I hope you are doing well and
will find the following article about xerophillic fungi by Harriet Burge and about yeasts by Karen Abella
Santo-Pietro both interesting and helpful.
With best wishes,
By Dr. Harriet
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
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
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?
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.
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.
By Karen Abella
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).
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
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.
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.
(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
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