Background and Purpose:
The fungi form a large, diverse group of eucaryotic organisms belonging to the Kingdom
Myceteae. Nutritionally, these organisms are heterotrophs.
After secreting exoenzymes into the environment, digested nutrients
are then absorbed. Most fungi exists as saprobes by decomposing
dead organic material, whereas some may also be parasites of living plants and
animals. Several species are true or opportunistic fungal pathogens
of humans.
The
molds consists of filaments called hyphae. Most molds possess
septate hyphae where each filament is structurally composed of
individual cells separated by septa or cross-walls. These septa
possess central openings that permit cytoplasmic streaming between the cells.
Other molds feature
nonseptate hyphae which lack
cross-walls. Hyphae spread across the surface of a food source and
penetrate into it. Besides anchoring the mold to this material, they function to digest and absorb nutrients.
Hyphae which grows above the surface
produce fruiting bodies that form spores. After spores
disseminate to another suitable substrate, they can germinate to initiate the
formation of new colonies.
The molds we will examine include
four species. Penicillium and
Aspergillus possessing septate hyphae and produce conidia
(asexual spores) at the end of aerial hyphae called conidiophores.
In comparison, Rhizopus
and Mucor having nonseptate
hyphae and produce asexual sporangiospores within a sac called
the sporangium. The sporangia located at the end of aerial hyphae
are called sporangiophores. The sexual spores of these molds
called zygospores are formed when
the hyphae from two different strains fuse together.
Clinically, all the
mold species we will
study here can cause opportunistic infections in critically-ill or
immunocompromised patients. In addition to histological stains
and serological tests, accurate diagnosis of these and other mycoses are dependent upon
isolating and correctly identifying the fungal pathogens through macroscopic and
microscopic examination of laboratory cultures. When isolated on solid culture media, each mold
grows into an interwoven network of hyphae collectively called the
mycelium. The overall size,
color, texture, and organization of the vegetative and aerial mycelia are useful cultural
characteristics when identifying molds. Equally important, we will
concurrently examine
each mold's microscopic features. These include the type of hyphae
(septate or nonseptate) and spore-producing structures they exhibit.
This plate
exhibits a wide variety of mycelia
grown from molds
sampled from the fur of a
badger
taxidermic specimen.
We will
also examine three species of yeast. Unlike the molds, these microscopic fungi
do not form hyphae. Instead, yeasts are unicellular,
typically having spherical or oval shape cells. Reproduction is mostly accomplished
asexually through budding. During this process the parent cell
forms an outgrowth or bud that eventually detaches, becoming a new
daughter cell. If buds fail to separate from the parent cells, this may
lead to the formation of short chains of cells called a pseudohyphae.
When grown on a particular medium, some yeasts may exhibit sexual reproduction
and produce sexual spores.
The yeasts species we
will study have been reported to cause various opportunistic infections in
humans. Correct identification requires the examination of both macro- and
microscopic features. These characteristics, in conjunction with other
clinical tests and observations, are needed for accurate diagnoses.
Some fungal species may actually have both a mold and yeast life cycle stage.
These species are said to be dimorphic or biphasic. This ability
is largely determined by temperature, but it is also influenced by other factors
such as levels of oxygen and nutrients. True fungal
pathogens to humans are dimorphic. These include Histoplasma capsulatum and
Coccidioides immitis. In their natural habitats, these saprophytic
fungi grow as molds in soils where temperatures are usually lower than 30ºC.
However, when their spores gain entrance into a warm human body (37ºC),
these fungi germinate into a more invasive, parasitic yeast phase.
Structurally and physiologically, yeast cells are better adapted for survival at
the body's warmer temperature and are relatively resistant to phagocytosis.
Some yeasts may actually reproduce within phagocytes.
