and cultures on the other hand, are easily accomplished since the growth on such substrates allows the presence of the fruiting structures. It is however difficult to identify coelomycetes to genus and species on substrates other than a plant host. This is because their fruiting structures are much simpler and lack many of the elaborate characteristics present in those growing on a natural plant host. As a result, identification to genus and species may be laborious if not impossible.
Figure 4: Pestalotiopsis spores from MycoAlbum CD by G.L. Barron
Conidiomata are not strictly limited to the morphological characteristics of pycnidia or acervuli. Various deviations from the two main types exist at different levels (ostioles shape, ostiole size, acervuloid shape, etc…). Terms such as pycnothyrial, sporodochial, pycnidioid and acervuloid have been adopted to refer to these conidiomata. In an effort to organize the variation, 3 orders have been described: Melanconiales, Sphaeropsidales and Pycnothyriales. The Sphaeropsidales are characterized by those having pycnidial conidiomata and the Melanconiales produce acervular conidiomata. The third order, Pycnothyriales produces pycnothyrial conidiomata.
Despite their existence as plant pathogens, not all coelomycete colonizations are considered deleterious. Coelomycetes have been used as fungal biocontrol agents. They have been successfully exploited for their ability to control or eradicate weeds as well as some disease-causing rusts from natural host populations.
Coelomycetes represent a large group of fungi that share a common feature in that conidophores and hence conidia are produced within enclosed fruiting structures. Currently, approximately 1,000 genera and 7,000 species have been described. They are very widespread and ubiquitous, invading a plethora of ecological niches. As mentioned, although they are mainly known as pathogens, they have been used in bioremediation of natural populations.
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A summary by Dave Gallup
The paper "Factors of Importance In Determining the Prevalence of Indoor Molds" (published in 1979) was authored by Janet Gallup, EMLab's founder, and three asthma doctors, Dr. Kozak, Dr. Cummins, and Dr. Gillman. The purpose of their research was to help identify factors that that are correlated with increased indoor mold counts in the hopes that controlling these as much as possible may help alleviate asthmatic reactions in mold allergic people.
They studied sixty-eight homes in southern California using an Andersen 6 stage sampler (which is a precursor to the Andersen N-6 sampler) and consequently evaluating airborne culturable counts of fungi. Three minute, 84 liter, samples were taken inside and out during relatively stable periods of weather. Due to budget constraints and a desire to standardize the process, only one inside room was evaluated, generally the family or living room.
The following factors were evaluated:
After the data was gathered, each factor was analyzed separately, including a statistical analysis performed by UCLA. Because the analysis was dealing with airborne fungi, the log of the fungal counts was used to achieve a normal distribution for analysis. The results are as follows:
Dust control: Compliance with dust control also had a statistically significant (p<0.03) correlation with indoor mold isolates. Using dust control methods reduced the levels of airborne fungi.
Indoor plants: No statistical difference could be found between the two groups evaluated (p>0.58). A second analysis was performed using data from two smaller sets of the homes that were evaluated that enabled a more definitive isolation of just the effect of the indoor plants and remove other factors. Again, no statistical difference could be found. Finally, a "plant transfer" study was performed in 3 homes and again, no difference could be found although admittedly the data set was small for this last evaluation.
Landscaping and landscape maintenance: Landscaping and associated maintenance had a statistically significant effect (p<0.046) on the indoor levels of airborne fungi. Homes that were "uncared for/natural" had slightly higher levels of fungi in the indoor air than either "cared for/average" or "cared for/lush." This is probably a reflection of higher fungal levels in the outside air.
Levels of organic debris: Homes with low or moderate amounts of organic debris had similar levels of fungi in the indoor air. Those with high amounts of organic debris near the home had statistically significant (p<0.02) higher levels of fungi in the indoor air.
Use of an electrostatic filtration system: Perhaps as expected, there was a statistically significant correlation (p<0.00005) between the use of a central electrostatic filtration system and lower airborne fungal concentrations. The use of portable filtrations systems was not evaluated.
Shade level: Homes that had low to moderate levels of shade near the home had roughly the same levels of indoor airborne fungi. Those with high amounts of shade proximate the home had notably higher levels of fungal levels inside (p<0.0003).
No significant correlations were found relating to any of the following: Zip code, age or number of occupants, age or size of the home, and the month the survey was taken.
Summary: Higher levels of airborne fungi were correlated with high levels of shade near the home, high levels of organic debris near the home, and landscaping that was essentially unmaintained. Note that this elevation in expected counts should track with higher outdoor counts and is not related to "water problems" as defined by most IAQ investigations. Low levels of airborne fungi were correlated with the use of good dust controls and the use of a central electrostatic filtration system. In fact, it was noted in the study that three homes that used a central electrostatic filtration system still had relatively low airborne fungal levels, even in the presence of a previous water disaster that was large enough to cause a recognized indoor mold problem. It was also noted that although there was no association between airborne fungal levels and the presence of indoor plants, there was a recommendation not to use wicker baskets as potted plant holders due to the fact that mold will grow on the wicker and they can also harbor mites.
The abstract is available on the web at: http://www.ncbi.nlm.nih.gov
This article was originally published on June 2006.