Fungi are eukaryotic organisms and include yeasts, molds, and fleshy fungi.
Yeasts are microscopic, unicellular fungi; molds are multinucleated, filamentous
fungi (such as mildews, rusts, and common household molds); the fleshy fungi
include mushrooms and puffballs.
All fungi are chemoheterotrophs
, requiring organic compounds for both an
energy and carbon source, which obtain nutrients by absorbing them from their
environment. Most live off of decaying organic material and are termed saprophytes.
Some are parasitic
, getting their nutrients from living plants or
The study of fungi is termed mycology
and the diseases caused by fungi
are called mycotic infections or mycoses.
In general, fungi are beneficial
to humans. They are involved in the decay
of dead plants and animals (resulting in the recycling of nutrients in nature),
the manufacturing of various industrial and food products, the production of
many common antibiotics, and may be eaten themselves for food. Some fungi,
however, damage wood and fabrics, spoil foods, and cause a variety of plant
and animal diseases, including human infections.
Yeasts are unicellular, oval, or spherical fungi, which increase in number asexually
by a process termed budding. A bud forms on the outer surface of a parent
cell, the nucleus divides with one nucleus entering the forming bud, and cell
wall material is laid down between the parent cell and the bud. Usually the
bud breaks away to become a new daughter cell but sometimes, as in the case
of the yeast Candida
, the buds remain attached, forming fragile branching filaments
called pseudohyphae. Because of their unicellular and microscopic
nature, yeast colonies appear similar to bacterial colonies on solid media. It
should be noted that certain dimorphic fungi can grow as a yeast or as a mold,
depending on growth conditions.
Yeasts are facultative anaerobes and can therefore obtain energy by both
aerobic respiration and anaerobic fermentation. Most yeasts are nonpathogenic
and some are of great value in industrial fermentations. For example, Saccharomyces
are used for both baking and brewing.
The yeast Candida
is normal flora of the gastrointestinal tract and is also
frequently found on the skin and on the mucus membranes of the mouth and
is normally held in check in the body by normal immune
defenses and normal flora bacteria. Therefore, they may become opportunistic
pathogens and overgrow an area if the host becomes immunosuppressed or is
given broad-spectrum antibiotics that destroy the normal bacterial flora.
Any infection caused by the yeast Candida is termed candidiasis. The most
common forms of candidiases are oral mucocutaneous candidiasis (thrush),
vaginitis, onychomycosis (infection of the nails), and dermatitis (diaper rash
and other infections of moist skin). However, antibiotic therapy, cytotoxic and
immunosuppressive drugs, and immunosuppressive diseases such as diabetes,
leukemias, and AIDS can enable Candida
to cause severe opportunistic systemic
infections involving the skin, lungs, heart, and other organs. In fact, Candida
now accounts for 10% of the cases of septicemia. Candidiasis of the esophagus,
trachea, bronchi, or lungs, in conjunction with a positive HIV antibody test, is
one of the indicator diseases for AIDS.
The most common Candida
species causing human infections is C. albicans.
This organism is usually oval and nonencapsulated, but under certain culture
conditions may produce pseudohyphae, elongated yeast cells 4–6 mm in diameter
that remain attached after budding to produce filament-like structures similar
to the hyphae of molds. The pseudohyphae help the yeast invade deeper
tissues after it colonizes the epithelium. Asexual spores called blastospores
develop in clusters along the pseudohyphae, often at the points of branching.
Under certain growth conditions, thick-walled survival spores called chlamydospores
may also form at the tips or as a part of the pseudohyphae.
A lesser known but often more serious pathogenic yeast is Cryptococcus
neoformans. Like many fungi, this yeast can also reproduce sexually and the
name given to the sexual form of the yeast is Filobasidiella neoformans
. It appears
as an oval yeast 5–6 mm in diameter, forms buds with a thin neck, and is
surrounded by a thick capsule. It does not produce pseudohyphae and
chlamydospores. The capsule enables the yeast to resist phagocytic engulfment. Cryptococcus
infections are usually mild or subclinical but, when symptomatic,
usually begin in the lungs after inhalation of the yeast in dried bird feces.
It is typically associated with pigeon and chicken droppings and soil contaminated
with these droppings. Cryptococcus
, found in soil, actively grows in the
bird feces but does not grow in the bird itself. Usually the infection does not
proceed beyond this pulmonary stage. In the immunosuppressed host, however,
it may spread through the blood to the meninges and other body areas, often
causing cryptococcal meningoencephalitis
. Any disease by this yeast is usually
Dissemination of the pulmonary infection can result in a very severe and
often fatal cryptococcal meningoencephalitis
. Cutaneous and visceral infections are
also found. Although exposure to the organism is probably common, large
outbreaks are rare, indicating that an immunosuppressed host is usually required
for the development of severe disease. Extrapulmonary cryptococcosis
conjunction with a positive HIV antibody test, is another indicator disease for
can be identified by preparing an India ink or nigrosin-negative
stain of suspected sputum or cerebral spinal fluid in which the encapsulated,
budding, oval yeast cells may be seen. It can be isolated on Saboraud dextrose
agar and identified by biochemical testing. Direct and indirect serological tests
may also be used in diagnosis.
i, once thought to be a protozoan but now considered a
yeast-like fungus belonging to the fungal class Ascomycetes, causes an often
lethal disease called pneumocystis carinii pneumonia (PCP). It is seen almost
exclusively in highly immunosuppressed individuals such as those with AIDS,
late-stage malignancies, or leukemias.
PCP and a positive HIV-antibody test is
one of the more common indicators of AIDS.
In biopsies from lung tissue or in tracheobronchial aspirates, both a unicellular
organism about 1–3 mm in diameter with a distinct nucleus and a cyst
form between 4–7 mm in diameter with 6–8 intracystic bodies, often in rosette
formation, can be seen.
- P. carinii cysts from bronchoalveolar larvage
- P. carinii cysts from the lungs
We will use 3 agars to grow our yeast: Saboraud Dextrose agar (SDA),
mycosel agar, and rice extract agar. SDA is an agar similar to trypticase soy
agar but with a higher sugar concentration and a lower pH, both of which
inhibit bacterial growth but promote fungal growth. SDA, therefore, is said to
be selective for fungi. Another medium, Mycosel agar, contains chloramphenicol
to inhibit bacteria and cycloheximide to inhibit most saprophytic fungi. Mycosel
agar, therefore, is said to be selective for pathogenic fungi. Rice extract agar
with polysorbate 80 stimulates the formation of pseudohyphae, blastospores,
and chlamydospores, structures unique to C. albicans
, and may be used in its
identification. The speciation of Candida
is based on sugar fermentation patterns.
- One plate each of Saboraud dextrose agar, mycosel agar, and rice extract
Trypticase soy broth cultures of Candida albicans
and Saccharomyces cerevisiae
- With a wax marker, divide a Saboraud dextrose agar and a Mycosel agar
plate in half. Using a sterile swab, inoculate one half of each plate with
C. albicans and the other half with S. cerevisiae. Incubate the 2 plates at
37°C until the next lab period.
- Using a sterile swab, streak 2 straight lines of C. albicans into a plate of
rice extract agar. Pick up a glass coverslip with forceps, dip the coverslip
in alcohol, and ignite with the flame of your gas burner. Let the coverslip
cool for a few seconds and place it over a portion of the streak line so that
the plate can be observed directly under the microscope after incubation.
Repeat for the second steak line and incubate the plate at room temperature
until the next lab period.
- Describe the appearance of Candida albicans and Saccharomyces cerevisiae on
saboraud dextrose agar.
- Remove the lid of the rice extract agar plate and put the plate on the stage
of the microscope. Using your yellow-striped 10X objective, observe an area
under the coverslip that appears “fuzzy” to the naked eye. Reduce the
light by moving the iris diaphragm lever almost all the way to the right.
Raise the stage all the way up using the coarse focus (large knob) and then
lower the stage using the coarse focus until the yeast comes into focus.
Draw the pseudohyphae, blastospores, and chlamydospores.
- Observe and make drawings of the demonstration yeast slides.