Environmental Reporter
Home > Resources > Education > Environmental Reporter > Xerophilic Fungi | The Yeasts

Xerophilic Fungi | The Yeasts

Xerophilic Fungi

By Dr. Harriet Burge

Xerophilic is a term that literally means “dry loving.”  Thus, xerophilic fungi are those that  can or prefer to grow in “dry” environments.  The confusing part of that definition is the meaning of “dry.”

To explain “dry” with respect to the fungi, we have to talk a bit about the physics of water adherence to surfaces.  Most surfaces on earth have some attached water molecules.  The first water molecules that attach to a surface adhere very tightly, and cannot be used by any microorganism as a source for metabolic water.  Subsequent layers attach less and less tightly until, finally, there is liquid water on the surface.

Another important concept is osmosis.  This is the process whereby water diffuses across a semipermeable membrane from a compartment (cell) with a higher water concentration (or lower concentration of dissolved material or solute) to a compartment (cell) with a lower water concentration (or higher concentration of dissolved material or solutes). 

An ordinary cell (for example a red blood cell) immediately begins losing water when in contact with a solution containing a higher concentration of dissolved solids.  The xerophilic fungi, however, can metabolically increase the concentration of dissolved solids in their cells when exposed to a concentrated solution.  This prevents water from leaving the fungal cell and allows metabolic activities, including growth and spore formation, to continue.

Because this is a metabolic process, many factors influence the ease with which the fungal cell can accumulate these additional solutes.  Nutrients must be available so that the fungus has the building blocks to make the solutes.  Temperature and pH are also crucial, as is the type of solute in the surrounding water.

What does this mean in terms of the indoor environment?  In the first place, the situation is not as simple as the fungus absorbing water from the air when the humidity is high.  It should be obvious that if this were the case, xerophilic fungi would be covering all the surfaces in outdoor Atlanta by the end of the summer.  Secondly, just because ambient humidity in an indoor environment is high, fungi are not necessarily going to grow.  Many interiors with high humidity do not promote fungal growth.   On the other hand, fungi always grow when indoor materials actually get wet! 

So, is water activity a useful measure for indoor air specialists? 

  1. Measures of water activity alone are unlikely to accurately predict whether or not fungi will grow in a particular environment.  Devices that claim to indicate whether or not a particular surface is likely to become moldy actually only measure whether or not sufficient free water exists on the surface to allow the spores within the little unit to germinate.  The conditions inside the unit (with respect to solutes and nutrients) are not the same as those on the wall.  Thus, if the spores do not germinate, mold growth is unlikely on the wall.  However, if the spores do germinate, growth may or may not occur depending on the many factors discussed above.
  2. If growth of xerophilic fungi has occurred, it is important to understand the factors that have allowed the growth to occur so that remediation recommendations can be made.  In general, the recommendation is generally to remove the growth and reduce humidity.  However, humidity control is not always straightforward, and perhaps substituting a material with fewer nutrients or with less water holding capacity, or with solutes that are incompatible with fungal growth would be preferable.


The Yeasts

By Karen Abella Santo-Pietro

The yeasts are described as unicellular fungi and are generally characterized by the absence of coenocytic (septate, divided into sections by thin walls) hyphae. They are usually small cells which reproduce by budding or by the formation of a cross wall followed by fission (Figure 1). During budding, the cell wall of the mother cell inflates and blows out to form a “bud”, which is subsequently released as a daughter cell. However, several yeasts exhibit formation of hyphae or pseudohyphae (chains of elongated buds).

Yeast cells at 600X magnification. Arrows denote budding cells

The initial single colony that is formed could easily multiply into smaller satellite colonies almost overnight and spread in different parts of the agar plates.  It is therefore important to only consider the original colonies (typically larger) when quantification is done as counting of satellite colonies may overestimate the total counts.  Yeast cells are very seldom encountered on spore traps and are more likely to be recovered during direct microscopic examinations of bulks, swabs, or tape-lifts.  If only the yeast phase is present, genus identification of yeasts is almost impossible during direct microscopic examinations.

The term yeast has no taxonomic standing and is simply a growth form in several groups of unrelated fungi. Some fungi may be dimorphic (two life stages) and exhibit a “yeast” stage that shifts to mycelial growth under certain conditions. Other fungi exist primarily as yeasts throughout most of their life cycles.

The yeasts include fungi with sexual forms (basidiomycetous and ascomycetous yeasts) and asexual forms (black yeasts). In lay terminology, “yeast” is sometimes reserved for the sexual fungi while “yeast-like fungi” is used to refer to asexual unicellular, budding fungi. The basidiomycetous yeasts can be differentiated from the ascomycetous yeasts by utilizing the urease test.  In this diagnostic test, urease (an enzyme that aids in the breakdown of urea) is present in the basidiomycetous and absent in the ascomycetous yeasts.

The basidiomycetes are divided into subclasses Hymenomycetes (mushroom-forming basidiomycetes), Urediniomycetes (rusts), and the Ustilaginomycetes (smuts). A few members grow in culture with budding cells. Examples of basidiomycetous yeasts include Cryptococcus spp. (subclass Hymenomycetes) and Sporobolomyces spp. (subclass Urediniomycetes).

Cryptococcus neoformans causes cryptococcosis, a disease most commonly manifested as meningitis and meningoencephalitis.  Cryptococcosis may also infect the skin, lungs, prostate gland, urinary tract, eyes, myocardium, bones, and joints. A future Environmental Reporter article will be dedicated to C. neoformans.

Sporobolomyces spp. (species type salmonicolor) are occasionally found on culturable air samples and are observed as pink or salmon-colored mucoid colonies. In this particular species, the daughter cells are formed upon stalks and are forcibly discharged by water pressure.  

The ascomycetous yeasts  (class Hemiascomycetes) are one of the main groups of the Ascomycota. They form asci (sac-like structures that produce ascospores) without forming fruiting structures. The asci are produced on hyphae or arise after conversion or conjunction of budding cells.  Examples of ascomycetous yeasts are Candida spp., Geotrichum spp., and Saccharomyces spp.

Candida albicans is the major agent of mycoses in the mucous membranes and urogenital systems (yeast infections) of humans. Geotrichum spp. are sometimes encountered in direct microscopic examination samples and are observed to form hyphae that fragment into arthroconidia (conidia produced in segments). Saccharomyces spp. (order Saccharomycetales) occur in liquids such as fruit juices or polluted water and have a fermentative capacity.  Saccharomyces cerevisiae is commonly used as baker’s and brewer’s yeast.

Black yeasts are hyphomycetes (asexual fungi) that produce a unicellular budding form resulting in a black, pasty, and sometimes mucoid colony. They include Aureobasidium spp., Exophiala spp., and Rhinocladiella spp.  Aureobasidium spp. is commonly seen in fungal culturable air samples. This fungus first appears as a small white to pink mucoid colony that grows rapidly into a brownish black colony with a white radiating fringe on the periphery. Exophiala spp. and Rhinocladiella spp. are sometimes seen in direct microscopic examinations of samples collected from wood, soil, or water. Exophiala spp. are observed to have one to four celled conidia forming from conidiogenous cells (structures that give rise to spores) with one to three small scars that produce conidia repeatedly. Rhinocladiella species are identified by colorless, single-celled spores which areeither borne on an apical denticulate rachis (zig–zag formation) or on a cluster of denticles.

In viable samples, the yeasts appear initially as a single, mucoid colony which can easily multiply overnight into smaller colonies dispersed throughout the agar plate (Figure 2). Thus, only the original larger colonies should be enumerated because including the satellite colonies may overestimate the total count.  Yeast cells are very rarely encountered on spore traps and are more likely to be observed during direct microscopic examinations of bulks, swabs, or tape-lifts. When examining direct microscopic samples, the generic identification of yeasts is almost impossible if only the yeast phase is present.Other distinguishing characteristics (e.g. the production of asci or the formation of conidia) are needed to make a definitive identification of these fungi.

Yeast colonies on MEA with 0.01% chloramphenicol


This article was originally published on February 2006.